Orange OR80 HT Fuse

[Ted]

I have here an Orange OR80 which keeps on coming back to me with a blown HT fuse.
One of the output valves is clearly going into thermal runaway.
I've replaced the bias electrolytics and all the main power electrolytics.
I can see no sign of parasitics, and it runs at full power (into a dummy load) on the bench for ever.
The first couple of times it came back, the overheated (new, Russian) EL34 was sufficiently damaged that it needed to be replaced (bias hopelessly unbalanced).
This time I put in a pair of NOS 6CA7s and although it came back with the fuse blown, I replaced the fuse and the bias was fine.
The fact that I have been totally unable to reproduce this fault makes me think that it may be something coming loose from vibration - the owner says that it always failed after he's thrashed it through a 4 x 12 for a while.
I've chopsticked it thoroughly while watching the scope at full power. I've thumped it hard. It refuses to fail.

I'm considering the possibility that it may be something in the rehearsal room - bad power? bad cab? bad speaker cable? None of these seem particularly credible.
Any ideas?

[Enzo]

Have you SEEN the tube overheat? What if the amp is falling into oscillation and the cranking tube burns itself up?

A dummy load is not a speaker.

Have you invited the customer to bring in the rest of the rig, I mean his speaker cab and the cord he uses? FX pedals and stuff won't hurt it, but bad cords sure can.

Destroys tubes? Always same socket or moves around?

[Ted]

Have you SEEN the tube overheat? What if the amp is falling into oscillation and the cranking tube burns itself up?

A dummy load is not a speaker.

Have you invited the customer to bring in the rest of the rig, I mean his speaker cab and the cord he uses? FX pedals and stuff won't hurt it, but bad cords sure can.

Destroys tubes? Always same socket or moves around?

Same position every time. I'm not in a position to let him crank 80W of tube goodness in my workshop - I was actually planning to take the head down to his rehearsal room and watch him break it.

[loudthud]

Does the owner know the importance of matching the load impedance? He might have one of those Marshall cabinets with a 4 and 16 Ohm jack on the back and just plug it into the one that sounds loudest.