First, please clarify whether you are talking about a mechanical hum coming from the unit itself or a hum that is audible on your speakers. If it's a mechanical hum, I agree with the above two responders. You could try tightening down on screws that hold the transformer laminations together, if the transformer is not potted. Or you could isolate the transformer from the chassis on rubber grommets. Neither is guaranteed to work but might work.
If it's a hum on the audio signal, then the issue is mostly likely due to equipment grounding, not a tube, although it's also possible you have a dirty tube socket. All that said, if you've never changed the tubes since the 90s, it's probably a good idea to do that too.
If it's a hum on the audio signal, then the issue is mostly likely due to equipment grounding, not a tube, although it's also possible you have a dirty tube socket. All that said, if you've never changed the tubes since the 90s, it's probably a good idea to do that too.