The first thing to do is swap the interconnect cables at the inputs of the power amps, left for right.
If the problem moves its either the cables or the preamp.
If the preamp stays put is either the amps, speaker cables or speakers.
Audio valve preamp
Does anyone know a tech that has knowledge of this preamp ,it has a dual chassis, the sound on the right channel is weak ,replaced tubes ,I had someone look at it ,put found no problem. There is no schematic for this preamp. I’m on Long Island I would be thankful if anyone can help me find a good tech...