Ampeg B15S humming

[rf7]

I narrowed this down to the V3 tube, but I can't make it go away. I pulled the feedback wire and the external speaker wire, but it still hums. I lifted C20 and grounded it and the amp is dead quiet. I lifted C19 and tied it to ground and I get the hum. I've tried 2 new tubes and 2 used tubes from another amp and it still hums. I need to buy a guaranteed quiet 12DW7 tube from someone I guess.

There are not a lot of wires running into this part of the circuit, and moving them about does no good. Any other things that I should check?

Schematic

[loudthud]

Check the hum balance pot. Center it and check for ~3.2VAC on each side of the heater supply to ground.

[rf7]

Yeah, I've done that. I even hooked up a 6V lantern battery as filament supply, and it still hums.

[loudthud]

Does the amp have a 3 wire power cord conversion? Could the placement of the ground wire be shorting the signal ground in the power supply to the chassis? Other than a bad filter cap or a busted ground wire, I'm out of suggestions. Does shorting the signal at J5 produce the same hum as grounding C19? I don't understand why D5 is used other than to make the pilot light turn off right away when the amp is put in standby mode.

[rf7]

Does the amp have a 3 wire power cord conversion?

Yes, it looks factory installed, there is no polarity switch.

Could the placement of the ground wire be shorting the signal ground in the power supply to the chassis?

I don't think so, it's just a wire running to a lug screwed into the chassis near the filter caps.

Does shorting the signal at J5 produce the same hum as grounding C19?

Yes, pretty much.

I'm not sure about that diode either. All of the grounds seem to be ok.

Thanks for the suggestions.

[Jack Hester]

Check the hum balance pot. Center it and check for ~3.2VAC on each side of the heater supply to ground.

I am in the process of de-humming a B-12-XY. I have hiss, a higher frequency hum (which may go away with the hiss, when I fix that), and a lower frequency hum. Adjusting the hum balance pot helped some. But, when I moved it from V1 (one of the power tubes) to V7 (one of the Channel 1 tubes), I was able to all but rid the amp of the lower freq hum. His hum balance is not wired like mine, as his wiper is wired in above signal ground. Mine goes straight to signal ground. Maybe if he moves the the outside taps (1 & 3) of the hum pot to the offending tube (V3), it may make a difference. It sure did on mine.

Jack