If your tube amp starts developing noise issues, the first things to suspect are the tubes. When troubleshooting, here’s the first thing to try:
Turn all gain, channel volume, reverb level and master volume controls all the way off.
If the amp still makes noise, the problem is very likely with the output tubes. Replace the existing tubes with a full set of matched output tubes. If you use the same brand and part number as the original tubes, there should be no need for re-biasing. On models with rectifier tubes, if the new output tubes don’t cure the problem, try replacing the rectifier tube.