Maybe there was a carbon track you didn't see on the tube socket that gave you the last problem. Any original output tube sockets left? might as well complete the set, replace 'em all.
What's going on with the owner's speaker cab(s) and cable(s), they're all 100% perfect? No shorts, opens, intermittent connections, corroded jacks/plugs? Does he plug the speaker into the main output - not the ext output (if one speaker)?
Another fave spot for failures in these 70's amps: there's an automatic impedance selector switch on the ext. speaker jack. If the contacts on this switch aren't making good contact, the connection to the speaker can go open circuit & boom goes the amp. Similar if both speaker outputs are in use, other switch contact better be working well or else. Sometimes these contacts are corroded from age, or burnt up from handling excess current. If that's the case, tough time finding a new replacement switch; it may be best to wire the output jacks simply in parallel and adding a heavy duty toggle switch to select output impedance.