microphonic...something?

[fossilshark]

So I just got this JCM800 2204 head working except the high gain channel refuses to work properly.

No matter what tube is used for V1 (including one right out of my everyday amp) there is a distinct feedback that is caused when I tap the chassis around a spot (right over V1s passive components) and only on the high gain channel, so I was wondering if I could have a microphonic tube socket or possibly a bad solder joint causing this?

The high gain channel also seems to have unity gain, its definitely overdriving but not quite as loud as the clean channel, could these problems be correlated?

PIN 1: ~298v
PIN 2: 0v
PIN 3: ~2.6v

side note, the lead series schematic (reissue I think) omits a 100pf cap from pin 1 to 3, what is the purpose of this cap?

[vampwizzard]

is it turret board or pcb? pcb could have a capacitance in it. sounds like a grounding issue though.

[shooter]

what is the purpose of this cap

microphonics 

n you listed most of the "usual suspects" for your problem,
poor ground connection, flaky shielded cable come to mind also

[fossilshark]

its a turret board, all P2P.

Grounding would make alot of sense, I have both input jacks grounded but should I also have the switching terminal on the high gain channel grounded? The low gain switching pin is connected to the .22uF off of pin 1 on V1, is this correct?

[vampwizzard]

dunno about the schematic but it sounds like an issue i have when a ground solder joint is cold.