I have a Marshall JTM312, the clean channel works fine, i just put new sovtek tubes in last year, but a day ago i tried to switch to the overdrive channel, and there was no sound, absolutely nothing. I think this is because the preamp tubes are gone, but im not entirely sure, any help would be awesome.
If it's just the overdrive channel going dead, it's probably your preamp tubes.
I'd swap out your tubes anyways.
Call me Wes.
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Summary: find a dumpster for your JTM312.

I had one. Fantastic little amp when it worked. Great clean channel, great drive channel. Poor design. The power tubes are mounted horizontally under the PCboards. Eventually the boards get cooked. The symptoms are that the drive channel crackles & fades as the amp comes up to temp. Then the clean channel starts doing the same. Then nothing. I took it to several amp techs who all confirmed the diagnosis.

There are no replacement parts from Marshall. Your choices are to find a working unit & cannibalize it or get out the soldering iron and try to troubleshoot/repair the board (no thanks). Eventually it will result in the same failure.

Save your time & money. I stripped all the electronics out of mine to lighten it up and now use it as an ext cab. Nice looking cab with a Celestion v30 in it. Looks great with my handwired 2061x sitting on top!

best of luck
ha, thanks for the warning but unfortunately the money tree in the backyard is dead for the winter.
Check the tubes. JTMs are great amps, but suck for design flaws and occassion crapping the bed. Use a pedal for od channel till you get it fixed.