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I've got a Classic 50-410. The Classic is a fixed bias amp and cannot be adjusted (unless you modify the rig). You may have a bad tube or a bad cap or diode in the power supply. I run the JJ EL84 (6BQ5). If I were you, start swapping out the EL84s 1 at a time (like Thumbsmagee said) with your old ones (if you still got them) to see if you can pin down which tube maybe bad. There also may be a bad 12AX7 in the Preamp section (uses 3ea) as well, so don't rule that out. I doubt you have any O.T. damage.