Failing Marshall 8080

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Have a 20yo Marshall 8080 been fine but plugged my fender strat and lost all power from the Boost Channel. Can finly hear the amp sound, but very little. The amp squeeled for a second after plugging in and then this. I presuem that means the tubes in the pwoer Amp have blown? Had similar happen a couple of years ago when I first bought the amp. Is it possible that the cable was faulty? Foudn an old cable that came with the Strat and havent used in years. Is it easy to chnage the tubes? last time cost me $400 and the amp woudlnt be woth that much anymore.
 
It could easily be tube related. If the clean channel works OK, then the power amp must be OK.

The tube itself could have failed, or one of its power supplies. Are the heaters in the tube glowing? And there is a small high voltage supply just for the tube, make sure a resistor in it has not opened and that there is B+ on the tube plates.

Otherwise, apply a signal at the input and just trace it stage by stage through the bad channel.
 
I'm currently working on a valvestate 8080 right now. It apparently fell on its nose, as most of the pots were open or intermittent. tubeampdoctor has them in stock.

I repaired a mixer for a lady singer, it had a large number of broken pot's - with the ones in the middle broken the worst. Once I'd found all the broken ones, it was obvious that it had been hit by a football - it was a perfect circular pattern, with the very centre ones damaged most.

It turned out she kept it in her conservatory, and has a young son who likes to play football :D
 
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