Have you tried reversing the tubes, channel to channel and see if the sound follows the tubes? If not, do so first. If that fails my guess would be a cap but you're going to need a techie to fix it.
6 responses Add your response
It absolutely sounds like a failing tube. Either do as Newbee has suggested or drop in replacements if you have them. It's always a good idea with a tube amp to have a set of backup output tubes around, since tubes always seem to go bad on weekends when you can't do much about it! Anyway don't freak out, it's all in the care and feeding of a tube amp!
Check tube sockets! The GTA SE-40 did not have the best tube sockets. With the amp cold, take out the tubes for the channel making the noise one by one and reinsert, as to not change the placement or order of the tubes.
Frequently it's the 6SN7GT input tubes that are the culprits. Sometimes tubes will "walk" slightly within the sockets as they heat up and expand, then cool and contract.