Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Banjan73 on May 29, 2026, 02:41:59 pm
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Hello!
Refering to my Ampeg SB-12 thread, i have done some measurements with multimeter and scope.
Well. Truth is I am no scope expert, and I have one question:
What is going on here?
I have measured the PI (cathodyne). Cathode side seems normal, but on the anode side, I have this strange graph.
Attached is pictures of both of them + combined.
Anyone keen to explain whats happening with that strange one?
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The bottom two traces are frequency doubling - You have bias shift of the cathodyne when overdriven - Add a grid stopper to the cathodyne, 270KΩ and up seems to work well, anything over 1MΩ will shave off more highs than you probably want. I typically use 330K-470KΩ for a stopper.
--Pete
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Thanks!
Easy fix. I will do that.
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DL helped me with the same problem, before n after shots;
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Nice!
I'll see into that tomorrow!
(Btw... AI came up with the same conclusion. But much of that is actually based on you guys)..