Welcome to The Board, and sorry to hear your troubles.
How's the other channels, volumes OK there? Are you using the FX Loop? Hard Bypass it to help troubleshoot.
How's your cables? Have you tried a different guitar? How old are the tubes? Are you going straight into the amp, or is there pedals out front?
There are a lot of variables so try the simple things first like cables and dirty jacks. Plug the guitar straight into the amp, FX loop Hard Bypassed. Try to tell us how you are running the amp (bold/spongy, tube recto/SS diode etc.) to help troubleshoot.
Dom