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General Category => General Discussion => Topic started by: EricS on July 30, 2019, 04:19:35 PM

Title: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on July 30, 2019, 04:19:35 PM
For the past two years, I've been gathering parts for a new tube amp - the Magnequest iron took the longest time to track down and several of you here have contributed - thank you.  I have built lots of solid state stuff, but this will be my first tube amp.  The design is Paul Joppa's parafeed update to Joe Roberts' Western Electric Model 91 300B that was published in Sound Practices in the Summer 1992 issue.  Paul has graciously granted his permission for me to post his design and my thread here.  Thank you, Paul!

This build will likely take me several months to complete from here.  Since this is my first tube build, I'm likely to need a fair amount of guidance in the form of "hey, dummy - don't do that!"  I have read Morgan Jones' "Building Valve Amplifiers" and have tried to incorporate his long list of physical layout guidelines, though I'm sure I have missed several fundamentals. 

Attached are Paul's schematic (I have added all of the parts values) and my first attempt at a 2-dimensional layout the physical components (quick and dirty Visio drawing - for easy rearranging over time).  The overall top plate as of now is 11" x 17" and each of the parts is drawn relatively close to actual scale.  The final layout will likely be a little smaller in footprint.  The blue components will be mounted on the top of the plate, the orange components will be hidden on the bottom of the plate.  I have aimed for both visual appeal (physical symmetry of top-of-plate parts) and what appears (to me, at least) to be a reasonable first pass at an electrical layout.     

Here is where I need your help:  What have I done wrong?  Which parts should not be next to one another?  How can I improve the parts layout to improve running wires or preventing unwanted interactions?

My ongoing thanks and appreciation in advance!
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Tom-s on July 30, 2019, 07:51:27 PM
The book you've read is the first thing i'd recommend to reread  :P. It covers all basics on tube amp layout. With every layout i try, i'd reread it several times and combine this with the BH manuals i own. To see what practices i could improve on, even after the breadboard. Searching the internet for completed versions of your amp also helps a lot ime.
I'd advice on revisiting the pages about transformer and choke orientation relative to eachother (there's always room for improvement).
The most important part is your grounding scheme, make this your first step in the build. The book is a good reference for this, but here's another good link: https://www.aikenamps.com/index.php/grounding
Good luck with your build.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on July 31, 2019, 03:57:59 AM
I think you have the PGP 8.1 and the TFA-2004 aligned.  To double check this, tile print your layout to scale, set the parts on the piece of paper, then take another photo.

I would also double check with PJ to be sure that L3 and L4 are separate parts.  I suspect they are intended to be different windings on the same choke.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Joppa on July 31, 2019, 07:49:22 AM
L3 and L4 are separate, since for this I wanted to specify readily available parts (except for the Magnequest items - I did this design for the MQ forum).
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on July 31, 2019, 09:51:02 AM
Do you have a part number for your R1?
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on July 31, 2019, 10:34:49 AM
Tom - thanks for the link to the Aiken Amps article on grounding.  I'm reading it now.  I also have been looking at a few of the Bottlehead builds that people have documented  in various places as well as some kit manuals of various origin.  I see that many of the Bottlehead kits feature star grounding.  The Aiken concept of multiple stars (PSU, small signal) looks interesting.  I'll have to draw out some wires on my diagram and see how to optimize major components to keep things clean.

Paul B - I used this resistor RH05050K00FE02 : https://www.mouser.com/ProductDetail/Vishay-Dale/RH05050K00FE02?qs=sGAEpiMZZMtbXrIkmrvidDNaDpN5VXc5qswg6VdgV68%3D  Not sure if it was the "normal" or "non-inductive" variety that I purchased, I'll have to check my invoice.  Good eye, my power and output transformers are not quite aligned - this is an easy fix.

Paul J - Thanks for the confirmation.  I take it from our previous email that asymmetry in the chokes for the DC filament supply is not really a problem if I'm trying to hit a specific voltage target.   Is stacking these chokes OK for conserving physical space under the chassis, or should they be mounted 90 degrees to one another to avoid interactions?  Or will arranging them 180 to one another provide some benefit if they are stacked or physically parallel to one another?
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on July 31, 2019, 10:57:44 AM
I'm glad you went up a size.  Those chassis mount resistors get really hot without a huge heatsink as you approach a small percentage of their rated power, but with a 50W one I think you'll be OK.

If the coils on the TFA-2004 and PGP8.1 are aligned in your photo, I would recommend rotating the PGP8.1 rather than the TFA-2004.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on July 31, 2019, 04:35:43 PM
PB  - Yeah, I thought moving from 25w to 50w for the bleeder resistor would be a good move.  The data sheet still indicates a pretty steep derating curve, even when the resistor is mounted to a 12" x 12" metal plate.  I also adjusted the PGP transformer orientation and added some detail in the diagram to indicate the direction of the transformer laminations for clarity.

I'm working on seeing what the grounding/wiring layout looks like with this parts arrangement. 
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Tom-s on August 01, 2019, 07:46:25 PM
Where did you find 40uf MBGO's? I'm planning on the same as cathode bypass for the SII 2a3, but only found 30uf as largest size (so settled on using 2/channel if it fits). A single 40uf would sure help.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on August 02, 2019, 05:09:51 PM
Ah - you've caught me.  My 40uF MBGO is actually two individual 20uF 160v caps.  Compared to the paper and tin foil caps, these are pretty small.  I like them because they have a nice mounting tab welded across the bottom.    I measured all of my Russian caps for leakage, the MGBO had about 1uA of leakage on my meter.  I also charged them up to 150vDC and let them sit for 24 hours.  After this time, two of them had only 8% of their initial charge left, one had 26% left, and the best of the lot had 40% of the initial charge after a day.  I'll see how they perform in the amp.  If they don't work out, the MBGO will get replaced with an ASC 45uF big can cap.   

The OKBG are paper and tin foil and performed much better.  I could measure no leakage on my meter and these all had about 70% of their initial charge (350vDC) remaining after 24 hours.  I figured none of this was very critical as they are all bypassed by resistors that will drain them much faster than their own internal discharge rates will. 

The Aiken Amps paper on grounding is an excellent resource.  I've read it several times now, making notes along the way, and have a trivia question for the crowd:  Is Star grounding or Buss grounding preferred?    I'm thinking this design lends itself nicely to using three star grounds, one on each of the main 20uF caps to make increasingly "quieter" ground points.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Bardamu on August 03, 2019, 06:04:02 AM
Hello,
This drawing from A Dutch side is the way i usually do it.
And i always follow the advice to keep wires between transformer, rectifier, input choke and first capacitor at a minimum length.
Greetings, Eduard
P.s i hope attaching worked
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on August 03, 2019, 07:11:46 AM
Interesting, that looks like two stars: one for the psu, a second for the amp section...
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Bardamu on August 03, 2019, 07:39:33 AM
Hello Eric,
Their explanation. The closer the cap is to the transformer/ rectifier combination the bigger the current will be so that connection should be short. The last capacitor in the power supply they regard as a part of the circuit itself. There should not be much of a ripple left to take care off.
Greetings, Eduard
P,s i think i will try to get the majority of the caps/ the number of mF have their return to the spot located close to the transformer and then have a cap like 10 mF right next to the output transformer??
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on August 03, 2019, 07:43:41 AM
The last capacitor in the power supply they regard as a part of the circuit itself. T
This is absolutely true in a series feed amp, but far less important in a parallel feed amp, as the parallel feed cap takes over the lion's share of this duty.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Bardamu on August 03, 2019, 09:27:03 AM
Hello Paul,
Is this comparable to my future amp as seen in the attachment where a so called ultrapath cap is used.
They say it takes the power supply caps  out of the circuit. But what is also does is injecting noise from the power supply into the circuit. So they say the power supply must be dead quiet.
So one must take care of this by using good parts in the supply. But the quality of the ultrapath must be top notch too i guess.
Greetings, eduard
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on August 03, 2019, 09:38:11 AM
I am definitely not sold on the ultrapath topology.  You end up moving where the cathode bypass cap is and requiring it to have a significantly higher voltage rating.  Ultimately to do so with a film cap ends up requiring the same part that you would use for a power supply filter stage anyway.  The observations about the issues of power supply noise are quite relevant.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on August 03, 2019, 01:25:21 PM
I've been playing around with wiring and parts layout and have two diagrams now.

The first is the original layout that I posted a few days ago - we'll call this one V1.  The primary problem that I see with this layout is that the wiring turns into a rat's nest pretty quickly.  I haven't been able to draw a wiring diagram for this layout that I'd call clean or neat.   

The second one -V2- will keep the wiring neater with fewer crossovers, shorter overall wire lengths, and what appears to be a "cleaner" grounding configuration, but requires running AC for the driver heater all of the way to the opposite corner of the chassis, past the output transformer & tube.  I'm thinking this might be a problem.  I'm also not sure where the best place for the speaker outputs would be on this design.

Which one of these do others prefer?  Or should I keep playing around and come up with a third layout?

   
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on August 03, 2019, 01:50:08 PM
If your heater wiring is well twisted, there's far less concern about picking up additional noise. 
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on August 05, 2019, 07:45:56 AM
Thanks for the confirmation, Paul.  I suspected this would be the case but don't have any experience with the tradeoffs.  I suspect it is better to keep the heater AC as far away from the rest of the circuit as possible, but I can't seem to come up with a configuration that lets me do this cleanly.

I've been working on a v3 parts layout that has a much cleaner grounding and overall wire layout than what I've posted so far.  In the mean time, I have two questions:

1) can I stack L3 and L4 (300B filament chokes) on top of one another to save a little space?  Or is this an example "don't do that" ?
2) is the position of the EXO-03 B+ choke OK in physical relation to the PGP8.1?   
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on August 05, 2019, 09:22:54 AM
You can certainly stack the filament chokes.

In the image you just posted, the PGP8.1 and TFA-2004 appear to be aligned,  I would rotate the TFA-2004.

The EXO-03 shouldn't cause you any problem, especially being all the way across the chassis from the power transformer and oriented as it is.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on August 07, 2019, 04:41:51 PM
I see what you are saying about the power and output transformers, Paul - thank you.

I reread the section in Morgan Jone's book about transformer alignment and I think I was misunderstanding his intention.  Most of the reference I came across online indicated aligning the transformer laminations at 90 degrees to one another and the more important part is the alignment of the coil winding axis (duh...).

I also found a helpful piece of advice on another forum that suggested laying out all of the parts as indented in the completed amplifier then energizing the power transformer.   Then connect each transformer of interest to a DMM set to ACmV and see how much voltage is induced for each transformer at its intended location.

Doing this led to some minor adjustments.  The output transformer seemed to be the most sensitive with 19mV being induced in the primary.  Rotating it 90 degrees cured this, reducing the voltage down to 0mV.  The B+ choke and the 300B filament chokes seemed rather immune to just about any placement relative to the PGP.  I moved the EXO choke for the 300B plate back a little and repositioned the grid choke a little to reduce their interaction with the mains transformer as well.  The resulting configuration is attached.  I'm thinking this is as good as I can do right now until I prototype the layout.

This configuration works fairly well for a neat grounding scheme with a local star ground on each of the three big caps across the top of the diagram.  I'll post a ground wire diagram shortly.

Please feel free to point out anything else that might look problematic at this point.

Thanks!

Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Bardamu on August 07, 2019, 11:23:05 PM
Hello Eric,
This trick with a dmm is well know.
I have done it in the past with my DDDAC where there is 1A running through the secundary of the power transformer. This was an R core transformer and it did not interfere at all with other iron nearby.
ALSO depends a lot on the number of Henry the other iron has. 
Sometimes i would use a bad transformer just to find out the optimum orientation of the an iron on another spot if that spot was kind of predetermined.
Usually i am using Lundahl chokes and they are all less sensitive to their surrounding and also disturb much less themselves.
It could well be that the filament chokes would radiate into the 40 H iron when they have some current running through them.
The Lundahls i used so far dont disturbe more in other orientation. What i would usually do is using the highest H anode choke i have ( 100 H 25 mA) to magnify the results.
In all my builds i use choke input and usually that choke is famous for its disturbance. To get the best results they always say to mount transformer rectifier choke and first cap close to each other. SO far never had a problem.
By the way i am also a fan of the russian caps. My favourite so far is the K75-10. I have used them in my small system in the cross over and recently i send a collection of caps to a friend in Czech Republic to use as bypass in power supply which is  5r4gyb, 30H, 16 mF Philips paper in oil, 10 henry and then a 80 mF oil containing Eyuda cap. I send him several caps to put in parallel with the Eyuda. His favourite was a 4700nF K75-10. At the time you could buy them for 3$ on ebay now they have gone up but they are still cheap compared to the usual audiophile caps. YES, i tried them too. Still have loads of MIT in the attic.
In the attachment my iron loaded DDDAC.
greetings, eduard
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on August 08, 2019, 04:48:54 PM
I've built lots of very nice solid state gear, so when I starting planning for this amp I wanted to build as much of it as I could with "more traditional" parts.  So, I'm curious to hear what the Russian PIO caps sound like.  Just waiting on the 300B tubes to arrive from Jac and I'll have just about everything I need to start prototyping.

This diagram is essentially the same as the last, but now includes the grounding scheme.  It seems to be relatively clean and neat so far...

As far as prototyping goes, do people tend to prefer wood or metal?  Wood will probably be easier, but if I use aluminum, it will probably be a better approximation of the final version. 
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on August 09, 2019, 04:52:54 AM
For prototyping, a piece of plywood is good.  The cheap Chinese ceramic wafer sockets can be mounted upside-down right onto the plywood, then you can solder to the terminals and plug the tubes in pretty easily.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on August 09, 2019, 05:33:50 AM
That's a clever trick!  It took a little while, but I was able to source all NOS Johnson sockets for my build.  I bet this same trick will work with them as well.

I presume when using a wooden base that a small 3"x4" scrap of aluminum sheet will suffice for each of the power resistors as a proof of concept?  The data sheet shows they're good for 15w dissipation at 100c in free air (no sink, derated to 30%).  Since I'll only be running them at 1/3 of this figure and adding a small sink, I'm thinking they should be able to survive... 

Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on August 09, 2019, 05:59:48 AM
For prototyping, you typically won't be running the amp long enough for this to matter.  Heat takes time to accumulate, and if you mount them to a piece of plywood, technically they are in free air and will cool somewhat well.  If you are concerned about hurting the wood, you can add some washers between the resistors and the wood to keep them from touching. 
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Bardamu on August 09, 2019, 06:33:56 AM
Hello,
The resistors will be used well below their rarings for sure.
BUT the will develop quiet a high temperature within a limited time.
I would tather have them connected with a sturdy solid core wire at both ends and let them be surrounded by an inch of air then bolting them to a piece of wood.
Once there is smoke you will carry your amp outside or use a fire extinguisher .
I always try to make a kind of main frame so the amp can rest on most of its sides to check with dmm and infra red thermometer.
Probably once it tumbles you will create multiple short circuits.
Better be safe than sorry.
Greetings, Eduard.
P.s one of the first things to add is a bleeder resistor.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on August 09, 2019, 06:47:59 AM
The resistors will be used well below their rarings for sure.
BUT the will develop quiet a high temperature within a limited time.
The datasheet suggests derating the power by 40% for free air mounting with no heatsink (we could assume the worst that this is to keep the operating temperature under 250C, which is the maximum listed on the datasheet). The OP has calculated that his dissipation would be 10-15% of the original rating, which is far below this, and also well below the temperature needed to ignite wood.  One of these 50W resistors running at 7W will be around 100-110C free air if the 40% derating means 250C @20W with 25C ambient.

In the absence of calculating the actual dissipation of these parts, then indeed it would be wise to keep them away from wood.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Bardamu on August 09, 2019, 07:11:20 AM
Hello,
In the past i have used these alumium body resistors but in order to keep them at a safe temperature they need quiet a lot of aluminium to coll them down. If there are a few on a small chassis ( with a lot of holes to mount other parts) things can get hot for the capacitors.
So i started using the big wattage Mills resistors, Sometimes in parallel but always surrounded by air and having some kind of perforation in the chassis under and above them. I also like it when the wires of the resistors can be used for the connection.
Greetings, Eduard
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on August 21, 2019, 03:31:40 PM
My 300B Meshplate tubes arrived for Jac in Germany today (hooray!), so I had the opportunity to experiment with heater wiring a little bit.  All of the amp parts are attached to my plywood prototype board, but the only things that are actually wired are the filament circuits.  The circuit is PGP8.1 --> Diodes (IXYS DSS 10-0045B 10A 45V) --> filament chokes --> 300B filament.  I have not wired up the 15R hum pot with 22R flanking resistors for the 300B filament.  The following are the measurements I found when I energized the PGP8.1:

With 155B (6mH 0R3) chokes after 5 mins:
    Driver tube filament measured 6.96vAC
    300B filament measured 4.75vDC

In my communications with Jac, he indicated that I should see 5.2v-5.4v on the filament of the Mesh tube right away after first power up and when everything warms up (about 45mins), the voltages should drop a bit to somewhere between 5.0v and 5.2v DC on the filament (5.0vDC is the desired target).  So, I swapped out both of the 155B chokes for 154B chokes.

With 154B (3mH 0R15) chokes after 5 mins:
    Driver filament measured 6.96vAC (same as before)
    300B filament now measures 5.1vDC  - I was happy to see this voltage come up a bit with the new chokes, but it's still a pinch low.

This all looked good to me, but then I found something that I thought was strange:  I was surprised to find AC voltage on the 300B filament!  Without the 300B tube in place, I measure 3.1vAC and with the 300B in place, I measured 1.64vAC.

Is AC voltage on the 300B filament normal?  Is it because the hum pot and grounding are not in place?  Sorry if this is a noob question, this is completely new territory to me.

Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on August 21, 2019, 04:09:02 PM
There will indeed be some ripple.  PJ will certainly offer some insight into how concerned to be about the 4.75V.  FWIW, Jac's website says 5V +/-4% the last time I looked at it, which is 4.8V minimum.  If you can look at the output of that supply with a scope, you should be able to discern what's going on a bit better.

Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Joppa on August 21, 2019, 04:54:30 PM
Yes, the choke filter reduces the AC but does not eliminate it. IIRC, the hum goes down 10dB or a little more.

You can add a capacitor at the power supply output (across the filament) to reduce the ripple if the hum is audible - 10,000uF at 6.3 to 10 volts would be reasonable. You can also increase the voltage if necessary by using a (much smaller) cap across the input (upstream side of the chokes). The high voltage power will alter the filament voltage, so don't get carried away until the rest of the amp is built.

