Ashdown EvoII500 valve drive issue

[sunshine_bassman]

Amp goes quiet when I turn on valve drive and turn the knob all the way up. Replacing the tube doesnt do anything. I have the schematic but don't know where to start.

[glebert]

Does the tube glow a little or get warm? I'd probably start by measuring voltages at the tube socket with the tube removed. Voltages won't line up with what is on the schematic, but I'd at least check that the plate voltage (DC) is there and heater voltage(s) (could be AC or DC).

If you can post the schematic without violating any agreement with Ashdown it would probably help.

[Helmholtz]

I have the schematic .

But we don't.

[sunshine_bassman]

Tube does glow, heaters are registering less than 1vcd and pin 6 is at 11.9v.

[sunshine_bassman]

https://music-electronics-forum.com/...5&d=1400193284

[Helmholtz]

Tube does glow, heaters are registering less than 1vcd .

Don't understand.
1VDC measured where?
Heater voltage should be 12VDC from pin 4 to ground.

and pin 6 is at 11.9v

Did you verify the 350V B+?
If yes, low plate voltage means either excessive plate current or - more likely - a bad (high resistance or open) plate resistor R48.

Excessive plate current could be caused by a leaky coupling cap or a leaky/shorting cathode cap.

[sunshine_bassman]

I'm measuring 1.9vdc at the junction of R48 and R50, where there should be 350vdc
2.4vdc from pin 4 to ground upon reassessment

[Helmholtz]

I'm measuring 1.9vdc at the junction of R48 and R50, where there should be 350vdc

You can't have 11.9V at pin 6 i.e. the lower end of R48 and only 1.9V at its upper end.

Is your meter battery fresh?

[The Dude]

I think you are counting tube pins wrong or wrong direction. These measurements make no sense.

[sunshine_bassman]

I realized R56 had blown so I replaced it, and got 375vdc B+ back, but now the amp doesn't have sound at all and pops when I engage the mute button...