Marshall JMP MKII non master volume

[DrGonz78]

At this point I would stop doing anything to the amp until I matched up a good schematic. Got to trace it out a bit and figure out which version is a match. Then post that version of the schematic for us here too.

[Chuck H]

Sorry for the confusion..I did use 220k ohm resistor

Start by removing the power tubes for now.

Can you alter the voltage on the power tube grid pins by adjusting the bias pot? If not, you need to repair the bias supply so it CAN be adjusted. I think what you have now is too much bias voltage or no bias voltage, depending on whether you have the resistor installed.

[g1]

Sounds like the bias is intermittently going to "full cold".
Some board flexing before may be why it straightened out for awhile.
Possible suspects are the bias trimmer, or bad connections at the bias trimmer or the 47K it connects to.

[pnut5150]

Ok, so far I have not found an exact schematic as mine..close but usually that is a 150k resistor instead of a 220k

[g1]

It's a 150K for 6550 versions. But that is not necessarily what you have. Some one could have put 6550's in there.
Look at EL34 schematics and see if your circuit matches.

[drewl]

The European versions with EL34's had 220k grid resistors, the us imports had 150k's for 6550's.

There are but minor variations of these amps from the plexi's until the the JCM800's.

its not rocket science, but a very simple tube amp.

I'd say pull the board out, carefully by removing the pots and check underneath for anything screwy.

Either the output tubes are flaky or the bias circuit isn't right.
Could be the diode breaking down, bias caps flaky or a flaky feed resistor.

[g1]

He hasn't lost bias. It's either been biased full cold or working.
Except when he had the bias feed resistor removed and the tubes red plated.
Or when he touched the live circuit with the soldering iron. That's a major no no, could do severe damage or hurt yourself. Power down and discharge caps before touching the iron to anything.

[pnut5150]

Ok here's what I have
With tubes pulled pin 5 on both power tubes read -069.2
Tubes in I read -069.0

[pnut5150]

Yes I can raise it up and down with the bias wheel

[pnut5150]

appears to be my schematic

[pnut5150]

The 47k resistor behind the bias wheel was reading 158.4k out of circuit..put a new 47k in and amp is running fine..no red plating. Using my DMM what should I set the bias for?

[g1]

What voltage are you now seeing on the power tube grids, when idling with power tubes in?

[pnut5150]

-38.43 & -38.45

[Chuck H]

That's probably fine. What are your plate volts now?

[The Dude]

Not to turn this into "20 questions" but here goes............Do you have a bias probe? If not, I recommend getting one if you plan to do much tube amp work at all- be it professionally or for a hobby.