Also, check your power line voltage. If it is low, then all the other voltages will also be low.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on August 22, 2019, 03:58:46 AM
Ah, it didn't occur to me that what I was seeing was ripple - I'm used to mV ripple figures in solid state stuff...  Makes perfect sense that a cap after the chokes will help reduce this.  I have a 10,000uF 50v cap in my parts box that I can attach across the filament as an experiment then I can check the waveforms with my scope.  I can also play around with various configurations of LC or CLC or adding small (0R1 or 0R22) resistors to hit the target voltage in a completed amp.  The Duncan Amp simulation provides some useful guidance on what to expect with different size caps in this domain. 

In my correspondence with Jac, he recommended that 5.0v with 0% tolerance is best for the EML 300B tubes, but 5.0v to 5.2v is a good target for a warm amp.  He also indicated that the tubes are more forgiving of slightly over-voltage than slightly under-voltage and advised not to run them below 5.0v.

Also good to know that total draw on the transformer might have some impact on individual secondary windings - I was wondering if things might behave differently once the entire amp is completed (more current, more flux, more heat, etc in the mains transformer).

Thank you!
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: 2wo on August 22, 2019, 09:35:30 AM
In your setup, the filement choke is reducing the ripple but not eliminateing it. This is likely to be fine. Many people use  straight  5v AC with  acceptable hum, it depends on how  sensitive your speakers are. I  would try it this way before making changes and if necessary you could go back and add additional stages of filtering. Also when you add the hum pot and associated resistors you may need to readjust the voltage at the filliment ....John
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on August 22, 2019, 03:13:55 PM
That makes sense to me, John.  I was just surprised when the AC ripple was half of the DC voltage.  Thanks for the insights.  Just as a quick experiment, addng the 10,000uF cap in conjunction with the 154B chokes reduced ripple to 80mV and provided a DC voltage of 5.1 after a few minutes.  The cap has the added benefit of providing a nice & soft ramp up curve for the filament.  This seems highly worthwhile to me.

I'm working on wiring up the rest of the amp now.  Given my schedule, this will likely take another few days to complete. 
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on August 22, 2019, 04:33:57 PM
Trivia question - how do people prototype the wire connections in their amps?  Is it OK to use these "European-style" screw terminal blocks to tie things together? Or should I be soldering all of the connections?   
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: grufti on August 22, 2019, 05:43:56 PM
WAGO connectors are perfect. You could actually build your final product with them for most connections. Flip open - insert wire - flip closed - again - and again - and again.

Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Deke609 on August 23, 2019, 02:56:22 AM
WAGO connectors are perfect.


Thanks for this! Those things look great. Just ordered some.


cheers, Derek
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on August 23, 2019, 04:35:03 AM
The Euro blocks are OK.  Often when I'm prototyping on wood I have a lot of components with really long leads, so I'll just solder those and end up cutting off the soldered part when I build the final amp.

The 5 lug terminal strips that we use are also super easy to screw into a piece of plywood with a wood screw.

I've attached a photo of a plywood prototype I made a while ago.  It also shows the little 9 pin breakout boards that I use that can be screwed to a piece of wood and wired up pretty easily.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Deke609 on August 23, 2019, 07:07:14 AM
500V bench power supply. Nice!
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on August 23, 2019, 03:10:14 PM
Those WAGO connectors look pretty cool!  I should pick up a set for other projects as well!  Looks like both these and the Euro blocks are fully insulated, so they don't leave high voltage exposed to the touch or accidental shorts.

Paul, you've gotta provide some details on that awesome looking tube on your wooden breadboard!  My prototype is much smaller - I am trying to proof the idea with the same physical layout that I'm planning for the final chassis.  Here are some images of where I am now - lots of flying leads at present and I made a crude aluminum sink for the big power resistors.  And, yes, those a custom-made, precision cap clamps made from folded layers of duct tape and held down with a screw & washer for the time being.  I guess I used up all of my plastic plumbers strap because I couldn't find it.

Question: Is it OK to leave just the filaments powered up on their own for 30-45 mins to see how things behave as the PSU warms up?  I was thinking this might be a good step before moving onto the high voltage wiring...
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on August 23, 2019, 03:15:40 PM
Oh yeah, you'll want some old clapped out tubes to use when you're getting all this going.  Someone's random single 300B where the other matching 300B died an early death (that's how I got my prototyping 300B).  That way you don't have to worry so much about damaging an otherwise expensive tube.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on August 23, 2019, 04:28:46 PM
I had a thought about an old/cheap tube a short while ago as insurance during the build process...  I'll post something and see if anything turns up.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on August 23, 2019, 04:35:38 PM
https://www.ebay.com/itm/Sovtek-300B-Tube/113835500802?hash=item1a811fe502:g:9SYAAOSw01pc9ycr (https://www.ebay.com/itm/Sovtek-300B-Tube/113835500802?hash=item1a811fe502:g:9SYAAOSw01pc9ycr)

https://www.ebay.com/itm/New-Psvane-300B-HiFi-Series-Vacuum-Tube-Plate-Current-Tested-GOLD-PINS-Hi-Fi/153017349251?epid=22010308575&hash=item23a08b7483:g:dvUAAOSw0Pxa9I2p (https://www.ebay.com/itm/New-Psvane-300B-HiFi-Series-Vacuum-Tube-Plate-Current-Tested-GOLD-PINS-Hi-Fi/153017349251?epid=22010308575&hash=item23a08b7483:g:dvUAAOSw0Pxa9I2p)

Those are the only options I see at the moment.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on August 24, 2019, 03:08:24 AM
Thanks for the link - just ordered a spare tube.  I feel better about experimenting with a $60 tube until I know the amp is working and stable!
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on August 27, 2019, 04:17:53 PM
Just got the filaments powered up on all three tubes for the first time.  With wall voltage at 123v, I'm getting the following heater voltages:

5AR4 Rectifier: 5.38vAC
6SJ7 Driver: 6.65vAC with a 1R resistor in series with the filament.  Was 6.9v all by itself.
300B Output: 5.1vDC
B+ was 407vDC with no measurable ripple (B+ was not connected to the circuit or to a cap)

This is as far as I'm willing to go with the circuit until my "spare" tube arrives in another day or so.

In the meantime, have a few questions:
-The B+ seemed a little low to me, I was expecting something north of 430v without a load.  Is the B+ voltage low because it wasn't connected to a cap?
-In addition to an AC mains fuse, I was planning on adding a B+ fuse.  Should anything else be fused?
-I was planning to install some solid state diodes on the tube rectifier as specified here: https://www.premierguitar.com/articles/the-immortal-amplifier-mod-1 (https://www.premierguitar.com/articles/the-immortal-amplifier-mod-1).  This seems like cheap insurance for a tube rectifier - has anyone else done this?
 
Thanks for the help!
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on August 27, 2019, 06:19:09 PM
With no cap at the input, the B+ voltage will be low.  The solid state diode mod isn't a bad idea, and I'm not so sure that I would fuse anything other than the mains.  If something faults badly enough to blow the B+ fuse, it may very well blow the fuse between the power cord and the primary of the power transformer first.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Joppa on August 27, 2019, 06:21:27 PM
Yes, the B+ voltage was low because it wasn't connected to a cap. Without a load but with a cap, you'll see more like 500-600 volts. Be careful with that!
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on August 28, 2019, 01:23:59 AM
Good to know on all fronts - thank you!  I was thinking B+ was low, but I didn't think adding a cap could bring it up as much as another 100-200v!!

In the mean time, I've been double- and triple-checking my wiring in anticipation of a first full power up.  The only wiring I have not done in the prototype build is to ground the transformer frames.  Is this a necessary step right now?

 
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Deke609 on August 28, 2019, 03:05:13 AM

The design is Paul Joppa's parafeed update to Joe Roberts' Western Electric Model 91 300B that was published in Sound Practices in the Summer 1992 issue. 

For others following along with Eric's very cool project, here's a link to the original Sound Practices article by Joe Roberts which includes the 1992 schematic based on a series feed typology (whereas Eric is building PJ's parallel feed reworking of the circuit): http://diyaudioprojects.com/Technical/Papers/Single-300B-Tube-Amplifier-WE91A.pdf (http://http://diyaudioprojects.com/Technical/Papers/Single-300B-Tube-Amplifier-WE91A.pdf)   [Edit: if the link doesn't work, check if the ":" is missing after "http" - insert it if it's missing and the link should work]


Eric: What a great project! Looking forward to more posts as it progresses.


cheers, Derek
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on August 28, 2019, 04:55:31 AM
For a plywood prototype, I wouldn't worry about grounding the frames of the iron.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on August 28, 2019, 06:40:29 AM
I won't worry about grounding the transformers, then.  Without having to charge the PSU caps, a fast-blow 1A fuse on the mains was fine for powering up just the three filaments.  Rough calculations show the total current draw for one completed channel is about 47w, so I'm guessing 1A rating will be fine.  Though, adding filter caps might require moving to a 2A fuse in order to handle power on surges. 

It also looks like my spare 300B is supposed to arrive tonight, so I'll give it one more careful cross-check with the schematic before powering up the entire amp.  It just might be able to make some music tonight!

Derek- thanks for posting a link to Joe's 1992 article.  I should have done this at the top of the thread.  Here is the thread where Paul originally posted this parafeed update to Joe's circuit: http://www.audioasylum.com/cgi/vt.mpl?f=set&m=73171 (http://www.audioasylum.com/cgi/vt.mpl?f=set&m=73171).  It appears about 1/3 of the way down the page and includes the schematic and some detailed parts notes.  I updated the schematic that appears in the first post with info from Paul's parts list.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on August 28, 2019, 07:26:58 AM
Just to share some resources, I found this very nice thread/guide concerning first power up of a tube amp:
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=3676&sid=6e4a1bc3ebc79835d7674cc4ad938c5a&start=0 (http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=3676&sid=6e4a1bc3ebc79835d7674cc4ad938c5a&start=0)

I also have a great DIY 4R/8R dummy load that I made for my solid state gear.  It features a set of binding posts for easy hook up, a regular wall switch that moves between 4R and 8R, and a 10:1 voltage divider that outputs to an RCA jack for distortion analysis with my PC.  The power resistors came from MJPA: https://www.mpja.com/16-Ohm-120W-Power-Resistor/productinfo/17789%20RS/ (https://www.mpja.com/16-Ohm-120W-Power-Resistor/productinfo/17789%20RS/).  Each one is good for 120w, so I can drive ~500w into a 4R load with this arrangement.  Don't think my 300B will develop quite so much power  ;)
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on August 28, 2019, 07:32:18 AM
That looks much better than all the clip leads I use to do the same thing!
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on August 28, 2019, 04:27:25 PM
I checked and double checked the schematic to my wiring.  Everything looked good, so I attached a number of DMMs, plugged it into my variac and brought up the voltage over a span of about 15-20 seconds so I could watch what was happening.  This is always the most nerve-wracking moment to me - the first full scale power up.  After a few tense moments, none of the magic smoke escaped anywhere and voltage readings on the PSU and a few other critical places looked pretty good to me.

Attached is the schematic with voltage measurements on it.  The voltages that appear adjacent to a component are the voltage measurements ACROSS that component.  The voltages inside the tube symbols are the plate voltages referenced to the top of the respective cathode resistors. 

With the hum pot, I was able to adjust hum at the speaker terminals down to 1mvAC !  I thought this was pretty impressive without having the 10,000uF cap in place for the 300B heater.  I increased the resistor in series with the 6SJ7 heater to 2R and the voltage measured exactly 6.3vAC.

The B+ is running about 15v high, but this still seems OK:  343v at the plate * 0.073mA of current = 25w dissipation.  Jac recommends running the 300B Meshplate between 22w and 28w dissipation - 25w sits exactly in the middle.

All of this looked great to me, with the exception of two related measurements: 
- The plate voltage of the 6SJ7 measured 223vDC referenced to the top of the R6 cathode resistor - this looks very high.
- The voltage across the R6 is only 2vDC - this seems very low to me.

I know the 6SJ7 tube is good, it was recently measured on a tube tester.  I did not swap it out for another tube, though...

Any insights on what is going on with R6 and the 6SJ7 plate voltage?
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Joppa on August 28, 2019, 05:47:32 PM
The 6SJ7 does seem to be out of spec. It could be very used, but it's also possible that it will return to proper functioning if it runs for a few dozen hours. If you have other 6SJ7s, give them a try! Sometimes the cathode deteriorates when a tube sits around for a few decades, and needs some time running current for the cathode to "form" again.

It's not so far out of spec that it won't work; you should be able to get sound out, though it won't be optimum.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on August 29, 2019, 02:56:22 AM
Ah - this is something I hadn't considered.  Fortunately, I've been gathering a small hoard of tubes over the past few years, so I have plenty of driver tubes to swap out and see what happens (though, they are mostly older Tung-Sol meshplates or some highly-visually-similar knockoff or rebranded Tung-Sol production).  I used one of my cheaper, no-name tubes for my first startup.

I was a little concerned with my very first power up because the voltage across the PSU bleeder and plate voltages on both the driver and output tubes went a bit negative (minus 1 to 3 volts or so) in the first few seconds as I was ramping up the variac.  I started to panic a little and shut it down to recheck the wiring again.  It took me a little while to realized it was because the rectifier wasn't warm and providing any B+ yet.  It seems that it takes about 15 seconds or so for the rectifier to start producing any voltage at all, then it ramps up over another 10-15s or so.  I knew the rectifier was a "soft start" variety, but I didn't have this in mind when I was powering up for the first time - What a relief!!!   I'm using NOS Mullard F31 rectifiers that tested very strong.

When the plate voltage for the driver tube didn't pan out, I figured it was time to do a systematic voltage measurement and then shut it down for the night rather than continue poking around as it was starting to get late.  I'll double check the driver wiring one more time and then I'll swap in a few of my other tubes to see what happens. 

Thank you for the insights, I hope you don't get tired of my questions!  (because I have a few more)

-When you talk about reforming the cathode, is it sufficient to just run the tube with a hot heater (plate disconnected)?  Or should it be fully connected to the rest of the circuit in order for this process to work?

-Is there any value to removing the 2R resistor that I put in series with the driver heater in order to help this process of reforming the cathode?  With the heater directly connected to the secondary, I get 6.9vAC which seems to be at the top end of the plus/minus 10% window in most datasheets.  I measure 6.6vAC with a 1R resistors inserted in series with the heater (now at plus 5%), and 6.3vAC with 2R in series (exactly on target).

Edit: I just found an interesting write up about how tube tests can lead to erroneous interpretations and evaluations.  I suspect I am a victim of the same when I tested my 6SJ7 tubes.  Link here: https://tubesound.com/tube-testing-how-to-use-a-tube-tester/ (https://tubesound.com/tube-testing-how-to-use-a-tube-tester/)
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on August 29, 2019, 04:34:51 AM
Cathode current means the tube is heated and electrons are being pulled from the cathode (B+ applied).

If you have a tube tester, just power that up and leave the 6SJ7 plugged into it with the "test" running.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on August 29, 2019, 06:05:14 AM
Thanks for the clarification, Paul.  This reinforces my desire to include a voltmeter in the completed amp.  Now I'm thinking I want to put a switch on the meter so I can flip back and forth between plate voltages on both the driver and output tubes.   Maybe a third position for the B+ as well...   And maybe tuck an hour meter under the chassis somewhere...
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on August 29, 2019, 06:57:28 AM
I would strongly recommend not doing that!  The plate voltage on the output tube is not a particularly useful piece of information.  It will always be just a bit below B+, and since you have a cathode biased amp, the amp will keep the idle current of the 300B relatively constant, so all you'll really see is movement based on your line voltage and other abnormalities that have little to do with what your amp is doing.

Similarly the driver plate voltage is going to wander a bit as well, and this also doesn't tell you a whole lot about what's going on.

Meters like this will just drive you crazy and detract from your enjoyment of your project.  Also finding switches that have a reasonably high voltage rating isn't all that easy.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on August 29, 2019, 01:00:57 PM
Hmmm...  I was thinking a meter would be a good way to keep an eye out for aging tubes and a situation like the one I just ran across with what appears to be a bum tube.   Rather than connecting the meter to the plates as I indicated above (and the accompanying high voltage), I was thinking of connecting it across the cathode resistors.  Thus the meter would see ~70v on the output tube and ~60v on the driver tube.  With a 1/2Meg resistor attached to the PSU bleeder, there would be less than 50v there as well.   I thought installing a meter would be easier and safer than periodically attaching DMMs to the inside of the chassis to see how things are going.

Is this a better plan? Or is adding a meter just a bad idea in general?

 

Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on August 29, 2019, 01:21:44 PM
Hmmm...  I was thinking a meter would be a good way to keep an eye out for aging tubes and a situation like the one I just ran across with what appears to be a bum tube.   
With a cathode biased amp, the difference between a strong 300B and a weak 300B is really hard to resolve with a panel meter.  It might be a difference of only a couple of volts, and of course the AC wall voltage has to be considered as well.

I thought installing a meter would be easier and safer than periodically attaching DMMs to the inside of the chassis to see how things are going.
That is something that folks generally do when something isn't going well.  If you're curious about the condition of your 300Bs, keep a spare set handy and plug them in every year or two to see if they sound remarkably better than what you're running.

Is this a better plan? Or is adding a meter just a bad idea in general?
Meters are super cool and very helpful when you have adjustable bias, but everything in this design will automatically bias.  If you want to add them for the sake of appearance, that's a great reason to have them!  If you decided to build a fixed bias 300B amp instead of a cathode biased 300B amp, then meters would be very useful. 
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on August 29, 2019, 02:39:53 PM
I see - thank you for your patience with me and for providing the additional details, Paul.   All are much appreciated!   I didn't realize the meter would be so ineffective with a cathode biased amp - I suppose the discussions I found online were in the context of fixed bias amps without explicitly stating so.

Back to my previous problem of the 6SJ7 driver tube not biasing up properly, I've been through four different tubes.  All exhibited EXACTLY the same behavior.  I'm thinking the problem is not the tube.  I double checked the tube socket wiring and haven't found any errors.

I'm now working on changing out the cathode bypass caps.  These, too, are NOS parts - Russian OKGB tinfoil, paper, and oil caps, though they've been tested prior to assembly.  I am using two 8uF 200v caps in parallel.  I tested them for leakage at 180vDC with my cap leakage meter and they exhibit no measurable leakage on a 50uA scale meter.  They also measure proper capacitance on my LCR meter.  But I'm changing them out anyhow for some modern poly caps just to remove a variable.

