Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Lectroid on January 08, 2026, 12:13:53 pm
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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?
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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.
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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/ (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.
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@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. :icon_biggrin:
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.
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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 (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.
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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 (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. :icon_biggrin:
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. :cussing: