Low signal at power tube input grids (Marshall TSL JCM200)

[Gaz]

Hi,

My friend brought me his JCM2000 TSL that was sounding weak, but otherwise fine. Sure enough it was only putting out 35W. I tried new power tubes (his were 7 years old), but only yielded a few more watts.

I've searched for common problems in these amps (and there seems to be a lot), but so far none of the usual suspects (i.e. shorted ceramic 'anti-oscillation' cap at the power tubes, fried speaker jack, dead OT, etc.). There are no burnt components and I have re-soldered some suspect joints with no improvement.

I just noticed that I get a good normal amount of drive from the PI, but the AC signal voltage at the power tube grids is low, like 1/5th of what's coming out of the PI. It definitely explains the low output, but I don't know what would cause this symptom. Any ideas? I've attached the schematic. This amp is so over complicated with all the connectors, it's hard to trouble-shoot.

Thanks for any advice.

Alex

[Enzo]

OK, you are going to hate me if I am right, but look over by the power switches. The four little push buttons there. The two next to the knobs are deep boosts, the one by the power switch is a mute, and the one next to it is a half volume switch. ANy chance that one is pushed in?

[Gaz]

Hate you? I love you! So hilarious.

So that's what those 680K resistors are for! I see now that they form a voltage divider with the 220K grid bias resistors like a fixed PPIMV.

Thanks a million, Enzo.

[Jazz P Bass]

Enzo is sharp, I'll give him that
Virtual Power Reduction.