I'm not sure what to check next.  At least the schematic is pretty simple, so there aren't too many things to check...   
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on August 29, 2019, 03:43:54 PM
This is reminding me of some frustration when attempting to deal with the loctal equivalent of the 6SJ7.  It could potentially be that the SJ7's curves are specified for 0V on the suppressor grid, but I ended up using a different tube and not taking the time to determine if that was causing the problems I was having.  Are all of your tubes showing about 220V on the plate and 100V at the screen?

Probably the easiest change to make would be a slightly lower cathode resistor, as changing the plate load or the screen voltage will make a not so subtle operational difference, but nudging down the cathode resistor won't because it is bypassed. 

As always PJ will know what to do!
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on August 29, 2019, 04:07:13 PM
Interesting...  I changed out the R6 + C5 combination and tried a 5th tube with exactly the same results.  I haven't measured each of the 6SJ7 pins for voltage relative to ground.  I'll do that next, perhaps it will provide some additional clues.

Unless there is some really obvious error I have made that I just can't see right now, my next thought was to start playing with resistor values...

Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on August 29, 2019, 04:20:27 PM
Looking back at Paul's notes concerning R6 - he did indicate the possibility of having to adjust this value in order to hit a target of 170v for the plate and that +/-20v would be sufficient.  Figuring he meant this more as a "minor tweak" I ordered 1k1, 1k2, and 1k3 values.  I'll swap 1k2 for 1k3 and see what happens...   
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on August 29, 2019, 05:38:39 PM
Since you have the appropriate plywood mockup, you could also get a 5K pot and wire it up as a variable resistor.  Start with it all the way up at 5K, then turn it down till you see the desired plate voltage.

Power down and repeat for every 6SJ7 tube that you have, then adjust the value til you reach a sensible average for the sample of tubes that you have.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on August 30, 2019, 01:34:15 AM
Ha - We were thinking exactly the same thing!  As I was sitting on the couch last night at 11p watching an old Hitchcock on TCM and fighting off the urge to power up the soldering iron again, I starting thinking about the pile of 1K and 5K trim pots that I have laying around for my solid state builds.  A few extension wires, a dab or two of solder, and I'll be able to experiment!  But I figured if I started at 11p, I'd never get to bed...

I like the idea of averaging for the sample of tubes that I have - I wouldn't have thought of this.  The rest of the circuit has been stable enough over the few hours that I've been playing around that I'm confident enough to hook up an old test speaker and run some music through it tonight or tomorrow.   Tonight is "movie night" with the kids in our theater (see my homepage link), so I'm not sure if I'll have the time to get to it or not...

Looks like it might soon be time to turn my attention back to the chassis.  I'm trying to figure out a parts layout and "sub-structures" below the top plate for mounting components that minimizes the "Chicken Pox" look of a million screw heads piercing the top plate.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on August 30, 2019, 05:04:22 AM
A builder here had the same visual disdain for lots of screw holes, so he ended up making an extra top plate that sat on top of the normal top plate to cover them all up. 

If you paint/powder coat your chassis black and use black screws, they will tend to disappear, or you could maintain the plywood style of build and mount everything to the bottom plate, then let the tubes and iron stick up!
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on August 30, 2019, 08:36:49 AM
I'm torn between doing black powder coat on aluminum or using brass to match those nice TFA-2004 bells.  Of course, they would REALLY stand on a black background.

Here is the wood that will be used for the chassis frame.  It's some nice cross-grain figured walnut that I found at the local flea market.  I split it and added a 1/4" stripe of tiger-striped maple.  There are three boards in the image.  From top to bottom are the rough glue-up, the middle one has the layers sanded smooth, and the third one is after applying a coat of pure Tung Oil, allowing it to soak in a for a few days, then top-coating it with some Formby's Tung Oil Finish (which I'm not sure if it actually contains Tung Oil or not).  I tend to like darker wood.  Still undecided, though, about that top plate  :-\

Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Deke609 on August 30, 2019, 08:52:11 AM
Wow. That is some beautiful wood. Nice inlay work. My vote would be for black powdercoat and black anodized screws/bolts.  But you might also try brass screws/bolts - they might actually look nice with a black chassis and the brass end bells.  Or go with black everywhere except brass on the on-top OTs or chokes (I don't remember which is going on top in your build).


That amp is going to look (and presumably sound - since PJ designed it) great!


cheers, Derek
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on August 30, 2019, 09:03:12 AM
The only parts visible from the top will be the three PSU caps (black), the PGP8.1 (bell painted and lacquered to match brass), the TFA-2004 output transformer (actual brass bells), and the three tubes.   Of course, the usual fair of input/output jacks, power switch, fusing, etc. will also be on top.  Everything else will be hidden below deck.  I'm thinking I will need to cut some slots for airflow around those big power resistors - they will be mounted directly to the underside of the top plate with a little thermal paste for good heat transfer.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on August 30, 2019, 01:41:20 PM
OK - now I'm totally confused.  I installed a 5k pot in place of the 1k2 resistor at R6 and made some voltage readings across the pot as I adjusted it.  As I adjusted the pot toward 0 ohms, the voltage across the pot (R6) fell.  At the same time, the screen voltage on Pin 6 between R8 and R7 also fell, to about 80v (was 100v with 1k2 resistor in R6).  I was expecting exactly the opposite..

Looking to achieve something closer to 60v across the pot (leaving the 170v target on the plate), I adjusted the pot toward 5k  and the voltage across it started climbing.  With the pot maxed out at 5k, the voltage across it increased to only 5.5vDC.   With a value of 5k in the R6 position, the driver screen voltage rose from its previous 100v to 110vDC.   

I was sure using a pot would solve this problem, but it didn't.  Any ideas??
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Raymond P. on August 30, 2019, 05:29:54 PM
Here's what I think is happening with the screen voltage -


With the pot at 0 ohms, the tube turns on harder, drawing more current, which then pulls the B+ voltage lower. Since the screen node is a voltage divided version of B+, it also drops. With the pot at 5K ohms, the opposite happens. Can you confirm all this by monitoring B+ as you vary the pot?
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on August 31, 2019, 06:23:20 AM
Thanks for the suggestion, Raymond.  Attached is a chart of various resistances at R6 and resulting B+ and voltages at the divider for the driver screen voltage.  The asterisk at 1200R is the original value from the schematic. 

I attached an old speaker and my iPad for the first time today and it makes music!  Woohoo!   It sounds good to me - only a single channel on an old speaker, but there is no obvious audio distortion or other distress that I could detect.  With the hum pot adjusted for 0.0mV AC at the speaker terminal, I can hear a faint hum from the woofer with my ear next to it.  The woofer was silent when I connected the 10,000uF cap across the heater.  The speaker output volume gets to about where I expected it would with ~8w of output from an underpowered input (max line level output on apple devices always seems low to me in the context of CD player/DAC output).  It sounds like the amp is functioning properly.  The only issue is that voltage measurement across R6.

Edit: the 6SJ7GT datasheet indicates a maximum plate voltage of 250v and a maximum screen voltage of 125v, so I don't think the amp or tubes are in danger with the 1200R in place for R6.   I've also attached a more complete voltage chart on the schematic.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Raymond P. on August 31, 2019, 09:55:20 AM
Doesn't look like there's significant change in B+ voltage to account for the screen voltage change. It looks like R6 varies the amount of current into the screen grid though (the difference in current through R8 and R7). I wonder where that current ultimately flows to, as it doesn't seen to be going out of any of the other pins of the 6SJ7.

Despite the mysteries, it's great your amp is working!
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on August 31, 2019, 02:58:52 PM
Looking at the schematic again and the associated voltage drops that appear between B+ and Ground in the driver circuit, adjusting R6 seems to have little impact on achieving the 170v target plate voltage.

Total noob question here:
Is there any immediate danger associated with me adjusting the 91k value of R9?  If I increased the resistance of R9 to something larger than 91k, would it drop more voltage, thereby decreasing the plate voltage and allow more voltage to develop across R6?

Does this sound reasonable, or am I missing something fundamental here...
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on September 01, 2019, 10:33:55 AM
I spent some time making measurements with some meters, a signal generator, and scope with the hopes that someone can either guide me toward making a change to elevate the driver cathode/adjust the plate voltage to the intended voltage, or just leave it alone and start enjoying the amp.  Overall, the amp with the PSVane 300B tube can drive about 8v into an 8R load with an input signal of 0.8vAC before distortion becomes evident on the scope.

Here is a description of the tables I've attached:

The first attached chart is gain measurements of the 6SJ7GT driver stage, measured at the "input side" of the coupling cap with an 8R load attached.  Looks pretty constant at about 37dB of gain for the first stage.  Don't know if this is where it is supposed to be or not...

The second chart is the overall gain of the entire amplifier into an 8R load as well as input signal level sensitivity. Overall gain for the entire amp into an 8R load looks pretty constant at about 21dB.  This seems a little low to me, I was expecting somewhere closer to 27dB based on some comments PJ posted some time ago on another forum.  It also took close to 0.75vAC input in order for the amp to reach maximum clean output on the scope.  PJ's prior posting also indicated an input sensitivity of about 0.35vAC, so this figure looks a little "off" to me...

The third chart is an experiment with different values of the Driver Plate Resistor R9.  I didn't know how safe this experiment would be, so I only powered up the amp briefly enough for the B+ to stabilize and make a reading, then I shut it down.  Changing R9 seemed only to impact the plate voltage and didn't do much to the cathode elevation. 

I also swapped out R10 for one of Mike's BPC-16 Ni chokes.  As PB predicted, this was a bad idea.  It both introduced noise (hum pickup) and absolutely KILLED the bandwidth of the amp.  With R10 set to 250k as per the schematic, the point at which output magnitude was reduced to 70% relative to the magnitude of a 1kHz sine wave is about 22kHz (this seems a bit low to me - I was expecting a tube amp to be somewhere near 200kHz bandwidth, but perhaps I am wrong here).  When R10 was replaced with the grid choke, the 70% output mark happened at 10kHz!  I put the 250k resistor back in.

I also have images from the scope for all of these readings.  I figured I didn't need to post 50 images until someone asked for something specific...

So, is there tweaking to do to adjust plate/cathode voltage levels?  A few of these readings seem to deviate from my expectations...

Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Raymond P. on September 01, 2019, 01:25:54 PM
Looking at your 3rd table, it does look like the current into the screen grid flows out of the cathode. (Previously, I was just looking at the voltages marked in the schematic.) If you could raise this current, the cathode voltage should rise. In turn the driver tube should draw less plate current, which would drop the plate voltage. You could raise the screen current by lowering R8 or increasing R7.


Anyway, all of that is just conjecture on my part, so proceed cautiously.  ;D
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on September 01, 2019, 04:39:22 PM
If you're going to put a pot in for the cathode resistor, you will want to adjust it very, very slowly to get the target plate voltage.  You are looking to be very, very close to your original value.  If you're off by more than a couple hundred ohms in one direction or the either, you will fall off a cliff so to speak in terms of operation. 
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on September 01, 2019, 05:12:07 PM
Ah, I think I'm beginning to understand a little better now.  Given the relatively large size of the bypass cap and the very small mA draw of the tube, I think I was adjusting too quickly to see the effect that I was looking for.  I'll experiment again tomorrow and be a little more patient as I adjust things.  I'll be sure to get a meter on the plate as well.  Somehow, I think I may have missed the obvious  :-[
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Joppa on September 02, 2019, 12:57:53 PM
If you're going to put a pot in for the cathode resistor, you will want to adjust it very, very slowly to get the target plate voltage.  You are looking to be very, very close to your original value.  If you're off by more than a couple hundred ohms in one direction or the either, you will fall off a cliff so to speak in terms of operation.
I've been out of town for the holiday. Fortunately, this post by PB covers what I was going to say.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on September 02, 2019, 03:40:42 PM
The 6SJ7 does seem to be out of spec.

You nailed it, Paul!  The tube I was using sure is out of spec!  Like most problems in life, this one is another a case of user error  :-[.  While I was adjusting R6, I was also measuring voltage drop across R6 expecting to see a much larger voltage change.  I had assumed voltage drop across R9 and R6 would somehow be constant and would co-vary, so I only put a meter across R6, not R9 that leads to the plate - duh...

So, while measuring the PLATE voltage at pin 8 (thanks for the kick, PB), adjusting R6 actually makes a big difference!  It was easy to dial in 170vDC at the plate once I started measuring correctly!  The problem is that until I was satisfied that the amp was working properly, I was using my most-used-looking driver tube and it seems that it is furthest out of spec - it takes a value of 873R to hit 170v on the driver plate.  The remaining 10 tubes are all clustered between 1200R and 1500R (average of 1327R) in order to hit a plate target of 170v.  Looks like individual tubes are just as variable as solid state transistors...   

I'll re-run my gain and bandwidth measurements and see where things stand next.  I'll also post an updated voltage map of the amp shortly.

Very happy now!  :)  Many thanks for the help and support, guys!  I hope everyone had a nice holiday weekend!

Question: Is there any value to placing a pot in series with R6 so that adjustments can be made as any individual tube ages? 

Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on September 02, 2019, 04:10:17 PM
Pentodes are a fair bit touchier than triodes are.  You can expect the plate voltage to be a little different for each tube you use, there's nothing wrong with that.  I think you could stick with 1.4K cathode resistors and call it good. 

I wouldn't bother with the pot, good working tubes should all bias up to an acceptable plate voltage.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on September 05, 2019, 03:26:01 AM
Now that I've got a better handle on the driver plate voltage and it is dialed in, it dragged down overall B+ by a few volts, which also reduced the output of the 6.3v secondary winding for the tube filaments.  Adjusting the driver tube back to ~6.3v was cake.  Adjusting the 300B back to 5.0v-5.2v range that Jac recommended is taking some more experimentation.

The original configuration was diodes --> 154B chokes (3mH, 0R15) --> 10,000uF cap --> 300B filament, but now my voltage is too low.  I see two paths and was hoping someone could provide a sanity check for me:

1) Put a smaller cap (1,000uF 35v) between the diodes and chokes to make a CLC filter.  This worked well and I was able to hit a filament voltage of 5.1vDC with 122.5v (pretty typical) coming out of the wall.  Using my variac, I was able to experiment with the impact of different sized caps and get an acceptable window of filament voltages over the typical range of my AC mains voltage.   I was pleased with this result until I measured ripple on the 1,000uF cap at 1.5vAC.  With an ESR of 0R016, this seems like a ripple current of 1.5v/0.016R = 93 AMPS ?!?!?  Is this calculation correct?  If it is correct, this poor little cap surely won't last very long before it pops....

2) Use two equal size caps (6,800uF ?) in the CLC (which results in filament voltage closer to 6.0v) then add a power resistor on each leg of the filament supply (because of the center tap hum pot) to drop the filament voltage down to the desired target.  This seems like a more stable approach, but I also want to do some additional measurements to make sure that it doesn't delay warming the filament relative to when the B+ arrives at the tube.

Any thoughts or reactions?


 
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on September 05, 2019, 04:42:31 AM
1) Put a smaller cap (1,000uF 35v) between the diodes and chokes to make a CLC filter.  This worked well and I was able to hit a filament voltage of 5.1vDC with 122.5v (pretty typical) coming out of the wall.  Using my variac, I was able to experiment with the impact of different sized caps and get an acceptable window of filament voltages over the typical range of my AC mains voltage.   I was pleased with this result until I measured ripple on the 1,000uF cap at 1.5vAC.  With an ESR of 0R016, this seems like a ripple current of 1.5v/0.016R = 93 AMPS ?!?!?  Is this calculation correct?  If it is correct, this poor little cap surely won't last very long before it pops....
The cap input filter is what should be done to bring the voltage up.  Depending on how much additional voltage you need and what rectifier diodes you are using, you could also use more efficient rectifier diodes to bring the voltage up.  While the 1,000uF cap has an ESR of 0.016 ohms, it's capacitive reactance at 120Hz is 1.3 ohms.  Still, for this reason, you'll commonly see a few caps in parallel to share this abuse. 
2) Use two equal size caps (6,800uF ?) in the CLC (which results in filament voltage closer to 6.0v) then add a power resistor on each leg of the filament supply (because of the center tap hum pot) to drop the filament voltage down to the desired target.  This seems like a more stable approach, but I also want to do some additional measurements to make sure that it doesn't delay warming the filament relative to when the B+ arrives at the tube.
That will work and is an approach we use.

You can also rectify the 6.3V winding on the PGP 8.1 with a choke input filter and a big cap.  1N5820 diodes and the Hammond 155B got me to 4.95V in my own 300B project. 
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Deke609 on September 05, 2019, 05:19:06 AM
  While the 1,000uF cap has an ESR of 0.016 ohms, it's capacitive reactance at 120Hz is 1.3 ohms. 


I don't mean to derail the thread, but for those of us newbies trying to follow along, does this mean that what we need to look at to avoid filter capacitor "abuse" is the ratio of ripple to capacitor impedance (reactance + equivalent series resistance)? And that, in the case of very low ESR caps acting as reservoir or filtering caps in a power supply, capacitive reactance dominates and is all we really need to look at for practical purposes?


many thanks, Derek
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on September 05, 2019, 05:28:56 AM
Thanks for the confirmation, Paul!

You can also rectify the 6.3V winding on the PGP 8.1 with a choke input filter and a big cap.  1N5820 diodes and the Hammond 155B got me to 4.95V in my own 300B project.

This is where I was until I adjusted the plate voltage on the driver tube.  Then my 300B filament fell from 4.95v to 4.50v.  Moving to the 154B chokes only got me back to ~4.75v.  Still too low.

I'll continue to experiment with the CLC approach with similar sized caps and a few dropping resistors.  I was hoping to avoid using any electrolytics at all - oh well...

Derek: I'm not quite sure how/where Paul determined capacitive reactance at 120Hz to be 1.3 ohms.  I don't fully understand the implications of this measure.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on September 05, 2019, 05:46:40 AM
Reactance is AC impedance.  Capacitors and inductors have impedance that varies with frequency.  The impedance of a capacitor is usually (hopefully) higher than its ESR.

