Ampeg Gemini II/7591 screen voltages
Posted: Sat Jan 12, 2019 6:34 pmI know there are a few amp builders on here...Any Ampeg gurus here or anyone have experience with JJ 7591s?
I have an early Ampeg Gemini II that I am fixing up and it's working great after a recap, new power tubes, and a few other part replacements but I am concerned about voltages, mainly at the screens. It's running about 489V B+ with 119VAC at the plug. The schematic claims with 117VAC at the plug it should have a B+ of 430. However the schematic also labels the transformer secondary as a 720VAC winding, which I am seeing very closely. So I have to assume the voltages shown are totally theoretical and not based on an actual load? Anyone else notice this?
This leads to my concern about screen voltage. Even upping the screen resistor to 1k I am still about 483V on the screens and the JJ Datasheet claims 440V max for screens. This amp had a 470r screen resistor before so the old EHX tubes apparently sat at 480+ V for years and didn't fail from it that I know of. Any thoughts on this? Any resolutions or should I just roll with it? Even dropping the line voltage to 110V it's still over the 440V max. I don't think I can up the screen resistor enough to drop almost 40V.
I have an early Ampeg Gemini II that I am fixing up and it's working great after a recap, new power tubes, and a few other part replacements but I am concerned about voltages, mainly at the screens. It's running about 489V B+ with 119VAC at the plug. The schematic claims with 117VAC at the plug it should have a B+ of 430. However the schematic also labels the transformer secondary as a 720VAC winding, which I am seeing very closely. So I have to assume the voltages shown are totally theoretical and not based on an actual load? Anyone else notice this?
This leads to my concern about screen voltage. Even upping the screen resistor to 1k I am still about 483V on the screens and the JJ Datasheet claims 440V max for screens. This amp had a 470r screen resistor before so the old EHX tubes apparently sat at 480+ V for years and didn't fail from it that I know of. Any thoughts on this? Any resolutions or should I just roll with it? Even dropping the line voltage to 110V it's still over the 440V max. I don't think I can up the screen resistor enough to drop almost 40V.