Peavey VTM-120 Possibly bypassed Heater Elev ckt?

[The Dude]

^^^^^^ That. If results offer no clues, I'd start at the front end and pull tubes, one at a time, to see if you can isolate the gain stage causing the problem. But, start with Enzo's plan.
One more add: Because tubes are new doesn't mean they are good.

[Just Mike]

SO isolate the problem. Does it do it with no guitar plugged in? If so, then leave the input jack empty. Correct, with no input and no volume there is hum.

Do ANY of the controls affect the hum in ANY way? Level, tone, whatever? No

Turn your guitar to zero and plug it into the FX return. Still hum? Yes

Tubes are all new, but I guess I can start pulling them starting with V1. Can/should I include the PI in this tube pulling exercise?

[g1]

Yes you can include pulling the PI.
That will leave only the power tubes in operation. Are the power tubes matched? Do you have a way of measuring the idle current for each power tube, like a bias probe or something?

[Just Mike]

Started pulling the preamp tubes. V1-still hums, V2-still hums, V3-voila! hum gone. Looked at schematic and hush my mouth- there's some 22µ filter caps under there.... Replaced those and the amp is dead quiet. Sometimes just having a sounding board and a little prodding makes all the difference. Thanks again!

[tedmich]

Started pulling the preamp tubes. V1-still hums, V2-still hums, V3-voila! hum gone. Looked at schematic and hush my mouth- there's some 22µ filter caps under there.... Replaced those and the amp is dead quiet. Sometimes just having a sounding board and a little prodding makes all the difference. Thanks again!

Yay! I just picked up a 1990 era Bravo and all the blue Nichicon axial electrolytics (well >80%) test bad. ! Replace ALL is the best strategy IME!