Problems with blown valve and fuses

Hi All,
I recently sold an HD-130-210. It was in good condition, working very well with a new OT from Mojo (a Heybower I believe), 4 new power valves and some replaced filter caps. Unfortunately the new owner has experienced problems - a 'blown' power valve and 3 instances of blown fuses. I realize that it is now his problem rather than mine but I do feel curious as to what could go wrong with such a reliable and quiet amp. Any ideas or suggestions? Any particular signs to look for?
thanks,
Sam.

Since it takes out only one tube and the fuse, I'm betting bad tube and/or insufficient bias voltage to control runaway and the one tube was just weaker than the oters. Those amps are bad about eating tubes anyway so if you have one that was cranked bias wise and one tube is a little softer than the oters it coud do it. Bad tubes socket will allow control grid voltage to drop on ONE tubes. Bad resistor on THAT socket wil do it also. You need to know if it lost control voaltage full time or intermittantly. Intermittant and I vote for loose socket. They ARE 30 yrs old.