View Full Version : Ampeg Gemini I - going from Sovtek 7591XYZ to JJ 7591 S

Tres Mellow
12-02-2009, 08:24 PM
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.

Tres Mellow
12-02-2009, 08:35 PM
I should add that this amp has had a cap job and all the electrolytics in the power stage have been replaced.

12-02-2009, 09:56 PM
A lot of people dropped those Sovtek's (which aren't true 7591's from my understanding) in to old cathode bias ampegs. I just had my reverberocket redone with the JJ 7591's and they work great. Oddly enough, after mine got a cap job the reverb now cuts out, so I too am focused on the reverb circuit.

Tres Mellow
12-03-2009, 06:16 AM
Thanks. I'm thinking someone just dropped them in, because the voltages read correctly according to the schematic. It sounds even sweeter with the 7591s, but I haven't been able to crank it up yet to see how they might change the breakup. It sounds like the Sovteks are basically 6L6s--is that right?

As for your reverb circuit, clean the input and outputs on your tank. That tank needs a grounded input and output and it doesn't take much corrosion to mess that up, plus the cables had to be disconnected to do the cap work. One other place to look is I had some touchy wiring on the terminal strip where the switch cables attach and then feed the signal to the 12ax7 reverb recovery. I had to re-solder that entire strip, particularly the resistor to ground. I also had a weak solder joint where the big 5watt resistor feeds pin 6 of the 6cg7 reverb driver tube. I don't know what would be the corresponding parts in a Reverberocket II.