Marshall JCM 900 No Output

I've got a Marshall amp that all of a sudden lost all of its power - its sounds like a 2 watt amp now for some reason. It's worked for many, many months. Could it be a tube issue, or perhaps the output transformer? What can I check?

Thanks,
Sig

first thing, check the send/return jacks. the shorting contacts go intermittent- usual symptom is total loss of output but i have seen a couple cases like yours.

As quoted above, check the S/R jacks first. If yours is like my old 50W it doesn't have those. Check the amp into another speaker cab to make sure you don't have a bad connection there. Then reseat the tubes - pul them out once or twice and reinsert them. After that swap the tubes. Usually when the power tubes go the HT fuse blows but you never know. Once all that is done you may need to pop it open and trace supply voltages but cross that bridge when you get there.

cheers,
kent

quickest way out of that dilemna during a show or recording is a short
1/4" inch cable to jump the send/return jacks - ordinarily, they are normalled but the jacks do go bad/get dirty/etc. - and the send/return jacks are directly in line between the preamp and the power amp.

i've seen this on a couple of 4100 model JCM-900's (that's the 100 watt w/ switching and reverb).