Some brands have a tendency to die or go microphonic very quickly - some of the Chinese valves are more susceptible. I've had the occasional brand new Sovtek and even NOS Philips be microphonic right out of the box. When in doubt, slap some JJs or another reliable brand in there.
Gearhead wrote
Better get the amp checked to be safe, such overloading would be easy to pick up on.
+1. Valves are very durable, but an amp can be faulty enough to still work, but go through tubes faster than it should.
Which valve is it BTW? Preamp or power amp?
EDIT: Just saw your new post. Power valves should always be changed in pairs and you should use matched sets, otherwise its more than likely that one will die much quicker than the other(s).
