Mesa boogie tremoverb PI problem

[Andrei_T]

Hi, I have a mesa treboverb on the bench with the following problem:

V6b , that's the firs half from the PI triode seems to draw less current than the other half. While I have 120V drop on the 91K plate resistor I have only 50V on the 82K plate resistor. And the voltage reaches 50V after about 30 seconds from powering on from stby. This means that the current flowing in the first half slowly rises to about 0.6mA I think.

There's also some crackle and pop noises.

What I did so far is replace the 82K plate resistor, the 1M input resistor with no luck, measured the coupling caps to the power tubes and they seem ok.
I also cleaned the socket and replaced the ecc83 with no luck. The next step I think would be to replace the coupling cap before the PI input and see what happens altho it did pass the leakage test...

Any ideas as to what might be the problem?

Thanks,

Andrei

[Enzo]

You did not mention the cathode and grid resistors. Check all of those.

[Andrei_T]

Thanks for the reply Enzo, I forgot to say in the original post that I have changed the grid resistor from the input of the phase inverter. The chatode resistor couldn't have been the problem since the second half of the PI is working just fine.

Anyway I found that the coupling cap or vibrato LDR upset the bias of the input PI triode and that was the cause of the problem. Which one I don't know because this was an intermittent problem and didn't appear after I raised and put back the coupling cap leg. Just to make sure I replaced the coupling cap and removed the ldr from the circuit since the amp's vibrato was not used.


Andrei