I own a Marshall AVT50H and whenever I play clean, disortion will fade in and out. It happens periodically, for maybe 5 seconds. I’m sure it isn’t switching channels because the light that indicates I’m playing clean doesn’t switch. I’ve been told that it could be cold solder joints, I didn’t get any more information then that. I was wondering what my problem is, should I worry? etc…blah.
Is the amp still under warranty? I guess not if you’re asking here…
Could you describe the distortion? Does it sound like overdrive or clipping, does it get louder or quieter? Who told you it was ‘cold’ (I presume they mean ‘dry’) solder joints? If it is that it’s pretty easy to fix.
It doesn’t sound dangerous, just annoying.
OK, done a google on the model. It’s got a valve pre-amp. Maybe the valve has gone microphonic or is just playing up. Try replacing it, it shouldn’t cost that much.
The distortion sounds exactly like the distortion I have set on the other channel. A few times the clean channel has actually stayed distorted without wavering. If I switch channels, it’s as if I didn’t. If I switch back, then it’s gone. Usually, it gets louder, then quieter and then louder and it continues like that for the 5 or so seconds. Someone on a lesser forum told me it might be a “cold” solder joint.
My guitar is under warranty, but the people I bought it from seem to be sleezy, and like to give me the run-around. I just want to be informed as to the problem, so I’m not paying to fix things that weren’t broken, or for parts that weren’t even replaced. Thanks for the info.
It isn’t a speaker buzz is it?
If you bought the amp new from a shop and it is still under warranty, and it is faulty (sure sounds like it) then they are obliged to fix/replace it for no cost (well in this country they are - it’s the law). If you open it up to fix it yourself* you void the warranty and lose the option of taking it back.
Sounds like a fault in the channel switching, hold off buying a new valve.
Have you tried contacting Marshall? They may be aware of this fault.
FWIW a dry joint is very easy to fix, you get a soldering iron and re-melt the solder. Would you feel comfortable doing this, or have a mate who would? Unless they’ve drastically changed their construction Marshall amps are very easy to get inside. Undo the screws on the base and slide out the chassis.
*and tell them/leave evidence
Small Clanger gives good advice -
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If you are comfortable and competent with electric stuff, open up the back without touching anything!! - and see if you see anything obvious. If you do, you probably should have the warranty place fix it so you don’t void your warranty, but at least you can clearly explain what you saw. If you absolutely must, fix it yourself.
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If you aren’t competent with that stuff - even though it is inconvenient, you must have them service it. Amps can be dodgy - they often have complex circuit boards, etc. - and you shouldn’t take chances. YOu can check Marshall’s website and see if there are other authorized service outlets nearby besides the one you bought it from - maybe you would trust them more.
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Check the Alt.guitar and rec.music.maker.guitar usenet groups in Google groups. Search on your model and on your problem and see what others have posted. There are a LOT of hacks out there, so don’t take everything you read to be the gospel truth, but you might find something out.
It sounds like there is some sort of solder/short issue which should be straightforward to fix, but do it right, okay?
Best of luck,
WordMan
All the more reason to get rid of that POS Marshall and get yourself a Mesa Boogie. You’ll never look back! 
Haha, I totally knew about not needing to pay when you’re under warranty. I think that was just an excuse for coming on this board and not phoning the place I bought the amp.
Well, if you knew what an AVT50H was, you’d know they’re like 1/4 the price of a mesa-boogie. Trust me, when I can afford a mesa-boogie I will have one.
Oh yeah. Thank you wordman and smallclanger, much appreciated.