This is an Ampeg J-12-D Jet from Tim Berry's studio.
It had some problems with heat. The 7591 output tubes were overheating and putting on an internal light show.
This peculiar activity was traced to a loose, non-conducting octal socket pin. It was pin #8, and was causing voltage on the suppression grid to vanish, which in turn caused the plate to short to the cathode, which resulted in the fireworks.
Tightening the pin resolved the problem. Unfortunately the amp managed to eat a new pair of JJ 7591s.
The J-12-D uses the 6BK11 triple triode Compactron tube, which is no longer manufactured, and NOS tubes are hard to find and very expensive. The 6C10 is a reasonable replacement, but it is not exactly the same, and still somewhat hard to find. Ampeg had a habit of using weird tubes in their amp designs (like the 7591 output tubes).
For mysterious reasons, the tremolo only works well if there is a 6BK11 in V1. If you put a 6C10 in V1, the Tremolo is very wimpy. I don't know why this is, but luckily Tim had a good 6BK11, so he has excellent tremolo in this amp.