what would cause this damage?

[Milkmansound]

I had an amp come in - output is wired like an 85W Twin. 6L6GCx4 fixed bias

On both sides of the output transformer primary, the plate wires coupling the tube sockets (2 on each side) had arched over to the chassis. All 4 power tubes sustained some damage. Everything else was fine.

My guess was that the speaker leads had come loose - maybe just on one side, and sent a huge reflection back to the plates of the tubes. However, the client said that they looked fine and had not come loose.

Another possibility is power surge or spike

Its very strange - I have never seen this kind of damage before on both sides.

thoughts?

I replaced the damaged wires, put in a new quad of tubes and the amp was back to normal

[vibratoking]

That would be my guess as well. Probably no load attached for whatever reason, or he tried to play it at 100,000 ft.

[Super_Reverb]

Some guys add a fairly large (by speaker impedance standards) across the output transformed to protect amp in the event that it is played with no speaker attached. A 220 Ohm power resistor soldered across speaker outs won't shunt significant current from speaker.

rob