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

[EricS]

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.  This seems like cheap insurance for a tube rectifier - has anyone else done this?
 
Thanks for the help!

[Paul Birkeland]

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.

[Paul Joppa]

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!

[EricS]

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?

 

[Deke609]

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   [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

[Paul Birkeland]

For a plywood prototype, I wouldn't worry about grounding the frames of the iron.

[EricS]

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.  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.

[EricS]

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

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/.  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 

[Paul Birkeland]

That looks much better than all the clip leads I use to do the same thing!

[EricS]

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?

[Paul Joppa]

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.

[EricS]

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/

[Paul Birkeland]

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.

[EricS]

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...

[Paul Birkeland]

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.