[Arran]

This is really in the wrong forum, it should be in the electronic restoration section. In any event I would check the other resistors in the set and see if any have drifted in value. I would also check the tubes, although I wouldn't expect any problems there. I would also check the polarity of the battery leads if they don't have plugs, I would also check the condition of the battery leads near where they go into the plugs.
Regards
Arran