You are probably right thinking it's bad 7591's. They run very hot and go bad easily. Do you notice a blue color inside the tubes? This would indicate a gassy tube which draws excessive current and distorts.
If you have a meter measure grid #1 voltage in relation to cathode. The 7591 has pin #6 as grid 1 and pin #5 is cathode.. Grid #1 should be 15 to 20 volts negative.
These tubes are expensive and I know of no substitutes.
Norm
: I lately found the volume of one of the channel is lower than the other and the distortion of that channel is very large in my Fisher amp. I also found the temperature of the output transformer and capacity of one of the channel is higher than that of the other channel. There are 4 7591 and 6 12AX7 tubes in my amp. I am quite sure all 12AX7 are ok. but the conditions of those 7591 tubes are not stable - effect from changing tubes of the two channels is sometimes significant, sometimes not. I’ve never set the bias and the ac balance because I don’t have the user instruction. So, what can I do to solve the problem? Please help..