Need Help with First Tube Build - WE91 300B Parafeed Derivative

[Paul Birkeland]

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.

[EricS]

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?

   

[Paul Birkeland]

If your heater wiring is well twisted, there's far less concern about picking up additional noise. 

[EricS]

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?   

[Paul Birkeland]

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.

[EricS]

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!

[Bardamu]

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

[EricS]

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. 

[Paul Birkeland]

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.

[EricS]

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... 

[Paul Birkeland]

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. 

[Bardamu]

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.

[Paul Birkeland]

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.

[Bardamu]

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

[EricS]

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.