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Unstable SRPP




I am having motorboat issues with the SRPP front end of this new amp. This is a TubeCad approved circuit.

It does not seem to be power supply related, It is now fed from a 250v RC dropper off the main filter cap.

I also seems to be speaker load sensitive but that baffles me as you can see the SRPP is not within the feedback loop? On the dummy load it's stable but still motorboats on warm up, perhaps that is also a clue?

Is C2 too large?

Any ideas?

P.S. The output stage is PP 6BQ5 feeding a Hammond 1608 OPT. Feedback is the 8ohm tap/configuration.


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Topic - Unstable SRPP - gusser 16:40:28 06/22/21 (30)
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