Power tube gone bad..It's rare for a small signal tube to go bad until they have 4000+hours..
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Sorry I was in a hurry when I replied & yes power tube would affect only 1 channel..
As I think about this I agree with reply from noromance that the possible culprit is the 101D which is used for Voltage Regulation..If that were going bad it would display as a lowered output....However the BUZZ points to a transformer..
Have you tried contacting Melody?