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Originally Posted by
tunghaichuan
Does one of the output valves look burnt? (EL84 in BJ).
It's kind of hard to diagnose these types of problems over the 'Net, but if one of you EL84s has failed, the amp will still produce sound, but you'll get some hum as the output transformer is push pull. When only one tube of the push pull pair is operating, you get kind of a quasi-single-ended operation.
You could check to see if this is the problem by replacing the burnt valve with another, but if something other than the valve was causing the problem, then you risk burning up the new valve.
The best thing to do is take it to a tech and have the amp evaluated.