I'm surprised you didn't get more voltage from the 6.3V winding, especially if you are using a 154B.  You are using 1N5820 diodes?  (and your bog standard testing 300B?)
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Deke609 on September 05, 2019, 05:58:20 AM

Derek: I'm not quite sure how/where Paul determined capacitive reactance at 120Hz to be 1.3 ohms. 
I think it's the textbook equation for cap reactance (that I've read a million times but never used): Xc = 1 / (2*pi*f*C) = 1 /(6.28*120Hz*0.001F) = 1.32Ohms

Edit: And in quoting the formula for cap reactance, I think I've answered my own question above about relative importance of reactance versus ESR. Since reactance varies inversely with capacitance, the smaller the capacitance the higher the reactance. This suggests to me that for low ESR caps, reactance will dominate unless the cap is insanely large - e.g., tenths of a Farad.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on September 05, 2019, 06:35:31 AM
Ah - I've seen that formula before - I'm still learning how to wield each of these great tools!  I'll get there eventually  :).

I was surprised by the low rectified voltage from the 6.3v winding as well.  I was expecting something north of 5.0vDC with a choke input.  I'm using 80SQ045NG diodes (https://www.mouser.com/ProductDetail/ON-Semiconductor/80SQ045NG?qs=%2Fha2pyFadujIGlmyYa9M8%252BWQ31K%252Bb%252BCtAaPPtZbg%252Bfg%3D) that have a 0.55v forward voltage drop.  I also tried using IXYS DSS 10-0045B diodes (https://www.mouser.com/ProductDetail/IXYS/DSS10-0045B?qs=%2Fha2pyFadug59d17CRjWjt%252BNDUp2lITVLL7Zh40I8PUxMJTDr5BNUA%3D%3D), but the final filament voltage was identical to the previous.  I'll add some 1N5820's to my next order - the curves on the data sheet look encouraging.

The EML mesh tube draws 1.5A for the filament, so this is dragging things down a bit.  There is a clear difference in voltage level between my PSVane tube and the EML tube.  All of my notes are at home right now, so some of this is from memory.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Deke609 on September 05, 2019, 06:56:11 AM
The EML mesh tube draws 1.5A for the filament, so this is dragging things down a bit.


That may be your culprit. I use EML tubes in all my BH amps and the additional filament current draw always drops filament voltage below Jac's targets and requires tweaking (which is pretty easy in BH amps b/c they employ dropping resistors at the filament that can be decreased). I just did some really quick and rough PSUD sims of a 6.3V filament power supply and switching from normal 300B current draw of 1.2A to 1.5A dropped voltage by approx. 250mV. 


cheers, Derek
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on September 05, 2019, 07:16:39 AM
Yep, I can confirm that missing 0.25v!  I was at 4.98v on the filament and doing my happy dance.  Then I adjusted the plate voltage back to target and switched to the EML tube and was suddenly the party was over.

I'll just use a larger first cap for the CLC to reduce the ripple current to sane levels and then insert some resistors to hit my target.

Caps on the filament PSU definitely reduce the audible hum from the speaker, even when the hum pot is dialed in.  At first power up, I had 5mVAC on the speaker terminal.  I could get it down to 0.0mV by adjusting the pot, but I could still hear it (I have no idea what the sensitivity my old test speaker is).  Adding a cap cured this by reducing ripple on the filament from 1v5 to 80mV.  Adjusting the hum pot now produces a very subtle difference because the hum is so very low.  These tests need to wait until the kids are in bed so the house is quiet.  The CLC takes it down to something closer to 35mV at the filament.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on September 05, 2019, 07:58:35 AM
Yeah, 1.5A is no longer a 300B, and you'll lose a lot of filament voltage because of that.  The Hammond 156B might be a better choice, or more likely a different set of tubes.  If you intend to design and amp to use both, aim to be slightly low on the EMLs and slightly high on a 300B.

Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Deke609 on September 05, 2019, 08:48:00 AM
How about one of these Hammond 8.5V CT @ 3A filament PT's for each EML 300B mesh? https://www.mouser.com/ProductDetail/Hammond-Manufacturing/166M8?qs=sGAEpiMZZMvwUzoUXIIvyQPvPmwnNFGywL5FidNAeTk%3D (https://www.mouser.com/ProductDetail/Hammond-Manufacturing/166M8?qs=sGAEpiMZZMvwUzoUXIIvyQPvPmwnNFGywL5FidNAeTk%3D)

There's also a 2A version for about 4 bucks less.

You'd have some extra voltage to drop, but that would give you lots of wiggle room for filter stages to get ripple super low.

cheers, Derek
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on September 05, 2019, 08:57:30 AM
It's easier to go down. The choke after the bridge just needs more DCR.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on September 05, 2019, 10:17:44 AM
I did contemplate a separate PS for the filaments. 

Then I started thinking about the quantity of large parts that already need to go under the hood...  And that it would be one more part to spew an electromagnetic field...  and how that might just create a bigger problem than I already had on my hands...  For now, I think I'll stick with some additional caps and resistors.

Darn that "no free lunch" thing  ;D
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on September 05, 2019, 12:21:24 PM
While the 1,000uF cap has an ESR of 0.016 ohms, it's capacitive reactance at 120Hz is 1.3 ohms.

Now that I have some time to think about this, does this mean that my previous calculation of ripple current was way off?  Is a more accurate calculation based on frequency specific reactance of 1v5/1R3 =  1.15A of ripple current?
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on September 05, 2019, 12:28:06 PM
If I had concerns about ripple current, I would model the supply in PSUD, then insert an AC ammeter between ground and the negative lead of that cap and actually measure the current.  You'll need a meter with a current shunt that's got very low DCR though.  You could also put a 0.1 ohm resistor between ground and the negative terminal of the cap in the absence of a low resistance current shunt in your meter, then measure the AC voltage across this.  You can also try a 0.01 ohm resistor to get even closer to the actual value.

Doing this will eliminate any unpredictable behavior from your meter attempting to resolve AC voltage riding on DC voltage. 
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Deke609 on September 05, 2019, 02:01:11 PM
Just doing some "just in case" thinking here.


@PB - Could the Kaiju DCF handle a 1.5A draw, maybe with heavier duty heatsinks?


It looks like the board could be scored and split into two separate reg boards.  Since the DCF was designed to provide about 5V DCV after approx 1.2V filament resistor drop, it might offer a solution to the voltage sag issue with the EML tubes.
 
cheers, Derek
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on September 05, 2019, 02:11:28 PM
You'd either have to ask PJ, or you'd have to buy one and try it out.  It's still not the most optimal solution, as it puts out a regulated 6.3V, which we drop with a couple of low value resistors to get to 5V. 
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on September 10, 2019, 05:22:27 AM
The mains voltages in my neighborhood seem to range from a low of 120v to a high of 125v, but seems to hang out most often in the 122-125 range.  So, I used my variac to see what happens to filament voltage levels with mains voltages of 120v, 122.5v, and 125v.  I can't seem to help myself, I'm a compulsive tweaker...   ;D

I have a variety of datasheets and found lots of online discussions that talk about voltage tolerances for tube filaments for lots of tubes except the 5AR4 rectifier that I'm using.  All of the datasheets I have found merely state 5.0v but I'm measuring more than that in my build.  Does the "typical" window of +/- 10% apply here as well?  Will the tube last longer in the long run with a +/- 5% window?  My specific tubes are NOS Mullard f31 tubes.

I made a voltage chart and attached it below.  Should I leave the rectifier heater as is, or is it better for the longevity of the tube to insert a 0R1 resistor in series that will bleed off 0v19 and shift the heater range down to 4.97v to 5.19v?

Thanks for the perspectives!
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Deke609 on September 10, 2019, 05:37:10 AM
... insert a 0R1 resistor in series that will bleed off 0v19 and shift the heater range down to 4.97v to 5.19v?

That's what I would do, assuming your measurements were taken with all tubes in place (power supply loaded). Then you'd be within 4% of spec.

cheers, Derek
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on September 10, 2019, 05:39:59 AM
+/- 10% is quite normal.  There's this image that's made the rounds, though its exact origin eludes me at the moment.

If you feel the need to bring that voltage down, I would do it with a wirewound resistor in parallel with the 5AR4 filament. 
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on September 10, 2019, 05:53:21 AM
Derek: yep, completed amp at full temperature, all tubes lit and properly biased

Paul: I presume you are recommending the resistor in parallel to place a more "even" load on the AC filament?  Does it achieve the same result if I put a smaller resistor in series with each side of the filament?

Guess I have a new round of experiments to try - I've been putting resistors in series with the filaments.  At least resistors in parallel don't require a power down/cool down cycle for each value I try...
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on September 10, 2019, 06:00:15 AM
Paul: I presume you are recommending the resistor in parallel to place a more "even" load on the AC filament?  Does it achieve the same result if I put a smaller resistor in series with each side of the filament?
Yeah, you would be pulling the winding down a little bit, and the paralleled resistor would pull the voltage down less with lower line voltages and more with higher line voltages. 

I would snag a pile of 47 ohm 1W resistors and just clip lead them in parallel with the filament until you get the desired filament voltage. 
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on September 10, 2019, 07:19:48 AM
Ah - so resistors in parallel is a nice way to reduce the voltage variability a bit further...  Cool trick! 

I presume this works equally well for AC and DC voltages?

Each time I build an amp, I keep thinking to myself that I should start a new bound-paper notebook so that all of my notes stay together and in order.  Then months/years later, I pick up some piece of paper and wonder "gee, which amp was this set of notes from???"

Sigh, I still haven't done this...  Maybe now is a good time to start!
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Deke609 on September 10, 2019, 07:33:19 AM
That's what forum posts are for!  :)
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Deke609 on September 10, 2019, 07:51:32 AM
@PB: Just for my edification: the parallel resistor approach will result in less current through the filament than the series approach, right? So depending on desired current draw of the filament, the series approach might work better is some circumstances, e.g., with some EML tubes?  Or have I mixed up something basic?

cheers and thanks, Derek
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on September 10, 2019, 09:08:01 AM
If I am understanding this correctly (I know, a little knowledge is sometimes dangerous...), both series and parallel resistors will help bleed off extra voltage, but the differences in physical arrangement will impact the voltage relationships.

The parallel approach will provide an extra measure of voltage "stabilization" or "buffering" by bleeding off less power (V^2/R) when mains are lower (slightly less voltage across resistor) and bleeding off more power when mains are higher (slightly more voltage across resistor).  Thus, the magnitude of the mains-induced voltage swing at the filament will be reduced with parallel resistors. 

Resistors in series with the current load of the filaments are more consistent in their voltage drop (current * resistance).

That is, if I'm conceptualizing all of this accurately  ???
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on September 10, 2019, 09:53:43 AM
My guess is that the 5V winding you're using to heat your rectifier wants to see 3A of current draw (it could also be that it has a 115V primary, which is worth asking about), so adding a little extra draw with the resistors will pull the winding voltage closer to its specifications.

I do not usually recommend this approach for DC supplies, especially choke input filament supplies, as the regulation of the choke input supply will make this not work as well. You are also limited to the ratings of the chokes themselves, so if you have a 1.5A filament and a 2A choke, you can only pull an extra half an amp (DC) before you're out of gas.  Doing this will also reduce the filter inductance a little bit, which is a negative side effect.  Series resistance here is the more workable solution.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Deke609 on September 10, 2019, 09:54:47 AM
Hmm. I misstated things in my previous post. I was remembering my tweaks to already existing series filament resistors in my SIIs and Kaiju where I decreased series resistance to get more voltage dropped across the filament.  But here we're talking about adding series/parallel resistance, not decreasing it. So ... in this case I think: (1) adding series resistance will reduce current and split the voltage drop between the filament and the series resistor(s) - thereby reducing voltage across the fil; whereas (2) adding parallel resistance will increase current draw, and this increased current draw will drag down the voltage at the power supply, and in this way decrease the voltage dropped across the fil.

But maybe I'm still confusing things.

[Edit - PB posted while was I posting and cleared up my issue: adding current draw should bring down voltage of the heater power supply]

Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on September 10, 2019, 10:54:55 AM
Digging back through some notes I made a long time ago, the information I have indicates the PGP8.1 has a 115v primary.  This explains why I'm over voltage... 
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Joppa on September 10, 2019, 11:05:23 AM
Actually, the primary is designed for a nominal 117 volts 60Hz input and the 5v winding is specified at 2.0 amps RMS.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on September 10, 2019, 11:10:04 AM
Actually, the primary is designed for a nominal 117 volts 60Hz input and the 5v winding is specified at 2.0 amps RMS.
Hmm, so bring the 5V AC current right up to 2A (with a 50 ohm resistor in parallel with the filament) and call it good?
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on September 10, 2019, 12:36:43 PM
Actually, the primary is designed for a nominal 117 volts 60Hz input and the 5v winding is specified at 2.0 amps RMS.

This is good to know!  I wonder if there were different iterations of the PGP8.1 as I found a reference to 2.5v windings each being rated for 1.25A.   Given my voltage levels under load, it appears the transformer does have 2A capability.  A resistor in parallel it is! 

Looks like it's time to turn focus toward the final chassis implementation...
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Joppa on September 10, 2019, 01:11:26 PM
I should be clear that Mike LaFevre de-rated the windings to 2.0A at 6.3v and 1.25A at 5v - I have quoted the original spec of the design which came from Mike's vast collection from various "golden age" companies. Mike is often conservative about specs. It's a long story...
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on September 10, 2019, 04:03:05 PM
Thanks for the details, Paul - this makes me feel better about pulling 1A9 for the rectifier out of the winding that is spec'd for 1A25 (at least on paper).

Playing around with various resistors in parallel with the rectifier filament, I'm inclined to think that this transformer can actually deliver a fair bit more than 2A on the 5v windings.  I kept adding resistors in parallel until I got something in the voltage range that I liked.  The best result was having 6R6 (three 20R 5w resistors) in parallel with the filament.  This is an additional draw of 0A75!  The increasing current draw didn't do very much to compress the range of voltages on the filament, but it did produce some pretty warm resistors.

I'm not sure that I'm comfortable pulling a total load of 2A65 on that pair of windings...   I'll experiment with various resistance in series tomorrow.  I'm thinking 0R1 or 0R2 might be sufficient to get the voltage where I want it.

Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Joppa on September 10, 2019, 05:31:39 PM
The heat generated by a winding is proportional to the current squared. So 2.65A on a 2A winding makes nearly twice as much heat as the materials are rated for. It won't blow up immediately, but it very well might start a fire. It's a bad idea to exceed the rating. Don't do it!
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on September 10, 2019, 05:43:09 PM
Yeah, I would just take the improvement that the 50R resistor gives and not go any further.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on September 11, 2019, 08:35:39 AM
Understood - I agree that exceeding current ratings can end in all sorts of bad outcomes that are best avoided.  I tend to be a fan of the Nelson Pass approach to (over)engineering circuit designs: Wear both a belt and suspenders.  I can arc-weld with the PSU of my 150w Class A solid state amps :)

Is there a compelling reason not to add series resistance to trim the rest of the AC voltage on the filament?  I've been doing lots of searching/reading and haven't found anyone saying that this is a bad idea...
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on September 11, 2019, 09:30:06 AM
If you split it up as two resistors, I can't see that it would be a problem, but it does feel overly anal to be dialing in rectifier voltage that tight. 
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on September 11, 2019, 01:32:31 PM
Yep, I absolutely LOVE to futz with and tweak stuff, so I'm definitely guilty as charged!  Besides, isn't the realm of DIY just an endless opportunity to tweak simply because we can?  ;D   On the plus side, now that my prototype is nearly finished and dialed in, I think I might have fewer questions ahead than behind. 

In all seriousness, I do very much appreciate your patience with me while I learn!  I'm REALLY excited for my first opportunity to hear a stereo pair of these amps!
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on September 12, 2019, 03:49:16 AM
Here is the final iteration of voltage measurements - I think I'm finished tweaking things and I'm pretty happy with where things are right now!  Overall, the amp is producing about 7.5-8w of output power into an 8R load, which is pretty much in line with the datasheet for the EML 300B mesh tube. 

I think I'll keep my prototype functional and build the second channel so that I'll be able to listen to the stereo pair sooner.

Guess it's time to work my way through those physical/aesthetic decisions that I've been putting off: color choices for the top plate (brass or powder coat black), methods to avoid a million screw holes in the plate, etc.  I'm thinking this is going to be much slower work.

Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on September 19, 2019, 04:25:29 AM
Did a quick color test a few days ago with a scrap of aluminum I had laying around.  Painted one side black and the other side with as close of a color match that I could find to brass (not ideal, but close).  Also used it to paint the bell on the PGP transformer.

I'm thinking black looks a bit nicer - there is a local guy who does powder coating, so I need to go visit with him for a little while.  Need to spend some time cutting and drilling first...
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Deke609 on September 19, 2019, 06:31:07 AM
I think they both look great. I'd be pleased with either.

cheers, Derek
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on September 19, 2019, 07:22:59 AM
I would always suggest a powder with a texture to it for these projects.  When you slip with your screwdriver installing hardware, those gloss powders are not going to be forgiving!  Gloss paints will be even worse than the powders. 
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on September 19, 2019, 02:18:45 PM
Good to know about scratching.  I suppose I had it in my head (honestly, I don't how half of the stuff up there got there in the first place...) that powder coating was somehow more resistant to scuffs and scrapes.  Now I can ask a better set of questions!

You guys rock - thank you!
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on September 19, 2019, 03:51:22 PM
It is definitely tougher than a solvent based finish, that's for sure.  It's also not all that easy to touch up. 
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on September 30, 2019, 06:38:04 AM
My next task is choosing some wire.  I'd like to use different colors to signify heaters, B+, signal wires, etc. rather than having the chassis filled with with all black wire that I used on my prototype.  This part is easy - there are "standard" color charts floating around for wire color in tube amps.  It also seems there is a preference for solid core wire (20g) over stranded and the logic that solid core wires tend to stay put makes good sense to me.

So, this leaves me with what I consider to be an aesthetic choice for wire: teflon or cloth-over-pvc for the jacket.  To me, the cloth seems potentially more in line with "vintage" tube gear kind of look-and-feel.  With the options that I'm looking at, prices are close enough for either type to be considered equal, so price isn't really a decision factor.  The only thing that I can think of is that teflon might be easier and cleaner to work with than cloth. 

Is there something that I am missing that might suggest that this is not strictly an aesthetic issue?

Any impressions or experiences that might favor the use of either teflon or cloth?   Any value in choosing 18g instead of 20g?

Thank you!
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on September 30, 2019, 07:29:55 AM
The teflon covered wire will be impervious to heat from your solder iron, while the cloth on cloth covered wire can burn and also can soak up flux and end up looking unsightly.  Both are not the easiest types to strip.

You definitely want to use solid core wire.  Aside from the benefits you mentioned, there's only one strand to get to pass through each hole, while stranded wire tends to fan out if you try to pass it through small holes.  Once tinned, it will pass through OK, but then making a mechanical connection becomes more difficult.

