I just got JJ 7591 S power tubes and installed them in my (new to me) '65 Ampeg Gemini I. It was running Sovtek 7591XYZs--I had assumed they were 7591As. When I looked up the 7591XYZ, I saw it uses a higher bias. I know on a fixed bias amp, a cathode resistor would (should) have been added to run the 7591XYZs, but the Gemini I is cathode biased. When I had it open to work on the reverb, I was getting voltages off the power tube sockets in the proper ranges (according to the schematic) with the 7591XYZs installed. I'm wishing I took pics now, but I don't remember seeing any new resistors, but I was more focused on the reverb circuit at the time. If it helps, the new JJs were glowing a nice yellow orange with no sign of red, so it looked like everything was ok. Thanks!