I bought a used Mesa Boogie Roadster last year. Never asked how new the tubes are. I have a GT10 going through the effects loop. I was playing a bit today and the whole amp got quiter, distortion and clean channels were real low all the sudden. After a few min it kicked back in. Is this the symptom of a tube going bad?
Possibly. It could also be a cold solder joint. Since it's mesa boogie I would reccommend not opening it up and taking it to a qualified and certified tech first.
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definitely get it checked out by a pro. A couple of months back my marshall had similar symptoms at a gig. Everyone I talked to said it sounded like the power valves were dead. I took it to an authorised repairer and it turned out to be a faulty plate resistor. I could have spent lots of time and money finding that out if I hadnt just taken it to them.