I have a marshall jcm 900 4100 half stack. Recently the volume of the amp has been fading out then returning to the set level. I have never owned a valve amp before, so would be grateful of anyone has any ideas about this problem.
I just went through this exact situation, and it turned that my cable (guitar to head) had a short in it. If you haven't already tried switching the cable, give it a shot.
you have to change your output tubes/valves
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