Hey there, I have on the bench our Marshall plexi reissue from the studio. This has been modded with a PPI master volume and variable NFB. Amp was working and then magic smoke escaped.
The component that blew was R1 on the V1b cathode bypass.
Replaced tubes (output tubes were fried). Amp sounds correct and has the right tone again but...
Issue now is there is a huge amount of noise coming from PI. With all knobs up and no signal injecting its about 14VAC coming out of the PI plates and about .7VAC going into the grids. Im testing without power tubes installed.
Seems like there is a ground issue somewhere but I can find it and not sure where to look. Ive chop sticked around and reflowed joints where I can access them, etc. Tried bypassing the dumb effect loop card but that wasnt it either.
One of the challenges is these amps are half hand wired and half PCB with the soldering under the PCB and the components on top. Not ideal for servicing. So to service the PCB solder joints you need to remove one half of the leads to the sockets or pots to be able to get the PCB up. Very frustrating. Also the coupling caps are through hole tight to the PCB so no leads to solder on to for replacements.
This was the other thing I thought might be the issue is the coupling caps on the PI could be leaking but I dont know if that would explain the massive amount of noise. Also this amp is not that old. The PPI master volume mod is... after the PI so its not the issue, unless its the source of some weird grounding issue.
I hate working on amps with PCBs and I hate even more working on amps modded by someone else!
Any ideas welcome.
Scheme:
https://www.drtube.com/schematics/marshall/1959-01-60-02.pdf