I would stick with 20AWG for this project.  20AWG is listed with a maximum ampacity of 11A for chassis wiring, which is about ten times the maximum that you need.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on September 30, 2019, 07:53:45 AM
Thanks, Paul!  This tips the scales toward teflon for me.  I'll get a color assortment from Steve at ApexJr.com.  I have a nice set of wire strippers that works well with teflon.  I use lots of CAT5 wire for projects and got tired of my old strippers nicking the wire and having it break all of the time, so I finally got a better tool.

I'm certainly no daVinci (who could probably still turn out a nice painting with a Wall-Mart brush), but having a proper tool tends to make certain tasks go more smoothly...
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: oguinn on September 30, 2019, 10:01:25 AM
I got the Stripmaster that Doc has recommended a few times and it's been really useful for stripping teflon wire.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on September 30, 2019, 02:41:07 PM
I had been using one of those "Self-Adjusting" automatic wire strippers (see image below) for a long time and it does an "ok" job, but it causes problems.  With smaller solid-core wire, it would often make nicks and with lager stranded wire, it often broke a few of the strands.  After putting up with it for years, I got tired of the problems.

Then, I purchased a Stripmaster for 16-26g wire.  It really works great! 
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Deluk on October 01, 2019, 01:41:13 AM
Stripmaster USA = $37  For me in the UK £96 Both Amazon.

Think I'll stick with the basic one I used for my Crack with no problems. I will be careful with the skinny Speedball wires!
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Deke609 on October 08, 2019, 11:44:58 AM
I'll get a color assortment from Steve at ApexJr.com. 

+1 for Apex Jr (no affiliation). I'd never heard of them before. But after reading Eric's post I checked them out and ordered 200 ft of 20 awg teflon silver plated copper wire for $56 USD - 100 ft red, 100 ft black.  It just arrived. I'm no metallurgist, but after cutting it, stripping it and examining the gauge, it looks pretty good. It's unbranded. Judging by the couple of cuts I made, the silver plating looks to be substantial.  A good deal.

cheers, Derek
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on October 08, 2019, 03:30:00 PM
Good to hear!  I've bought tons of stuff from Steve over the past decade or so - never been disappointed.  I haven't put in my order for wire yet, been busy with work over the past few weeks.  I also need to visit my local metal guy and order some aluminum plates so I can start drilling holes and working out the mechanics. 
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on October 09, 2019, 12:17:03 PM
I've been exploring various resistors for trimming rectifier filament voltage and I came across a notation that I've never seen before.  For these resistors, the "Maximum Working Voltage" is listed in the data sheet as the square root of (3xR) - circled in red in the attachement.  If the ohmic value of the resistor is 0R05, does the mean that the maximum voltage that can be applied across the resistor is the square root of 3 * 0.05, or 0.387v ??

This seems VERY low for a 3w rated part.  I'm a little reluctant to put them somewhere where they are likely to see 450+ volts across them while the PSU comes up at power on.  Am I interpreting this correctly?  Something seems wrong to me...

Thanks for helping me understand!

Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Joppa on October 09, 2019, 12:28:59 PM
Ohm's Law: current = volts / resistance = 0.387/0.05 = 7.74 amperes

Power is volts times amps = 0.387 * 7.74 = 3.0 watts

It's just another way of saying it's a 3-watt resistor. Your rectifier is not going to draw 7 amps, so no sweat.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on October 09, 2019, 01:04:38 PM
I've been exploring various resistors for trimming rectifier filament voltage [...]  I'm a little reluctant to put them somewhere where they are likely to see 450+ volts across them while the PSU comes up at power on. 
Filament voltage is 5V.  You're using these to trim down a 5V supply.  They will not see 450V across them.  A 0.05 ohm resistor across a 450V power supply would dissipate 4 million watts ;)

(In reality your fuse will just vaporize instantly if you tried this)

Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on October 09, 2019, 01:15:08 PM
Got it, my thanks to both of you.  My primary concern was that the heater circuit also carries the B+ supply from the rectifier to the PSU caps, so it will have ~450vDC across it until the caps ramp up to steady state.  I didn't want to put a resistor there that was not rated for that voltage difference...
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on October 09, 2019, 01:45:34 PM
My primary concern was that the heater circuit also carries the B+ supply from the rectifier to the PSU caps, so it will have ~450vDC across it until the caps ramp up to steady state. 
The heater circuit is floating and it has 5V AC across it.  It is completely unaware of the DC voltage in question.  It is true that the DC current needed to charge those caps will have to pass through these resistors, but that current is limited by the power transformer, the 5AR4, the size of the caps/chokes, etc.  When you turn the amp on, there will be 0V DC at the first filter cap and the 5AR4 will have zero emission (no current flowing) to charge up that cap.  The voltage seen by the filament dropping resistors at this time will just be 2A*0.05V=0.1V.  You may see a bit more than that since the 5AR4 filament is cold and will draw a bit more current until it's hot.  Without the filament being hot, the high voltage AC humming along on the rectifier plates is not in the equation.

A quick simulation in PSUD (which does not model the slow warmup of a tube diode) shows the current required to charge the power supply peaks briefly at about 2.25A, then settles down to 164mA almost immediately.  I worked on bigger amps with 1A fuses between the rectifier and the first filter cap as an extra safety measure, which is another indicator of how gnarly the charging current is when a tube rectifier is used. 
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on October 10, 2019, 03:32:11 AM
Ah - now all of the pieces are coming together for me!  There will be a current and voltage peak across the 0R05 resistor when the empty caps are first charged, but because of the low resistance of the resistor and the "slow" voltage ramp up of DC voltage out of the 5AR4, there will never be a 450v delta across the resistor...  This was my mistaken assumption.

I can't quite get the PSUD simulation to duplicate the exact voltages that I've measured in the amp (I'm unsure about resistance in the B+ winding in the transformer and I've estimated the reactance of the caps at 60R for 120Hz), but it seems close enough that I have a better picture of what is going on now.  The peak current on the 0R05 resistor is the same as the peak current going into the first cap of about 2.4A.  The voltage across this resistor closely follows the pattern of the current draw, which settles down to a very small amount once the PSU achieves steady state.  I'm seeing a peak voltage across this 0R05 filament resistor of less than 0.15vDC before it settles down to somewhere in the 0.02v range for steady state.  Even if I'm off by a few orders of magnitude (which I'm pretty sure I'm not), things still look good to me.

Many thanks, Paul and Paul!  I keep thinking I should go take an intro EE course from my colleagues across campus...  I've got many of the concepts in my head but sometimes I don't quite see how they all interact with one another.

Next up is re-reading Morgan Jone's chapters on transformer mounting to the chassis plate and looking over some of the great builds I've seen documented here so I can order all of the mounting hardware, grommets, shoulder washers, etc that I'll need next.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Deke609 on October 10, 2019, 04:09:33 AM
That's pretty cool. A couple of milliseconds of 2+ amps @ 120 mV - so approx. 1/4W.  And the quoted specs for your resistors indicate that they can withstand up to 15W of dissipation for 5 seconds and a constant 3W dissipation.  You're golden!
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on October 10, 2019, 04:45:41 AM
Yeah, that was totally non-intuitive for me: any old 1/4w resistor will work for this application!  It's my strong desire for way over-engineering things that pushed me toward a 3w part for this purpose. 

The pair of 0R5 Mills MRA12 wirewound resistors that I put on the 300B filament to tame the voltage get toasty warm at 60c.  1.5A of current pass through those guys, so they dissipate about 1.125w of power each.  Derating for temp, they're still good for 75% of their rated power at 100c, so things should be good here.  The 300B filament PSU is a CLCR (1,000uF, Hammond 155B, 10,000uF, then 0R5) and I keep wondering whether the better solution is to torture the resistors at the end or torture a smaller first cap in order to hit a 5.0-5.1v target.  Using the CLCR approach, ripple on the heater is about 9mV - this wasn't really my goal, but is a nice side effect of clamping down on the voltage.
 
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on October 10, 2019, 05:34:32 AM
You also have the DCR of the power transformer high voltage winding limiting current, as well as the impedance of the tube rectifier itself.

For the amp I last worked on with the rectifier to B+ fuse, it had blown because the owner had the amp plugged into a power strip and he turned the power strip off with the amp running, then back on a few seconds later.  This was enough time to discharge the caps but not so much time that the tubes actually cooled off much.  Still, this was a 1A fuse fed by 5AR4s (two per amp) into a 100uF filter cap, so I have reasonable confidence that in your particular amp, you're unlikely to even see 1A of peak draw.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on October 10, 2019, 06:53:24 AM
That's interesting - there is a good deal of value knowing that the simplifications built into PSUD actually present more of a "worst case" scenario that we are likely to find in actual application.   Cool stuff!
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on October 10, 2019, 07:00:49 AM
They can go the other way too.  There isn't really anything in PSUD to track how hot a solid state diode will get.  I've had some actually fall out of a PC board because they got so hot and melted the solder that was holding them in.  The same goes for ripple current rating in capacitors, PSUD will at least give you a number for that, but you have to pay attention to the parts that you're buying or you'll let the stinky goop out. 
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on October 10, 2019, 07:54:28 AM
Ha- shoulda figured the error would cut both ways  :D
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on November 27, 2019, 02:26:53 PM
I just received the aluminum sheets that I'm going to use as the top plates for the amp.  I ordered three that are 11" x 17" - one to prototype and make a mess of, and two for the finished amps.

Now that the amp is verified working properly (no longer using a bad driver tube), I made some additional measurements of bandwidth and gain.   The bandwidth measurements are particularly interesting:

With the 250k grid leak resistor: -3dB bandwidth points are 20Hz to ~17kHz - not really sure that I can hear above 17kHz ;-)
With the "Nickel" BCP-16 Grid Choke: -3dB bandwidth points are 21Hz to 11kHz - wow, what a nosedive!
With the "High Nickel" BCP-16 Grid Choke: -3dB bandwidth points are 23Hz to 11.5kHz - same dramatic dropoff
Not sure if the sound quality from the chokes is better, but based on bandwidth, the 250k resistor will stay for sure!

Clipping behavior is also pretty interesting with the various configurations for grid leak:
With 250k resistor, max input signal is 0.68v AC RMS before onset of clipping: max output is 7.43v AC RMS into an 8-ohm load
With both Nickel and High Nickel Grid Chokes, the max input signal before clipping is 0.47vAC RMS, but I got just a pinch more output at 7.50v AC.
Curious that the chokes deliver (marginally) higher input sensitivity and higher output than the Grid Leak Resistor did...

Overall output level is just shy of 7w into an 8ohm load (~20dB gain).

Overall noise/ripple measurements are very nice!  All of the following measurements were made with the RCA input shorted:
Speaker output: 0.4mV AC noise level
300B plate voltage: 0.0mV AC ripple
300B filament voltage: 0.04mV AC ripple with CLCR configuration
6SJ7 Driver Plate voltage: 0.05mV AC ripple

This amp is VERY quiet!  With my ear on the speaker driver, I can't hear any hum at all!

I also attached an updated voltage chart.  My AC mains tend to run 123 to 125 most of the year.

Time to start making holes in the top plate and assembling the wooden bases...

Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on November 27, 2019, 04:13:50 PM
If your gear will do a graph of frequency response, that may be useful to look at too.

It is interesting that the HF rolloff is so pronounced, but a wheezy 6SJ7 isn't going to be happy driving much in the way of capacitance!

Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Deke609 on November 28, 2019, 04:26:13 AM
With the 250k grid leak resistor: -3dB bandwidth points are 20Hz to ~17kHz - not really sure that I can hear above 17kHz ;-)
With the "Nickel" BCP-16 Grid Choke: -3dB bandwidth points are 21Hz to 11kHz - wow, what a nosedive!
With the "High Nickel" BCP-16 Grid Choke: -3dB bandwidth points are 23Hz to 11.5kHz - same dramatic dropoff
Not sure if the sound quality from the chokes is better, but based on bandwidth, the 250k resistor will stay for sure!

@Eric - Before committing to the 250K R, it might be worthwhile experimenting with different (higher) interstage coupling cap values.  For example, going to 1.5uF will reduce the impedance by a third, and going to 0.2uF would knock it down by half -- that might bump up your high frequency response? Maybe PJ and PB will provide some expert guidance on this. I seem to recall that playing with the coupling cap value may require changes to the grid resistor value.

[Edit] - I see that PB has already suggested this may not work:

...  a wheezy 6SJ7 isn't going to be happy driving much in the way of capacitance!

But maybe there's a bit of wiggle room for bumping up capacitance.

cheers, Derek
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on November 28, 2019, 04:54:49 AM
I'm a little confused by the bandwidth with the 250k resistor.  I thought I made a previous measurement where I got closer to 22k - but this was with my 6SJ7 tube that was out of spec....

Paul -  The figures that I measured were eyeballed with my scope and a signal generator.  I used a 1kHz frequency as a baseline, then adjusted frequency up and down until the output magnitude was ~70% of the 1kHz signal.  It is my understanding that the 30% down mark represents the -3dB point (but perhaps I am incorrect here). 

I have a PC with a sound card and have both REW and ARTA software installed.  I'll see if I can use some combination here to do a signal sweep with the amp.  Is this what you are looking for?

Happy Turkey Day to everyone in the US - it's time to put ours in the oven.



Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on November 28, 2019, 06:15:57 AM
@Eric - Before committing to the 250K R, it might be worthwhile experimenting with different (higher) interstage coupling cap values.
Bigger coupling caps will lower the LF response a little bit, but they won't restore the missing treble.

The bigger issue is that the gain of a pentode is approximately its Gm at whatever operating point you're using times the loading impedance.  With a plate loading resistor and a grid leak resistor, the loading impedance will be resistive until you get down to where the coupling cap start to roll things off.  With a grid choke instead of a grid leak resistor, that loading impedance will move around a bit.

With a triode, the plate impedance of the triode in a properly designed circuit is much lower than what's going on with the grid choke, so those frequency response aberrations are likely invisible. 
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on November 28, 2019, 04:03:27 PM
I tried a different method of measuring bandwidth earlier this evening.  Instead of using a signal generator and my scope and doing a manual signal sweep with a dummy load attached, I used a PC with a sound card and Room EQ Wizard software.  I was able to produce the attached image of a frequency sweep.  This shows essentially flat response from about 13Hz all of the way out past 20kHz (with a small bump around 10kHz) using the 250k grid leak resistor.  I can't make REW sample out past 20kHz.  I don't know why my signal generator and scope showed more reduced bandwidth, though both are quite old...

So, then I started to compare the 250k grid leak resistor with the grid chokes and something went wrong but I don't know what.  I removed the 250k resistor and inserted the Nickel Grid Choke with alligator clips.  I powered up the amp again and the 1/2A fuse flashed immediately.  I checked the wiring and didn't see anything wrong, I didn't find any inadvertent shorts or lose wires.  I removed the choke and restored the 250k grid leak resistor.  I powered up again and the new 1/2A fuse began to glow red as the B+ came up (after about 20 seconds) and then it blew.  Before the fuse blew, B+ voltage on the 300B plate and the 6SJ7 plate were right where they should be.  Somewhere, something is drawing extra current...  Not sure what is going on, I'll have to check more carefully in the morning.

Anyhow, bandwidth looks better with this measurement technique.

Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on November 28, 2019, 05:51:28 PM
If you unplug the 300B during any of this, it's easy to accidentally put it in the socket improperly, where the fat pins aren't in the fat holes.  If you do this, you short B+ to the cathode resistor through the 300B filament and things get crispy in a hurry.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on November 29, 2019, 11:16:23 AM
Paul - That is a danger of running the prototype with the tube sockets upside down for sure, but I didn't remove any tubes during my test.

I'm trying to track this fuse problem down by isolating as much as I can and bringing additional parts of the circuit in one-by-one.  Here is what I have found so far:

- With no tubes in, the fuse remains intact - this tells me the transformer is fine.
- I put the tubes in one by one and powered just the filaments (with B+ disconnected) and this worked fine - no problems with any of the three filament supplies.

Before I go any further, is there any danger with powering up the amp with B+ ONLY on the driver tube?  Or with B+ ONLY  on the output tube?  These are the last two parts of the circuit to test.  All of my resistors in the circuit check out...

Edit to add schematic.

Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on November 29, 2019, 03:31:32 PM
When you have a cap coupled amp like that, you can run one tube and not the other.  Without the 300B in, the B+ will be quite a bit higher than it should be.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on November 29, 2019, 04:40:35 PM
Thanks for the confirmation, Paul!  I was thinking it might work out ok (specifically because of the cap that you pointed out), but didn't want to screw something up if I was wrong...
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on December 01, 2019, 11:29:32 AM
Figured it out- another self-inflicted problem that does not really exist.   ::)

It looks like I blew the first fuse by poking around with my volt meter probes.  I think this had something to do with connecting one lead to the 300B plate and the other lead to the "high side" of the cathode bypass cap C4/R4 combo.  I should have measured each with respect to ground and then subtracted, oops...   

The original fuse that was in place was a 1/2A slow-blow.  My best guess is that subsequent fuses were 1/2A fast-blow, because they always toasted as the B+ was ramping up and approaching 400v.   I was unable to tell by the markings on the fuse whether they were the slow- or fast-blow type.  After making lots of measurements, using my variac, and blowing through a string of additional 1/2A fuses, I finally replaced the fuse with a 1.0A fast-blow and it seems to be surviving now.  I've rechecked all of the voltages, comparing them to the original voltage map that I made and everything seems fine now. 

My best calculation for the power consumption of this amp is about 50w steady state (throw in the additional heat from a few hot resistors and it appears that a 1/2A fuse was right on the edge: the slow-blow survived, but the fast-blow didn't.


Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on December 01, 2019, 11:53:57 AM
You may find that you still blow the 1A fuses occasionally.  You could just toss a 3A fast in there instead.  It will allow things to get a little crispier if there's a fault, but it will still blow.  You could also look at using an NTC thermistor to mellow out the startup surge. 
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on December 01, 2019, 01:46:07 PM
Just measured total current draw for the first time (duh...).  I pulled the fuse and put my meter across the terminals.

Power on surge: 0.52A  (64w)
Drops to 0.34A (42w) after the filaments are warm
Steady State 0.76A (93w) after B+ is up, so a 500mA fuse was definitely too small. 

Paul, you are right (as usual!) - it just might open a 1A fast-acting fuse from time to time.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on December 22, 2019, 05:17:39 PM
With a few days of unstructured time finally on the horizon, I'll be starting to prototype my build on a 1/8" aluminum sheet.  The tube sockets that I have are NOS Johnson ceramic sockets that mount from the underside of the top plate. 

