Hi, i read your post with great interest because i have the same type of problem. I have used the following amps all with the same problem: Vox AC15, Marshall JVM, Mesa Boogie Express, Laney Vc15, Marshall AVT 50 watt amp, Same problem -- after two or three hours the distortion just disappears. Does not matter what guitar, what pedals, etc. In frustration I took my Mesa Boogie to a really superb tech who tested it, turned it on with a load for 5 hours, etc. He said that the tubes did not malfunction and there is no other internal problem in the amp. Don't need to change tubes, they are ok. So what in the hell happens??? I think it is simply that the tubes overheat and get "tired" and somehow lose their punch. If the amps are turned off and allowed to cool down completely then they are fine, The problem appears when you run the amp for a couple hours with high distortion/overdrive. Maybe the answer is to take the Mesa and the Marshall to the same gig and alternate between them changing amps after every set? Anybody got any other ideas?
This is such a common problem it is strange there is not more info out there about it.
