BF Showman Driver Stage Hum

[booj]

Just installed a Mojotone output transformer in it. (Which was not a drop in fit)
After biasing, was checking it out with a spealer checking out for any circuit noises and found I'm getting about 35 mv of 60 HZ hum from the output. Then hum is definately coming from the driver. It doesn't change when I pull V1, 2, or 3. With the driver gone it's totally quiet.
I've swapped out tubes, checked all circuit components by de-soldering one end and checking with an ohmeter or a cap checker. I wound up changing out the 500pf to pin 2, because a mica cap read better on the cap checker.
I've even unscrewed the output transformer and spaced it from the chassis a good 1/4" to see if there was a change.
The 6.3 volt winding has a center tap and reads equally on both legs (about 3.4v. ea. leg)
I've inspected the fiber board underneath and it looks uncluttered and clean.
Questions:
Is it possible that the way the filament winding is connected to the socket a problem like this could happen? It's backwards from the preceding preamp tube socket.
Also, is it possible the tube socket itself could be bad? That's about the only thing I haven't completely disconnected or changed.
Thanks!
Showman Amp AB763 Schematic.pdf

Showman Amp AB763 Layout.pdf

[booj]

Ok. I got impatient and pulled the driver socket and cleaned it with alcohol, then re-connected it with the heater wiring the same polarity as the preceding stage.
No difference.

[booj]

Final solution was a compromise. I still don't know the cause of the hum but I've got it reduced to 18mvac by laying the 500pf cap flat against the fiberboard and installing a 22uf cap from the wiper of the bias pot to ground.
I was just kind of hopoing for an "oh yea I've seen that a dozen times" type response.
Maybe there's a problem with the power tranny, or the output tranny isn't matching well to the circuit? It has reached aetheral bizzarity

[ThermionicScott]

I wonder if the bias supply cap is bad, then. If it's not doing its job, then it could modulate the output with 60Hz ripple, since the bias supply is a half-wave rectifier. Some folks like to replace that 25uF with a higher value anyway.

- Scott

[tubeswell]

Hi booj

I had a similar sort of problem a while back with a 5F6A something clone. Turned out to be a bad presence pot - muckily soldered - in the LTP tail, which clipping a meter onto the tail end of the coupling cap going into the LTP seemd to quiet (until I figured out by replacing just about all the parts in the PI, what it was). Not saying that its a bad pot, but it could be a bad soldering joint. Have you tried re-flowing all the joints?

[Enzo]

Take your meter probe set for DC voltage and explore the eyelet board itself for stray voltage. Poke the meter probe into the board material right next to some eyelets, but not touching them. Specifically eyelets that are wired to B+ or plate leads.

Fender eyelet boards getting conductive is a time honored, "what the fuck" for many a technician. The waxy muck on the surface absorbs moisture from the air and gets conductive. Pretty strange the first time you park your meter probe on the "insulated" board only to find voltage present.

Worth a quick look.

Have you tried a different 12AT7? If the voltages are like the schematic, you have almost 100v on the cathode, you could have some heater/cathode unhappiness.

[booj]

Thanks guys. The bias caps are new. 22/160 each. I did try sevearl different tubes. (The customer put all 12AX7's in) A 12AT7 lowered the noise just a bit, but not too much. Andk I did clean and re-solder everything in the driver circuit including the tube socket.
Thanks Enzo, for the board probing idea. I did that and pretty much got a consistant .2 volts wherever there was a high voltage. That makes me suspect that the fiber board is indeed very slightly conductive. And it doesn't take much to cause a problem.
However, like I said it's pretty much a done deal now. I will tell the customer to keep the amp in a dry place, plus hopefully with a little use the condition will get better instead of worse. Now that's some black magic voodoo!

[Enzo]

I find i can often bake the conductivity from the eyelet boards with a heat gun. The board has a certain waxy semigloss appearance, and blowing the real hot air on it, you can see the appearance sort of thaw and get dull or matte looking. And at that point it usually is no longer conductive.

[booj]

Well, I warmed up the board a bit with a heat gun. Didn't get too carried away with the process. I wasn't seeing any difference in the appearance of the fiberboard, and since the thing is working ok now I consider it a done deal. I have however posted pictures. Everybody likes pictures!