A little help: Voltage levels for a 6SJ7?

[Lectroid]

I could use a sanity check on my Bell & Howell conversion.  I gutted and rebuilt it recently but I haven't gotten the rebuilt version working yet. The 6SJ7 tube isn't amplifying.  So I'm working through the possibilities.  Tried 3 different 6SJ7s, all JANs.  A signal is present at the 6SJ7 grid pin, but nothing comes out. Voltages seem reasonable to me, see below.  But are they?

That's the sanity check I need:
Are the voltages on the 6SJ7 high enough for the tube to conduct and amplify?  If it is, then I can ignore that as an issue and keep looking.

Is there a reason why B&H didn't run that tube at higher voltages?  It seems to me like doing that should increase the preamp amplification, but what do I know?

Thanks!

PS Please note: he first build had four power supply nodes with filter caps.  On this one, the "D" node has no filter cap.  That my have an effect?

[SEL49]

PS Please note: he first build had four power supply nodes with filter caps.  On this one, the "D" node has no filter cap.  That my have an effect?

Node D must have a filter cap. Otherwise, that 15K dropping resistor simply becomes part of the 6SJ7 plate load resistor. Put a filter cap at node D. It may even fix the problem with the 6SJ7.

[bmccowan]

I do not know what the plate voltage on that 6SJ7 in the B&H is spec'd at. But you are getting no sound through the tube. Having 80V at the plate should not be the problem. Testing amps with that tube in V1 with a variac produces sound at a much lower plate voltage than 80. So as you suggest I would look for the problem elsewhere. Such as the filtering, as SEL suggests.
BTW - this is a good piece on converting a Filmosound 179 https://muttaudio.wordpress.com/2025/02/01/giving-the-fender-treatment-to-a-filmosound-179-amp/
I converted one a few years ago - I checked my files and I did not record the voltages.

[Lectroid]

@SEL49, bmmccowan,

I added the missing filter cap back in and that fixed the voltage problem, thank you.  Still have the no-sound condition to troubleshoot but I can use the practice.   

My new 6V6 tubes arrived so I will retest voltages and post them here.

Thanks also for the link about the 179 conversion.  Their version runs the 6SL7 hotter than the B&H circuit runs it.  I've been wanting to experiment with pushing that tube a little harder once the amp is running better.  The Mutt project gives me a lot of food for thought.

[bmccowan]

Lectroid - this thread has interesting discussion of Filmosound 179 conversions. It includes info on one I completed. Rather than Fender specs I went with preamp circuits similar to Gibson and Valco amps I like. The amp sounds good, but was very sensitive to wire dress due to the crowded chassis and maybe some sloppy work on my part. I will not rebuild another one - too crowded for geezer fingers and eyes.
https://el34world.com/Forum/index.php?topic=29633.msg326372#msg326372
BTW - when I have encountered a no sound situation it is usually because I have accidently grounded something that should not be grounded.

[Lectroid]

Lectroid - this thread has interesting discussion of Filmosound 179 conversions. It includes info on one I completed. Rather than Fender specs I went with preamp circuits similar to Gibson and Valco amps I like. The amp sounds good, but was very sensitive to wire dress due to the crowded chassis and maybe some sloppy work on my part. I will not rebuild another one - too crowded for geezer fingers and eyes.
https://el34world.com/Forum/index.php?topic=29633.msg326372#msg326372

Thanks for looking that up.  I don't think I'll ever do another one. 

BTW - when I have encountered a no sound situation it is usually because I have accidently grounded something that should not be grounded.

This amp has had random fluctuating voltages since rebuilding so I've been suspecting something like a mis-ground.  I put in my new 6V6GTs this morning and took new voltage numbers, below.  They looked good to me.

About five minutes after I finished that, the entire power  supply failed.  I'm getting 122V at the inside contacts of the IEC inlet, but 0V out of the PT's secondaries: no HT, no 5 VAC, no 6.3 VAC.  That PT was brand new two weeks ago, but maybe that phantom voltage leak I engineered was enough to toast it? 

I'm going to go guzzle some wine and forget about electrons.