question about my amp.

stevesmith1987

stevesmith1987

New member
I have a Marshall JCM 2000 DSL 100. At our last show it got this loud irritating buzz coming out of the effects loop and my distortion was really thin and muddy.

When I got home I replaced all the preamp tubes. The buzz disappeared but the distortion was still terrible. I checked the bias on the amp and it was all out of whack. I readjusted it and now its fine.

Is there anything that can cause the bias to get off track like that on it's own? I don't understand how it happened.
 
Re: question about my amp.

A failed/failing component in the bias section could easily make this happen. Keep an eye on your bias for a while and see if it shifts
 
Re: question about my amp.

OOops, sorry for the earlier post....missed the bit where you changed out all the preamp tubes.
 
Re: question about my amp.

The bias on the JCM 2000 is adjusted by a set of micro pots. I wonder if over time the vibration shook them out of place. I used it for my gig tonight, it sounded great.
 
Re: question about my amp.

I have a Rivera S120 that used to do this from time to time. I would smell burning tube and it would sound like crap. Other times, it would sound great for hours.
I changed all of the capacitors in the bias supply and it's been fine ever since.
 
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