Hey there,

My Marshall JCM 900 2x12 combo (Hi-Gain, Dual Reverb, model 4502) is having some sudden problems. I recently bought a head, so the combo has been standing around in the rehearsal room for a few weeks now and hasn't been played since I checked it out for selling it. A friend of mine borrowed it and texted me it wasn't working properly, so I checked it out again and here's what I found;

I've practically lost all power, the volume is about 1/10th of what it used to be, but the power tubes are glowing and the amp gets enough warm up time. There is a strange hiss coming from the amp in standby mode, but the tubes aren't overly microphonic. It was working fine 3 or 4 weeks ago and neither of us have heard the amp popping or making other harsh noises. I've tried it with different speaker cables, cabs, etc, but to me it seems the power tubes are dead, but they glow as they've always done.

All tubes were replaced in April 2011, I'd estimate they've had about 300 to 500 working hours. The amp is typically operating switched to 50 watt (the high-mode) and volume at 12 o'clock. It barely makes it out of the rehearsal room, although it is stored in a closet there, so it is moved around on a weekly basis.

Should I only replace the power tubes, or are there likely other problems as well? As I'm selling it, I'm not too keen on having it checked out by a pro or a company and cut even more loss on selling the amp.

Thanks in advance!
Gibson Explorer
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Well, it could also be a preamp tube. You should have spares; a pair of power tubes and at least one preamp tube. If you had those you could have already found the problem via systematic substitution.
Blown tubes will often continue to glow.
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