Since I imagine it's tough to make a hole larger after discovering that I didn't include enough clearance, how much additional space should I include for clearance around the tube base?  If the base of my 300B tube is an actual 35mm in diameter, is a 38mm hole sufficient, or should I go with a 40mm hole?  The smaller hole will look neater though the larger hole will potentially provide more cooling.  If I go with the smaller hole, is it necessary to make a perimeter of smaller holes around the tube hole for cooling?
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Deke609 on December 23, 2019, 05:07:13 AM
Eric - I don't have an answer to your question, but I just had to comment on the beauty of your metal work! Those look like very precise and clean holes. Are you sending it out, or doing it yourself?  If doing it yourself, can you describe your method?

cheers and thanks, Derek
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on December 23, 2019, 05:19:29 AM
I'd recommend looking through a few tube datasheets to see what the base diameter is listed as.  You may find a little variation, but I would imagine 3mm is good enough.

With your mounting plans (which are good), you'll want a rubber washer in the stack so you can tighten down the socket without cracking it.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on December 23, 2019, 02:19:26 PM
Derek:  Sorry that I didn't specify, the mounting scheme shown in the image is not my work nor is it my amp.  It's a screen grab from a video that I found of someone showing the "proper" way to mount the Johnson ceramic sockets.  The technique is to use a nut as a spacer so there is airflow around the tube socket and up through the tube mounting hole.  I'll be making all of my holes on my drill press, not sure how precise I'll be able to make things...

PB: Thank you for the suggestion of using rubber washers!  I likely would not have thought of this on my own until I heard something crack while tightening the screws.  Maybe I'll use a short stack of rubber washers between the socket and the plate and another rubber washer on the other side of the socket to provide a little "give" so the socket is not rigidly coupled to the top plate. 

Yea or nea on a perimeter of air holes around the socket?
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on December 23, 2019, 05:24:16 PM
The amount of heat generated by the pentode driver and all the parts around it is pretty minimal, so I wouldn't go too nuts with ventilation.  The 300B cathode bias resistor and that Japanese style PSU dropping resistor are going to produce most of the heat under the chassis.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on December 28, 2019, 03:15:49 PM
Thanks for the input, as always, PB! 

I had great plans of constructing one of my amps this week, but it looks like things like repairing the washing machine (needed a new clutch), repairing the clothes dryer (needed a new motor), and an impromptu several day, 1,400 mile round trip drive to South Carolina in a few days (to start selling my mother's house), my time has been more limited than I thought it would be.  Nevertheless, I've been able to actually build 2 wooden bases.  They are pretty large, each one measuring approx 18.5" by 12.5".  I might need to build a new audio rack as well...

It was a bit of a trick to cut the 45 degree angles as the longer pieces of wood were too long to use the stop on my saw so I could cut them all exactly the same.  Instead, I had to hand measure, mark, position, and cut each one by eye.  The strap clamps worked VERY well to close up the corners nice and tight - I picked them up on sale at my local hardware store.  The four corners were hit with a 1/2" roundover bit and the top edges received a 1/4" roundover.  They were sanded with a 220 grit with my new orbital sander (what a wonderful toy!) and I applied some 100% Tung Oil to darken the wood.  I'll let this soak in for a few days and then I'll apply some low gloss Formby's Tung Oil Finish (which I'm pretty sure doesn't actually contain any Tung Oil at all).  I'll probably end up with 5-7 coats of Formby's before they are done.

I also made one more base out of rough plywood to use as a "construction base" so I don't mess up the nice ones while I'm working on the circuit.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Deke609 on December 28, 2019, 03:53:03 PM
Those look great already! Nice work.

cheers, Derek
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Doc B. on December 29, 2019, 09:42:37 AM
Re washers and dryers - we bought our washer and dryer when we moved into our house in 1987. A few years ago the agitator in the washer stopped working and the spin cycle switch under the lid stopped working. I started to look into getting a new washer and I was shocked to see so many reports of recent production washing machines with failed computers, leaky seals and trashed shocks. A little more digging turned up a few recommendations by old time repairmen to stick with older washers  as they are much more reliable and actually do a better job of cleaning. So I spent a few bucks replacing a few worn parts and the rather ugly old washer and dryer are still going strong in 2019. I think one could say vintage laundry appliances are more like vintage audio gear in terms of reliability and functionality than, say, vintage motorcycles are (NOT).
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on December 29, 2019, 02:07:46 PM
Doc - this is exactly what I've been reading about clothes washers and dryers.  The newer ones are all computer controlled and the computer consoles seem to be updated every two years or so.  Once a new model comes out, the controls for the previous model seem to be unobtanium, so everyone recommends keeping the old ones running for as long as possible.  Just feels that I've been spending all of my time fixing the old stuff lately...
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on January 13, 2020, 06:28:07 AM
I had some time during the holidays to get just a little bit of work done.  I was hoping to have second round prototype fully built and operational by now, but I'll take what I can get.

I taped up the aluminum sheet and carefully drew out the position of all of the holes.  For the lager holes, clamping the metal to my drill press kept things from moving around while I was drilling.  If you "nibble" slowly enough, the hole saw doesn't require too much oil in order to make nice holes.  This is the "construction" base that I made from Baltic Birch ply that I had laying around.  This way, I won't mess up the nice walnut base with constant movement during construction.


Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on January 13, 2020, 06:29:20 AM
Cool!
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on January 13, 2020, 06:45:55 AM
And here is the underside construction.  The goal is to keep the screw heads on the top deck to a minimum.

I picked up some aluminum U-channel and 2"x1/8" bar for support of "below deck" parts.  It took WAAAAY more time than I thought to do cutting and test fitting, but it looks like things will work out the way I had hope it would.  The tube sockets are mounted with neoprene washers all around to protect the ceramic and I think I'll pick up some nylon insert lock nuts to keep things from loosening up after construction.

The U-channel is attached to the socket and cap mounting screws.  Then the 2" aluminum sheet will be mounted to the U-channel to hold the remaining parts.  It looks a bit more clean underneath than I expected it would given the added hardware. 

My original plan was to attach the large wirewound resistors directly to the top plate for maximum heat dissipation, but now I'm wondering if I can insert one more 2" strip and put them there instead, then use some thermal paste between the aluminum parts to spread the heat.  Not sure how toasty this would make things or not...

Question 1: How much clearance should I leave in my holes if I plan to powder coat the top plate?  I presume the thickness of the powder coat will need to be accounted for- but maybe not.  Is using "the next size up" drill bit sufficient for clearance?  My hole for the IEC power entry "just fits" and holds the jack in place without screws - I'm presuming I should file it open a bit more...

Question 2: Does anyone have a part number for this type of a solder tab?  When I search on "solder tab", I keep finding the ones that are meant for soldering rechargeable batteries, not ones with mounting holes in the middle.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Deke609 on January 13, 2020, 07:16:17 AM
Quote
Question 2: Does anyone have a part number for this type of a solder tab?  When I search on "solder tab", I keep finding the ones that are meant for soldering rechargeable batteries, not ones with mounting holes in the middle.

They are stupidly difficult to search for. Here's a DigiKey link (note: you need to search under "interconnects"!? - who would think to do that?): http://www.digikey.com/products/en/connectors-interconnects/terminals-solder-lug-connectors/401?k=connectors (http://www.digikey.com/products/en/connectors-interconnects/terminals-solder-lug-connectors/401?k=connectors)

Even if you buy a hundred, they are still ridiculously expensive for stamped sheet.  There must be a cheaper source.

cheers, Derek
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on January 13, 2020, 09:01:17 AM
Score!  Thanks, Derek!  I've been going crazy trying to find these darned things...
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on January 13, 2020, 09:41:23 AM
The solder lugs are indeed ridiculously expensive but they are brass, so perhaps that helps to account for some of the cost. 

You'll have to post the resistors you're using and the heat each dissipates to really know how much of an issue you may be looking at. 

For the powder coating question, I keep a chainsaw sharpening file close at hand, and I can pop that into any holes that get too small from the powder and just file it out.  You can also run a thread tap through them.   
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on January 13, 2020, 10:41:51 AM
PB: Both resistors are rated at 50w dissipation: https://www.mouser.com/ProductDetail/Vishay-Dale/RH05050K00FE02?qs=sGAEpiMZZMtbXrIkmrvidDNaDpN5VXc5qswg6VdgV68%3D

The 50k PSU bleeder resistor will dissipate 425*425/50,000 = 4w of power all of the time.
The 1k cathode bias resistor will dissipate 70*70/1,000 = 5w of power all of the time.

Both on the same sheet will produce 9w of power.  My mounting options are to attach them directly to the 11x17 base plate (most optimal cooling) or to put them on another strip of aluminum mounted between the B+ choke and 300B tube socket.  This strip will be attached to the U-channel, which is then attached to the base plate.  If I put thermal paste between everything, it might work out? 

I think I'll need to make some temp readings once everything is wired up and running.

As for powder coating, I presume it won't chip/flake off of the top surface if I need to take a file to the edge of the hole - is this true?
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on January 13, 2020, 10:47:03 AM
The free air rating of the 50W resistors is 20W (it's in the datasheet), and I'd derate that a little more, but you're still plenty safe.  The heatsink area to get 50W of dissipation out of those is 1 square foot of 0.060" AL per resistor, so you won't be anywhere close to that with two resistors. 

The heatsink paste is unlikely to be necessary. 
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on January 13, 2020, 01:37:11 PM
Thanks for the feedback, Paul!  At this point, I'm thinking I'll be in good shape with everything mounted on the sub-plate.  Now I just need to find some time to drill more holes.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on January 19, 2020, 03:48:56 PM
Just one or two more parts to source and I think I'll have everything I need for a full prototype build on an aluminum plate (though, I'm sure I've left *something* out and will have to wait for yet another order to arrive).

Everything is now attached to the base plate except the power and output transformers and I still need to come up with a way to mount the coupling cap between the signal tubes.  I'll probably create some sort of metal clamp with a rubber liner and connect it to the mounting screw for the 300B plate choke.

I did a little prep work on the PGP transformer.  I gently sanded the laminations and painted it with a gloss black enamel and painted the bell end to make it look like a brass finish.  I'll post an image of the transformer when the paint dry.  Almost ready to start the wiring. 
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on January 19, 2020, 04:46:37 PM
You're running out of room ;)
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on January 20, 2020, 04:00:06 AM
Yep!  My choice of caps ate up a fair amount of real estate under the hood.  Without these, I think the chassis could have been half this size...  I'll need to clear some space on the audio rack for sure!
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on January 23, 2020, 04:33:45 PM
I managed to fashion a custom clamp for the coupling capacitor out of a chassis clamp for a larger film cap.   It may not be super elegant, but it's functional and holds the cap securely.  In the few minutes of spare time I have each day, I think I'm ready to start the wire layout to see how things go.   

I finally found the shoulder washers I need to isolate the power transformer when I mount it.  Is there any value to adding a neoprene washer to the mix for this transformer (for vibration isolation), or should I just stick with the shoulder washers for electrical isolation?
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on January 23, 2020, 06:45:05 PM
The PGP8.1 shouldn't vibrate.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on January 24, 2020, 05:13:15 AM
Good to know, Paul - thank you.  I have experience with n=1 of Mike's power transformers so far.  I've been through lots of transformers in the past, some from the same manufacturer where one example buzzes/vibrates but the next one doesn't, so I've learned to be a little cautious. 

Is there any reason to electrically isolate any of the other iron from the chassis and ground it separately with a solder tab and wire, or should I just bolt it all to the metal chassis and not think about any more?
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on January 24, 2020, 05:15:07 AM
We do that with all of the chokes and transformers on our products, but I haven't ever seen anybody else in the industry that does. 
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on February 04, 2020, 02:03:30 PM
I'm in the process of wiring up the prototype to see what changes might be necessary before I begin a final build with powder coated plates.  I managed to find some rubber grommets for where the output transformer wiring passes through the top plate.   I also need to get some longer mounting bolts for the PGP transformer.  After adding in the shoulder washers and the thickness of the top plate, there's no exposed threads for the nuts.   

Is it OK to use a zip tie to bundle up the remaining wires from the output transformer after putting some shrink wrap over the ends?
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on February 04, 2020, 02:19:42 PM
Yeah, I find for MQ iron that slipping a little heatshrink over the exposed wires and then wrapping them into a loop that you can secure with a ziptie keeps them out of the way.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on February 04, 2020, 02:59:17 PM
Thanks, Paul.  I was thinking this was the case, but didn't want to cause some bizarre interaction effect from stray wires floating around or bundled where they shouldn't be...
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on March 29, 2020, 11:45:12 AM
Managed to find a spare few hours over the past couple of days to finish my aluminum-plate prototype.  I used color coded wire and tried to make things more neat than my plywood prototype was.  The result is that the new one works, but produces about 39mV of AC hum at the speaker terminal, which is audible at the speaker.  Ironically, the one with flying wires is still nice an quiet.

Need to move some wires around to solve the hum problem.  My first guess is that it's from driver filament AC passing by the plate coke for the 300B...  Need to drill some holes in my support structure and pass wires under the platform so they are better separated.  I didn't take wire paths into account with my physical layout.   

Green is AC heater wire, red is B+, black is ground, and yellow is signal wire. 
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Thermioniclife on March 29, 2020, 12:07:14 PM
Hello,
The twisted green pair of heater wires passing close and parallel to the power switch and fuse holder could be problematic.
Looking at the photo I would just try to bend / flip those toward the 2 Hammond 155 chokes. Also it is preferred to keep heater / filament leads as close to the chassis as possible and use the perimeter of the chassis for said wiring.
Give it a try, it will only take a few seconds to get those wires away from the AC input.
Best wishes, Good luck and Good job.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on March 29, 2020, 03:54:53 PM
Thanks, Lee.  I'll give it a try.  The twisted pair for the rectifier heater is arranged closer to the 120v mains because I figured it would create trouble if I tipped it toward the 155B chokes which are for the 300B heater.  I'll try this first.  If this doesn't do it, I think I'll play with the AC heater wire path for the driver tube.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on March 29, 2020, 05:08:04 PM
Is your chassis plate earthed?
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on March 30, 2020, 01:55:07 PM
Lee:  I moved the AC heater wires for the rectifier further away from the 120vAC mains as you suggested.  They are now parallel to the HV secondary for the B+.  This reduced the ripple on the speaker output from ~39mV AC down to somewhere closer to 5mV!  What difference this made!  I didn't consider that AC would interfere with AC - I was trying to keep it away from DC supplies.

Paul: before grounding the chassis plate as you suggested, I measured it with respect to ground and found 22v AC on the plate!  Yikes :o  Grounding the top plate took the ripple on the speaker output down to the level I have in my wooden prototype of about 0.5mV AC.  This seems to wander around a bit from 1mV to 0.2mV with no particular pattern.  Either way, it is now at a level that I can't hear at the speaker, so I don't care anymore.   

All of the other measures are great so far, the AC heater for the input tube is 6.4v, I have 5.15vDC on the 300B heater, and about 5.10vAC on the rectifier heater.   B+ is about 422v and the supply for the input tube reads about 167v.  All of these are within ~2-3% of target with 125v AC mains (pretty much normal all year 'round where I live), so I'm pretty happy with this!  My main goal was to verify the layout and the wiring wouldn't cause problems when switching to a more appropriate long-term chassis. 

The next step is to start drilling and cutting plates for the final implementation and get them powder coated (when things open up again, that is).

The empty hole in the chassis plate was intended for a volume pot.  Not sure if I actually want to implement this, or just make it a dedicated amp and use a preamp for volume control. 

Thanks for the help guys!  I hope everyone is well!
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Thermioniclife on March 31, 2020, 01:34:19 AM
Glad to hear that you got it sorted out.
Best wishes to you and yours.
Regards,
Lee
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on June 10, 2020, 05:11:42 PM
After a long time not being able to work on my amp, I have been able to make some progress again.  I was experimenting with positioning the RCA input next to the input tube (away from the AC mains input and speaker output) vs putting the RCA input next to the output transformer and speaker output terminals.  There is very minimal difference in speaker output noise levels, both measure mostly less than 1.0mV AC noise on my DMM.

I think I prefer having all of the inputs/outputs in a line along a single edge of the amp because it makes routing wires in and out of the amp visually more neat on the equipment rack.  Is there any reason that I should not do this? 

With either RCA input placement, the amp sounds VERY quiet on my Tang Band 1772 speakers (advertised sensitive of 95dB - this is probably optimistic after my correction network to tame the treble).  I have attached an image of each RCA input configuration, as well as an image of the output noise level on my scope with the RCA input "open" (not shorted).  The scope is set to display 5mV per division on the y-axis with a time window of 2ms per division on the x-axis.

Thanks for any insights.

Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Thermioniclife on June 11, 2020, 12:44:28 AM
You shouldn't have any problems with the input and output next to each other.
I did this on a pair of 300b mono's that I built a while back.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on June 11, 2020, 06:02:19 AM
Thanks, Lee.  I've been playing around with the prototype top plate and I'm happy enough to drill holes anywhere, but I'm becoming a bit more conservative as I'm thinking about the work required to make two final plates and get them powder coated.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on July 28, 2020, 02:48:53 PM
Work and projects around the house have been keeping me pretty busy lately, so my progress has still been slow.  But, I'm coming down the home stretch.  Got my aluminum back from the powder coating shop last week and thought it turned out very nice.  I went with a textured black (thanks for the suggestion, PB!) with speckles of gold in it for the base plate and a gold-ish color for the power transformer bells in an effort to come close to the brass bells of the output transformer.  Both were a dual-layer powder coat process.  I also stripped the shrink wrap from the PSU caps and painted them with a gloss black enamel so they wouldn't have silver circles on top of them.

I had an interesting problem with the aluminum plates.  The sheets that I received from the shop were slightly bowed, so I took them back and asked them to run the sheets through their rollers to flatten them out.  A few minutes later, I had nice flat sheets in my hands.  But when I picked them up from the powder coat shop, the bow had returned - I guess the heat from the powder coating process caused them to spring again.  Thus, there is a slight bow to the top plate that flattens out a little with the weight of all of the components on the plate.   Guess I could always mount an "L" bracket to the inside of the wooden chassis and drive a screw through the top plate into the L to pull it flat again.  I think I'll leave it alone for now...

