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That being said, I'd swap the tubes out first to rule them out.
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Second, check out the preamp tubes (Carefully! With the amp on/warmed up) take a wooden chopstick lightly tap each tube starting with V1, if you can hear noise or get feedback on any you know that’s the culprit.
Next I would check out the output tubes and verify the bias voltage is within spec and that the 6v6’s are balanced correctly..
If all that checks out fine, you’re going to need to get into a more specific and difficult series of tests.. but a couple other simple things you can do is just visually look over the inside of the chassis.. often times with these older amps you can see burned, charred, or melted components (old CC resistors in particular) Also remember to take a look at the speaker and make sure that you haven’t blown it!