Our Tech Tip from a couple of days ago dealt with troubleshooting a tube amplifier that developed noise issues. As we said in that Tip, the first thing to try is to turn all gain, channel volume, reverb level and master volume controls all the way off. If the noise persists, the problem is likely in the output tubes.
However, if the noise goes away when a certain gain or volume control is turned off, then the problem lies in a stage before that control in the circuit and the noise is likely in a preamp tube. One technique for finding the problem tube is to get one known-good preamp tube and substitute it for each preamp tube in turn until the bad tube is located.
If you’ve had your amp for a long time and used it heavily, it would probably be a good idea to replace all your preamp tubes. Save the old ones (at least the known good ones) to serve as spares in the event of a problem with the new tubes.