XTC has a hum

[smdb]

My 20th XTC has started to hum fairly loud when I turn the amp on from standby. You cannot hear the hum when the amp is on the standby position. It's fairly loud and bums me out!! Any suggestions on what it could be? I have tried different outlets and power chords, still hums.

 

[Ventura]

Bad power tube(s) or preamp glass. Hum is more than likely powertube though; after that, it's OT.

Mo

 

[smdb]

Bad tube! Thanks!