Noisy cap?

[glebert]

I've got a friend who asked me to look at a Weber Smokin' Joe II he built that has a hum. He's built 5 or 6 of these amps without problem, so he's not a total noob. I can find the noise when I probe the circuit, see pic below. The hum at the speaker goes up and down with the post PI master volume but the magnitude doesn't change prior to the MV. AC shorting out at the PI at pin 7 totally kills the noise. The other strange thing is that C10 seems to be microphonic. Is that a thing? Could this cap be picking up noise?

Changing tubes has not had any impact.

TIA
Greg

[Jon Snell]

If C10 is ceramic, that is common to produce noise. Much like a conderser microphone.
I would lift C9 out and see if the noise, of a very odd frequency 160.4HZ, goes away.
If it doesn't, the noise is comming from the phase splitter.
I would look around for bad solder joints.
If the noise stops, it is comming from V1. Again do the obvious first, check the ground soldering, input jacks etc.

[Helmholtz]

The period of the hum signal is slightly above 8ms, indicating a fundamental frequency of 120Hz.

A microphonic cap might pick up a little vibrational hum, but I don't think that's your problem.

[glebert]

Jon was right, C10 wasn't getting a ground.

Thanks guys.