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I agree with yogiboy. Since it's a custom made amp, the designer might be using a fuse or resistor that might have popped when the spark hit. If so, that would need to be replaced. Normally if something like that happens, the amp wouldn't work on that channel, but it would depend on the topology of the amp design and how it's implemented.
I would look for a blown EL34 tube screen resistor. Should be easy to see.
Follow pin 1 on each right channel EL34 tube for the resistor for each tube.
Bias is way off.
Possibly a bad driver tube.
How many hours do you have on the power tubes? Average life, I believe, is about 2000 hours.