I've attached a few images of the top of the amp.  I haven't finished the wiring yet, so the amp won't be functional for another few days.   
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on July 28, 2020, 04:04:40 PM
The bow will go away over time and comes from the production of the sheets themselves.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Deke609 on July 28, 2020, 04:41:19 PM
Eric - It looks amazing! Nicely done.  Happy to see a new post about your progress. Looking forward to the finish and listening impressions.

cheers, Derek
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on July 28, 2020, 05:32:36 PM
Good to know that the bow will go away over time.  I was a bit disappointed to see it return after the heat curing of the powder coat process.

Can't wait to hear the stereo pair of them!
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Thermioniclife on July 29, 2020, 08:18:41 AM
Nice job Eric, Sweet inlay and finish. Something to be proud of.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on July 29, 2020, 04:28:02 PM
As I'm nearing completion, I have a question about using/setting the hum pot in this design.  My DC filament supply for the 300B is a CLCR (4,700uF, Hammond 155B, 10,000uF, 0R5 10W).  The hum pot is a 15R 5w pot flanked by 22R 5w resistors.   AC ripple on the 300B heater is essentially not measurable on my DMM (0.0mV).  As a result, adjusting the hum pot makes no audible difference at the speaker output (I'm using Tang Band 1772 drivers right now somewhere near 92-93dB efficiency after I tame the treble). 

Given that I can't *hear* any difference, where should I set the pot for best/safest operation?  Should I just leave it in the middle of the range?

Thanks for any insights!
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on July 29, 2020, 04:29:53 PM
I would just put it in the middle.  You may be able to measure a null with a scope or FFT, but if you can't hear it I wouldn't bother!
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on July 29, 2020, 04:59:57 PM
Thanks for confirming, Paul!
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on January 07, 2021, 02:44:32 PM
Happy New Year!

Wow - I didn't realize it's been since July that I've worked on my amps!  The fall semester kept me pretty busy, but now I've got just a little bit of time on my hands again and I'm hoping to get these amps finalized.

I may have made a mistake in the context of my mental health, but I recently picked up a nice USB Analog to Digital Converter so I could more easily do some sound/noise measurements.  Maybe there is something to that old "ignorance is bliss" sort of thing ;-)

Anyhow, I measured the noise floor for my prototype amp.  The graph below is what I am measuring with the input shorted and an 8R load on the output.  Removing the input shorting cap at the RCA in makes no difference in the noise profile.   But, since I have no context, I'm wondering what the noise floor "should" look like for an amp like this.  Is this a good result, or do I still have some work to do? 

To me, it looks like I have some power supply noise to deal with, as evidenced by the peaks at 60Hz, 120Hz, 180Hz, 240Hz, and 300Hz.   The odd harmonics seem to be a bit higher in magnitude than the even harmonics.  Measurements at the speaker terminal indicate about 42mVDC offset and 1.3mVAC hum.  The noise floor on the ADC all by itself is about -120dB.

Any input would be appreciated! 

Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on January 07, 2021, 03:18:20 PM
1.3mV of hum would be OK with me.  That's not quite as low as it could be with a DC supply, but about 10dB quieter than an AC heated amp. 

The FFT image you posted is useful in some ways, but not necessarily as useful as knowing the actual voltage. 

Incidentally you have two power supplies that will make 120Hz noise, not 60Hz noise.  The 60Hz noise you're seeing could be a ground loop issue or magnetic coupling between the PT and your output iron.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Deke609 on January 07, 2021, 03:33:39 PM
Hey Eric! Welcome back! I was wondering what happened to you and this amp.

Could some of that noise be residual power supply ripple appearing at 300B grid via the plate of the driver tube? If so, maybe consider a cascode constant source as driver load?

just a thought.

cheers, Derek

Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on January 07, 2021, 04:27:42 PM
Paul:  Seems like I'm in "the ballpark" of measurements.  What specific voltage measures would be more indicative of what is going on here?  Are you talking about ripple voltage at the plates of the input and output tubes?  Or filament voltage on the 300B?  Is it safe to just put my DMM from plate to ground for a ripple measurement?

Derek:  I didn't realize it had been so long!  The fall semester seemed to bring about 10M zoom meetings (waaaay more than my "normal" number of meetings during a semester).   The past few weeks have been spent catching up on things that got put off over the past few months.  Almost caught up now ;-)  I'm really looking forward to wrapping this project up while I have a little bit of time!
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on January 07, 2021, 04:51:28 PM
Your FFT measurement shows 60Hz.  Your power supply ripple frequency would be 120Hz (on the B+ and the filament supply), ergo you do not have an issue with power supply ripple, you have either magnetic coupling between the power transformer and signal handling iron, or some kind of ground loop.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on January 09, 2021, 03:09:25 PM
I solved my problem after poking around for most of the day today!   :) :) :)   With Paul's guidance (thank you, Paul!) on ground-related issues, I took a more methodical approach today when I noticed something very strange.  The noise profile I was seeing was present even with the amp turned off!  Turns out, just having it plugged in was causing all of the noise I was seeing in the FFT graph see first image - "Power Off- Plugged In- No Cheater"

With Paul's hint of a grounding issue, I pulled the plug from the wall and inserted a "cheater plug" that has no ground lug on it, and plugged it back in.  The amp was still off, but the noise floor dropped all of the way down to -115dB (except a little peak at 60Hz).  See the second image "Power Off - Plugged In - Cheater Plug"

Since I wasn't really excited to run an amp with a 450v power supply without a ground connection, I tried something else.  I inserted a CL-60 thermistor between the AC Ground/Chassis Ground and the PSU/signal ground.  So the aluminum plate is still directly bonded to mains earth, but the rest of the audio ground is separated by the thermistor.  It measures about 15R when cold and can handle more than 5A of current, so I figure it's plenty robust for this application, especially since the amp itself is protected by a 1.25A fuse.  This way, any potential fault will still find its way to mains earth for safety, but the signal ground is a bit isolated.

So, the final image here is my noise floor with the amp powered up with the CL-60 thermistor in place.  See "Power On - With CL60."

Playing with the hum pot impacts the 120Hz and 240Hz peaks.   By adjusting the pot, I was able to reduce the 120Hz peak from -66dB down to -80dB.  At the same time, the 240Hz peak went from -83dB down to -92dB.  This is easy to see in the FFT, though is just *barely* audible through headphones.  The amp is now *VERY* quiet.   The overall AC hum at the speaker terminals is now about 0.65mVAC - less than half of what it was before!  With a little more tweaking tomorrow, I think I can get these two peaks down just a bit more.  I can't seem to do anything with the 60Hz peak, though.  It pops up as soon as the switch is turned on. 

I am just thrilled with this amp now!  Now that I've solved this issue, it's time to rebuild the second amp on my power coated aluminum base.   

Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on January 17, 2021, 07:44:04 AM
I have completed my second amp on the finished chassis and tried a different grounding scheme.  With my first monoblock, I directly grounded the aluminum plate to AC mains earth and separated the rest of the PSU and audio ground with a CL-60 thermistor.  In the second monoblock, I put the CL-60 right at the IEC connector, so the *entire* amp is isolated from AC mains earth right from the start.   This lowered the noise floor even further.  The CL-60 measures about 15R.  As an experiment, I substituted in a different type of thermistor that measures closer to 26R and this just raised the noise floor again, so I went back to the original one.

The result is a 15dB reduction in noise at 60Hz, a 9dB reduction at 120Hz, a 4dB reduction at 240Hz, and an 8dB reduction at 360Hz.  The overall noise floor from about 500Hz to 20kHz is also about 5dB lower as well.  They are darn near "dead silent" with my headphones plugged into my FocusRite analog to digital converter.  Not really sure what that little spike is near 1kHz - seems to be related to a tube or two settling in - it was there when I made my screen cap, but isn't there now.  It's interesting to hear the metal in the tubes creaking and expanding while listening through headphones as the amp warms up.

At this point, I'm willing to call them done!  Pfew - my first purchase for this set of amps was in Dec of 2016.  I'm listening to stereo music through them right now for the first time and they sound just great!  Oh the glory!

I'll post of picture of the new twins after I've had some time to listen to them...

A deep and heartfelt "Thank You!" for everyone's help and input (and patience with me) along the way.  Special thanks and appreciation to Paul Joppa for the excellent circuit and to Paul Birkeland for the construction help!  I couldn't have pulled off my first tube build without all of the guidance!

Edited to fix my usual typos...
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Thermioniclife on January 17, 2021, 08:43:38 AM
That's interesting, I have used ntc thermistors in the past with varying degrees of success but I always used them on the hot mains input. Does it get hot in the audio ground as it would inline with the mains hot lead?
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on January 17, 2021, 08:46:23 AM
There isn't any current flowing, so there will be no heat.  It will definitely get hot if there's a fault in the amp though. 
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Thermioniclife on January 17, 2021, 08:50:07 AM
Thats what I thought but how does it lessen hum. would a resistor of equal value as the ntc in a cold state do the same thing?
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on January 17, 2021, 09:02:51 AM
would a resistor of equal value as the ntc in a cold state do the same thing?
Yes.  The issue with a resistor alone lies in the fault condition, where drawing a lot of current across the resistor just develops a lot of voltage across it and may not be sufficient to blow a breaker.  That's why in our products we use a resistor with a pair of antiphase diodes across it.  The diodes limit the possible voltage drop in case things go really wrong.  An NTC thermistor will also drop in resistance significantly if you try to pull much current through it, though I would expect the diodes to work more quickly.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Thermioniclife on January 17, 2021, 09:16:20 AM
on which products do you use a resistor and antiphase diodes on the audio ground? I have a Crack, C2a, Mainline, S3x and BeePre.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on January 17, 2021, 09:21:57 AM
Moreplay and BeePre 2 have it.  It will get built into the rest of the heapdhone amps and preamps down the road as the kits get refreshed.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Thermioniclife on January 17, 2021, 09:24:14 AM
As Artie Johnson from Laugh in would say, Very Interesting
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on January 17, 2021, 09:32:29 AM
My thought in using the thermistor is that it was "similar" to a resistor in that it would provide a path to ground while simultaneously providing a bit of ground isolation.  But it is superior to a resistor because it can pass a TON of current without burning up.  I've pulled more than 400w through a standard CL-60 in my big Class-A solid state amps without issue for more than a decade.  A standard 3w or 5w power resistor might open when a fault occurs (potentially leaving the fuse intact), thus providing no protection for the circuit or the user. 

I didn't try diodes with a resistor as Paul indicated, I was doing quick experiments and this is what worked most easily for me, so I'll leave it in for now.

Here are some images of the finished monoblocks.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Thermioniclife on January 17, 2021, 10:08:25 AM
Sweet, mirror image twins. Love the bases and the pinstripes. Beauty's to behold enjoy!
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on January 17, 2021, 10:09:50 AM
Yeah, big class A power amps have a pretty nasty turn on surge.  Those monoblocks are looking lovely.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Deke609 on January 17, 2021, 10:15:21 AM
Ditto. Those look amazing Eric. Nicely done!

cheers, Derek
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on January 17, 2021, 10:32:55 AM
Thanks, guys!  It's been quite a bit of physical work in addition to wiring and troubleshooting.  Only have a few hours on them, but they sound great!  Midrange is something special and there seems to be more bass than with my solid state stuff.  Need to start enjoying them now...
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: johnsonad on January 17, 2021, 10:48:27 AM
Those look fantastic Eric, nice work!  I'm glad you did this project.  I remember PJ posted on AA about it years ago.  You're the first person I've read of that finished them, enjoy!
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on January 17, 2021, 11:05:41 AM
Aaron: I know Al Sawicki, also an AA member, built this design several years ago.  He's also been offering some build advice for me as my project has progressed.  So that's two that I am aware of. 
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Karl5150 on January 17, 2021, 12:39:31 PM
The inlay in the bases and the pinstripes play well together, visually. Great work, enjoy.
Karl
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: 2wo on January 17, 2021, 05:40:15 PM
I have been following your build from the beginning. Very handsome....John
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on January 18, 2021, 10:53:15 AM
Thanks for sharing your kind thoughts, Karl and John.

Here are a few under the hood shots of the finished amp. 

Paul, are the specs for the grounding resistor bundled with anti-parallel diodes something you can share, or is this information proprietary?  I'm curious to explore other grounding configurations.  Not sure why I'm seeing increased noise in the 20-40Hz range with the thermistor in place, though I am unable to hear it in my super-sensitive headphones.  Maybe it's not worth playing around with...
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on January 18, 2021, 11:00:26 AM
The antiphase diode/resistor/cap thing certainly isn't proprietary.  Ideally you'd want a 10-20 ohm 5W resistor, a pair of diodes across the resistor anti-phase wired (non-schottky diodes that can do 20+A peak current), and a little 0.1uF ceramic cap to finish it off.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Doc B. on January 18, 2021, 11:47:08 AM
Nice looking!
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Jamier on January 18, 2021, 01:24:07 PM
They look fantastic! I guess the parts collecting was an adventure in itself.

Jamie
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on January 19, 2021, 09:30:55 AM
Thanks, Doc and Jamie!  It was a pretty solid 2-3 years to track down all of the necessary parts.  I got lucky and scored a pair of TFA-2004 Mexico Jr. M4 pin stripes direct from Mike.  The output transformers came from the great folks on the forum here.

I spent some time experimenting this morning with different grounding schemes (thermistors vs. diodes, resistors, caps) and none of the variations made much difference.  I did note that when using diodes, the value of the resistor made a small difference of 2-3dB here or there, but we talking about something like -94dB vs -96dB.  I could see it in the FFT, but certainly couldn't hear the difference with my headphones.  For the record, I tried 4R, 10R and 20R.  The 10R provided the lowest noise floor (at least for my environment), but only by a very small difference to the others.  Overall, there is no appreciable difference between diodes+resistor and a thermistor to isolate the ground.

I'm not sure what to make of the FFT data in the 10-40Hz range, though - it just seems to flop around like the end of a whip.  I don't know if my AC mains ground is just more "dirty" than others, or there is some house-borne contamination, or this is just part of what happens with this amp.  I  measured one of my solid state amps the same way and I'm seeing about -90dB in the 10-40Hz range as opposed to -55dB to -65dB with the tube amp.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on January 19, 2021, 09:36:28 AM
I haven't seen that kind of response myself, what happens if you run a frequency response sweep instead of the FFT?
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on January 19, 2021, 05:26:42 PM
I wonder if this is an artifact of new computer hardware I'm using.  I have an old computer with FFT software and a sound card that I'll try for a comparison.  I should also be able to do a frequency sweep with REW.

This 10Hz-40Hz behavior is similar on both amps.  For each of these measurements, the input RCA is shorted and the output is connected to an 8R resistor, then connected to my FocusRite analog to digital box.  All metal parts in the amp are grounded to the chassis except for the caps. 
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on January 20, 2021, 07:48:21 AM
Here is a 5Hz to 20kHz frequency sweep of the 300B amp that I made with REW.  Using the 1kHz dB level as a reference (about 61dB), it looks like the -3dB points (58dB) are about 21Hz and  about 16.5 kHz.  This is rather what I expected it would look like...

An FFT made with my old computer set up looked pretty much like the ones I've already posted: high noise floor in the 10Hz-40Hz region.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on January 20, 2021, 08:45:24 AM
Now I'm thoroughly confused   ???

I made an FFT with my old computer that looked similar to my previous graphs - output peaks in the 10-40Hz region.

So, using the integrated sound on my new computer (just built it last week), I'm getting what looks like much better FFT performance.  Maybe the FocusRite (ebay purchase) that I was using previously was abused by it's first owner...  Or the built in sound in my new computer is just not as sensitive...  I'm really not sure what is going on here.  I think I need to make a few more comparisons.

Anyhow, here are three new FFT plots using the built in audio of my new computer:

1) Noise floor of internal sound card, no audio connections
2) Noise floor of 300B amp, power on, input shorted
3) Noise profile of 300B amp, 1kHz test tone

At first I thought the odd harmonics in the third FFT graph are characteristic of the pentode driver tube, but after exploring other amps, it looks like this is an artifact of the built-in sound on my computer.

Do these graphs look better?
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Joppa on January 20, 2021, 09:50:22 AM
The low frequency behavior looks very believable - the gentle rise (about 10dB/decade below ~400Hz) indicates i/f noise, which most electronic devices have.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Deke609 on January 20, 2021, 10:04:45 AM
Here's a neat plot of 1/f noise of an op-amp that lines up with PJ's assessment.

And here's a link to the article from which the plot is taken: http://www.analog.com/en/analog-dialogue/articles/understanding-and-eliminating-1-f-noise.html# (http://www.analog.com/en/analog-dialogue/articles/understanding-and-eliminating-1-f-noise.html#)

Weird phenomenon.

cheers, Derek
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on January 20, 2021, 10:51:42 AM
Thanks for this sanity check, Paul and Derek!  I've never come across that before.  Sounds like it's time to stop tilting at this windmill and just enjoy listening  :)

Here is a set of final measurements I made of filament voltages for both amps.  I used my variac to adjust AC mains from 120v to 125v (typical range for my neighborhood seems to be low-122-ish to high-123-ish.   Excuse me while I cue up some of my favorite vinyl!

Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on January 20, 2021, 04:03:25 PM
And here's a link to the article from which the plot is taken: http://www.analog.com/en/analog-dialogue/articles/understanding-and-eliminating-1-f-noise.html# (http://www.analog.com/en/analog-dialogue/articles/understanding-and-eliminating-1-f-noise.html#)

Interesting read - never heard about this before.  It also looks like a pretty sophisticated circuit/approach to a solution...  I think I'll pass and just learn to live with the noise that I can't hear  8)

This is making me think there is nothing wrong with my FocusRite ADC.  I suppose the audio system in my new computer has some software/circuitry to mitigate 1/f noise.

 
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Deke609 on January 20, 2021, 04:37:17 PM
Thanks for this sanity check, Paul and Derek! ...

Any sanity here is not my doing. I'm just a grinning fool trying to clap along to PJ's beat.   ;D 

Enjoy the vinyl listening! [Now where's a clapping monkey emoji when you need one?]

cheers, Derek
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on January 24, 2021, 07:23:24 AM
Since this project is "mostly" wrapped up (isn't there ALWAYS a tweaking phase?!?), I thought I'd share my final parts list and schematic as implemented.  It's a joy to listen to, but I have to say that half of the fun is in gathering the parts, as Jamie suggested.   

Time to find my next project, though I already have 4 amps that don't fit on my oversized equipment rack... 
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Deke609 on January 24, 2021, 09:23:19 AM
Eric - Thanks for sharing the project. It's been fun to follow along.

