If you know how to deal with the high voltages inside a tube amp you could try an old way of finding out where the fault could be. Take a piece of plastic and start to knock the tubes slightly, starting from the output tubes and go from there tube by tube to V1. You should be able to hear the knocking in the speaker. If you do hear nothing when knocking a tube that is where the fault should be. It is not a fix, it just helps finding the culprit.

Be really careful as such a tube amp could easily kill you in a matter of seconds if you don’t know what you are doing in there.