Metropoulos Forum

Hi, I was wondering if someone might have some insight on a marshall jmp mk2 problem..

the amp suddenly developed this subtle, but weird kinda oscillation sound, like a phaser with the rate turned all the way up and a touch of an octave sound too. i opened it up and the 1k plate resistor had broken off of the middle two power tubes. i resoldered them and the noise remained. i swapped out every tube, no change.

about 19 yrs ago i replaced all elecrolytics and filter caps except maybe the two for the bias supply. plate voltage is about 450. using the 1ohm resistor/voltage method, the highest bias read around 50mv and the lowest around 34mv. i tried bringing down the bias so that the highest was was 34mv, which is about what it should be, but then the lower ones were in the 20s and still no change to the sound. all resistors read fine, including the plate resistors and the 5.6k grid resistors.

any ideas? what could cause the bias values to have such a broad range from one tube to another?

thanks for reading!!

The bias readings and the oscillation probably have nothing to do with each other. I would pull the power tubes and check pin 5 negative voltage to make sure they are all the same. If so, the bias variation is probably just difference between tubes.

Regarding the oscillation, you could pull one preamp tube at a time to narrow down where it's coming from. Try moving the preamp grid wires (pins 2 and 7) around to see if that affects the oscillation.

Thanks for the advice! I already swapped out all 7 tubes entirely, but I'll try your other suggestion. It's just weird that this condition just came outta nowhere and seems unrelated to what tubes are in there.. The head has only been used in the studio for the last 11 years and hasn't been abused like it was on the road for the previous 15 years

Finally figured out that one or both of the bias caps went bad.