I checked out your parts and prices list. Substracting the price of the EML 300Bs, that's $2200 for a beautiful pair of well-designed (and no doubt great sounding) parafeed monoblocks with really nice components. Quite a deal! (Although it looks like you got your MQ iron at a steal - including from Mike!).

Looking forward to reading the build-log of your next project!

cheers, Derek
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on January 30, 2021, 03:59:44 PM
Its funny, after playing around in the tube domain for a little while, I am inclined to agree that this is a bargain for a great pair of 300B monoblock amps.  At the start of this project, though, it was a bit of sticker shock for me.  I have built a number of solid state Class-A amps and the price tag on these has ranged from a low of < $100 (built with recycled/scavanged parts) to a high of about $600 for a premium-parts build.     
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on March 20, 2021, 12:27:37 PM
I've been enjoying my new 300B monoblock amps for a little while now and wanted to confirm an operational detail.  After several hours of operation, the PGP 8.1 power transformer is running pretty warm.  I can measure close to 73 or 75c. 

Is this temperature level normal and/or OK?  Transformers in my solid state stuff run much cooler.
Thanks,
Eric
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Joppa on March 20, 2021, 02:41:08 PM
That's pretty warm, all right. I assume this is surface temperature?

Can  you check the cathode voltage (wiper of the hum pot) - and what is the cathode resistor value? If it's operating correctly, it should be within the spec of the transformer, but it's an old design and I don't know what the expected surface temperature is. In modern practice, you'd like to stay below 60C.

Thinking a little  more - what is your powerline voltage? B+ volts? The transformer was originally intended for 117 volts, IIRC - I'll check my notes.

How is the ventilation?
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on March 21, 2021, 04:24:33 PM
Yes, this is surface temperature for the power transformer measured with an infrared non-contact thermometer.   Temp measures cooler closer to the top (in the mid 60s), warmer down by the top plate (low 70s). 

300B cathode resistor is 1k 50w aluminum body wire wound.  Last time I measured cathode voltage, it was about 68v.  It has never measured more than 70vDC.  B+ is about 420vDC measured from plate to ground.   The 300B dissipates about 23-24w.  All of this seems pretty much right on spec to me.  Each monoblock draws about 0.75A or about 93w.  AC mains is fused with a 1.25A slow-blow.

My AC mains tend to range between 122-125vAC.  If the transformer is intended for 117v, this is likely why I had to add voltage trimming resistors to all of my heater circuits.

My chassis doesn't have very much ventilation.  The chassis is wood all around the base, it sits on a few felt pads that are maybe 1/8" thick.  The aluminum top plate does not have any vent holes in it.  The power transformer sits on top of nylon shoulder washers, so there is a small gap between the laminate stack and the base plate for the transformer - maybe 1/16".  It was on for about 6-7 hours when I measured temperatures.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Doc B. on March 21, 2021, 04:33:26 PM
IIRC the PGP8.1 was wound for 115V mains.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on March 21, 2021, 04:55:16 PM
Hmmm...  I added 1R0 to the secondary for the 300B heater, 1R5 to the secondary for the input tube heater, and 0R1 to the secondary for the rectifier heater. 

Would a better approach be to add some resistance to the primary of the power transformer?  Perhaps a CL-60 of one variety or another might bring the primary voltage down a bit.  Seems like a 10R CL-60 might drop 5-7v off the primary.

I would certainly welcome other ideas for trimming primary voltage.  I'd like to avoid cooking the power transformers...
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Joppa on March 21, 2021, 06:54:39 PM
I found the spec sheet (from Freed):

DocB is right, it was wound for 117 volts input; it would be prudent to take 5 to 8 volts off of that; you could likely take those resistors out of the filament/heater lines then. Personally, I'd do it with a homemade autoformer stepdown in its own box, since it would generate less heat than a resistor.  You can make one with a 6.3v 2A or greater filament transformer, which should handle both amps.

Temperature rise is specified as 45 to 50 degrees. If room temperature is 20C (68F), that's 65-70C at the transformer, so it's operating within spec though at the limit. For what it's worth, we used it in the pld B-Glow and ParaBee at 70mA and never had any issues.

More ventilation would help, especially getting a little airflow over the 1K cathode resistor. I'd start with 3/8 or 1/2 inch clearance at the bottom - it's easiest.

What are you using for a rectifier? The 5v winding is rated 2.0 amps, which is why I specified a 5AR4 rather than the usual 5U4. Actually, the spec sheet suggests a 5Y3, which would give you a bit less B+ but would be easier on the transformer. Other possibilities are 5R4 or 5V4.

It is considered normal to take 3-4 hours for the power transformer to come to final temperature.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on March 22, 2021, 03:14:39 AM
Thanks, Paul, this is very helpful!  An outboard autoformer sounds like an easy solution and I'll likely be able to remove some of my filament trimming resistors.  The irony is that it took me a while to dial in those voltage levels to my ac mains ;-)

I'm running a NOS Mullard 5AR4 f31 rectifier - probably from the late 1950s or early 1960s or so.  I'll see if I can find some nice looking feet for the chassis - this should help a little as well.

Now that I have these working, it would be a shame to over cook things...
Title: ~
Post by: EricS on March 22, 2021, 04:58:23 AM
I always hesitate a bit when asking questions that reveal my lack of knowledge, but here it goes anyhow:    ???

Lowering voltage on the primary will certainly lower voltages on the secondaries, include B+ which I expect will decrease from ~420vDC to somewhere near ~400vDC (I'm expecting an approx 5-6% decrease in voltages across the board).  How will this impact the bias point of the 300B tube?  Will it draw more current at the lower voltage point in order to maintain its "desired" operating point of about 23w? Or will I need to tweak something else?

Thanks!
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Joppa on March 22, 2021, 05:19:02 AM
The cathode resistor will stabilize the bias, reducing it in rough proportion to the voltage change. In the old days it was called "automatic bias" or "self bias" for that reason. Nothing needs to be changed for less than +/- 30% voltage changes.

A way to visualize this (that works for me - YMMV!): If the plate voltage drops, the current will drop, reducing the bias voltage which increases the current.

It's a form of "negative feedback", effective at DC but not audio because of the bypass capacitor. You could call it "passive servo bias" as well. Don't you love it when something simple has several different names to confuse the unwary?
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on March 22, 2021, 06:00:24 AM
You can draw the load line for the cathode resistor on a set of 300B curves knowing that you have a 1K resistor, you can go to the 100V grid bias line and put a dot at 100mA, then another dot at 50V grid bias and 50mA, then connect the two with a line.  Your amp will always operate on that line, so you can find the spot that's a little under 400V (you'll lose some voltage across your plate choke) and see what current should be drawn through the amp. 

The bucking transformer is indeed a good idea and a 6.3V 1A transformer would do that job nicely per amp, or a shared 6.3V/2A unit for both.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on March 23, 2021, 03:27:56 AM
PJ: Thanks for the explanation.  I've read a little bit about self bias circuits and was wondering if this change would sort itself out. 

PB: Thanks also for the explanation using the load line graphs.  This is an area of understanding tube amps that I haven't invested the necessary time to understand, so I appreciate the primer you've provided.

Seems like I have a whole new round of experimentation ahead with a bucking transformer and re-tweaking my filament supplies.  I think the best place to start might be with my variac to see what happens when the mains voltage is dropped by 6 or 7 volts. 

Thought I was done with tweaking for a while  ;D
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on March 24, 2021, 01:46:00 AM
I've been poking around looking for a suitable filament transformer for bucking use.

Found a Hammond P-T166M6 (link is here https://www.hammfg.com/electronics/transformers/power/166 (https://www.hammfg.com/electronics/transformers/power/166)) that is rated at 117V primary, 6.3v center tapped secondary rated at 3A, with an 18.9VA overall rating.  At first glance, this seems appropriate to me (6.3v 3A secondary), but the overall rating is only 18.9VA which seems small to me.  Does the transformer itself need a rating that is closer to 200VA if it's going to power two amps that each draw close to 100w from the AC mains?

Or, should I choose a 12v or 12.6v secondary and only use half of the secondary winding instead of using both as I would with a 6.3v center tapped secondary?

Thanks for the guidance!
Eric
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Joppa on March 24, 2021, 04:50:28 AM
The transformer is not handling the full power; only the 6v winding carries any load current. So it does not nee to have a large VA rating. A 3A secondary can handle 3A of load current.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Deke609 on March 24, 2021, 05:19:54 AM
Further to PJ's post above, Rod Elliott has a good article on the theory and wiring of buck/boost transformers: https://www.sound-au.com/articles/buck-xfmr.htm#s30 (https://www.sound-au.com/articles/buck-xfmr.htm#s30)

cheers, Derek
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on March 24, 2021, 08:50:14 AM
Excellent!  Thanks to both of you!  Derek, I found that article last night shortly after I posted my question to the forum.   Should have known this is a topic Rod would have an article for...

I made a new round of measurements last night using my variac to reduce AC mains by 5-7v and see what happened with the rest of the circuit.  I think I'll need to wait until I have a transformer in hand to see how much actual voltage drop I get, then see if I need any new resistors to trim heater voltages.  I see the Hammond transformers are also specified with 117v primaries, so I it's not clear to me exactly what kind of voltage drop I'll get from a 6.3v secondary.  One step at a time...

Eric
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on April 04, 2021, 09:23:02 AM
I picked up two Hammond transformers, one is a 10vCT and the other is 12.8vCT.  My logic is that which ever one I didn't use would have utility in some other project.  The 10v one is better suited to my target voltage drop and as I expected, using half of the 10v winding gives a bit more than 5.0v drop. 

I wired both of them up and connected 200w worth of incandescent light bulbs (same draw as both amps) to measure the resulting voltages.  The 5.0v winding provides 6.8v worth of voltage drop with a 200w load on it.   My AC mains tend to range from 122v to 125v and the corresponding range after voltage drop is 115.2v to 118.2 which seems just about perfect of the primary of the PGP8.1 transformer.  I can get rid of the dropping resistors on the rectifier filament, but still need to trim the 300B filament by about one volt and take a pinch off of the input tube filament.   Need to wait for more resistors to arrive now... 

Almost there  8)
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on April 30, 2021, 03:07:14 AM
Finally had some time to return to this project after spending 5 weeks cleaning up the mess in my basement left behind by a washing machine that overflowed for 45 mins on the floor above  :'(  I've been through a few gallons of bleach lately killing the smell of mold and mildew, but it's almost back together again...  Now I have a $5 water alarm on the washer.

Anyhow, I finally wired up the the 10Vct transformer for my amps.  Got one amp adjusted last night.  I was able to completely remove the filament voltage dropping resistors from the rectifier.  I still need dropping resistors for the 300B and the 6SJ7 filaments, just smaller ones (which means they run a little cooler as well).  Here are some of the final voltage measurements at this point:

AC Mains:  124.6v
Mains after bucking transformer: 117.8vAC
Rectifier Filament:   5.04vAC (target 5.0v)
300B Filament:        5.12vDC (target 5.1v)
6SJ7 Filament:        6.4vAC    (target 6.3v)
B+ to ground before plate choke:   418v
B+ to ground at 300B plate:           398v
Voltage at wiper of humpot:            66.5vDC
300B plate voltage:           332v
300B plate dissipation:       22w

This looks better to me.  Haven't let it run for an extended time yet to measure temp reduction at the power transformer.  Will likely get there this weekend after I tweak the other amp.  I'm wondering if this might help tame some of the sub-400Hz noise I was seeing on the FFT of the speaker output - maybe the PGP was being pushed too close to the edge with 125v mains?

My plan is to build a nice box to house the bucking transformer.  I've also found a nice vintage meter with a 100v-130v scale and an old analog hours meter that I'll build into the face of the box.  A switch will energize two "lower voltage" outlets for the tube amps as well as a few more "normal voltage" outlets for my turn table, phono stage, and preamp.   
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Joppa on April 30, 2021, 05:07:20 AM
Thanks for the update! Those voltages look just about perfect.

And sorry to hear about the laundry disaster; what a mess that makes. Glad you were able to recover.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on May 04, 2021, 09:57:19 AM
One amplifier has been modified to operate with the bucking transformer, the other has not.  Here are the final PGP8.1 temps after about 4 hours of use today:

With bucking transformer:       69c
Without bucking transformer:  72c

I just realized I neglected to measure ambient temp, oops.   Somehow I was expecting a larger temp difference.   I understand with solid state transistors, lowering the operating temp by 10c will generally lead to double the service life of the device.  Is there a similar rule-of-thumb for transformer longevity?
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on May 28, 2021, 02:29:23 PM
I have now modified both amps to run on something closer to 115-117vAC.  Temps are down a bit, the PGP8.1 power transformer runs a bit cooler at the top than it does at the bottom of the laminate stack.  I also noticed that the temp is hotter on the side the faces the rectifer tube by a few degrees.  After 4 hours of runtime, temps are closer to 65-66c (non-rectifier side), exhibiting a 44-45c rise over ambient. 

I ended up building a new box to power my analog rig: it has a single switch that controls two outlets on the back.  The first is standard household AC mains pass through, the second one uses the bucking transformer for a 115-1117vAC supply to the 300B mono amps.  This outlet has a separate 2A slow blow fuse to prevent overloading the Hammond filament transformer.  Inside the box are also my outboard power supplies for my turn table and my turn table preamp, both connected by an umbilical cord. 

I made a nice front panel to match the amps and installed a vintage voltmeter and a vintage hours meter.  I liked it very much until I plugged it and found that the mechanical hours meter makes a bit more whirling noise than I want.  I'm thinking I'll look for a more modern replacement that will run more quietly.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on May 28, 2021, 05:19:13 PM
If you find a quiet hours meter, I would be very interested!
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Joppa on May 28, 2021, 07:03:30 PM
That looks really nice - I like it!
Title: Re: Digital Elapsed Time Meter ....
Post by: Tubejack on May 29, 2021, 01:26:30 AM
.... not cheap ....

https://www.rammeter.com/panel-meters-instrumentation/elapsed-time-hour-meters?display=842 (https://www.rammeter.com/panel-meters-instrumentation/elapsed-time-hour-meters?display=842)

https://www.mcmaster.com/elapsed-time-meters/display-type~digital/ (https://www.mcmaster.com/elapsed-time-meters/display-type~digital/)
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on May 30, 2021, 04:55:17 AM
Someone suggested that I insert a resistor and see if that reduces the noise level. It did reduce the noise, just not enough to make me happy.  Any resistor greater than 10k results in the meter not moving at all - it just buzzed. Smaller resistors in the range of 1k to 7k actually allow the meter to move and certainly reduce the noise, but still not to a level that I find acceptable.

Bummer. I think I'll need to look into digital meters and find a way to mount one inside the housing for the mechanical meter since all of the digital ones have a smaller diameter face plate. 

Thanks for the links, Tubejack.
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on June 09, 2021, 04:16:58 PM
OK, I feel like I cheated by replacing a period hour meter with a new digital one, but it is much more quiet than the original one that I chose.  I think it's time to call this one done and move onto other projects  :D
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on December 30, 2021, 07:58:27 AM
When I first built this amp, I wired the output transformer for 8-Ohm output.  I am contemplating adding another output terminal and a DPDT switch to the top plate of the chassis so that I can switch back and forth between 4R and 8R speakers.  Is a 6A/125v switch sufficient for this function?  Is there a reason that I’m not considering why I should not implement a speaker output switch like this? 

This is the specific switch I was thinking of using:
 https://www.onlinecomponents.com/en/gc-electronics/35072bu-11403409.html (https://www.onlinecomponents.com/en/gc-electronics/35072bu-11403409.html)

Thanks for any input and Happy New Year!
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on December 30, 2021, 12:15:47 PM
That is super duper overkill.  You are talking about a place where there's roughly 8V RMS appearing with about 1A of current, and you're unlikely to throw the switch while using the amp, so contact current ratings are far less important than doing something like switching mains!
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on December 30, 2021, 02:29:24 PM
Gotcha.  The primary attributes that caught my attention are that it was black, so it would match my overall color scheme, and it would fit in a hole that already exists in my top plate. ;) Maybe I could mount a smaller switch under the hood…
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: Paul Birkeland on December 30, 2021, 03:14:13 PM
In order to feed your potential for absolute OCD, some find it's nice to pay attention to contact plating material.  Here's a black switch with gold contacts.  Maybe it matters, maybe it doesn't, but you never know till you try.
https://www.mouser.com/ProductDetail/Marquardt-Switches/18236101?qs=4EOvy6wCNnuu0isWrGp0Lw%3D%3D (https://www.mouser.com/ProductDetail/Marquardt-Switches/18236101?qs=4EOvy6wCNnuu0isWrGp0Lw%3D%3D)
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on January 01, 2022, 04:03:02 PM
Thanks for the link and the laugh, Paul!

Happy New Year  8)
Title: Re: Need Help with First Tube Build - WE91 300B Parafeed Derivative
Post by: EricS on June 14, 2023, 02:19:23 PM
As it turns out, my mains voltage varies way more than I though it did.  Jac at EML specifies a pretty tight voltage window for the 300B heater, so I ended up using two different bucking transformers and implementing a switch to move between them as needed.   

Using half of a 10VCT transformer drops about 9VAC from the AC mains and is ideal during the winter months when the AC grid stays pretty close to 125vAC.  During the summer air conditioning months, the grid droops quite a bit and the 10V bucking transformer drops too much voltage, taking the 300B filament below the 5.0vDC minimum.  Half of a 5VCT transformer drops about 5VAC from the AC mains and works great during the summer. 

To keep an eye on the filament voltage, I picked up an LED voltmeter from Amazon.  I like this one because it has a calibration pot on the back:   https://www.amazon.com/bayite-Voltmeter-Motorcycle-Polarity-Protection/dp/B00YALV0NG/ref=cm_cr_arp_d_product_top?ie=UTF8 (https://www.amazon.com/bayite-Voltmeter-Motorcycle-Polarity-Protection/dp/B00YALV0NG/ref=cm_cr_arp_d_product_top?ie=UTF8) 

The display is a bit bright, so I put a strip of LED dimming tape across the display:  https://www.amazon.com/dp/B00CLVEQCO/ (https://www.amazon.com/dp/B00CLVEQCO/)

The tape tames the brightness and makes it much more subdued, but still easy to read.  Mounting the LED to the inside edge of the chassis bottom makes it practically invisible unless you squat down and view it from directly in front.  While I didn't do any output FFT measurements yet, I cannot hear any adverse effects such as hum or distortion as a result.  Now it's super simple to keep the filament voltage in the recommended 5.00v to 5.25v range.