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Voltage sag problems in tube amp (SS rectifier)
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trobbins:
Yes the 100R is the AC feedback signal being applied to half the PI stage, and so adjusting the 100R, or the feedback resistance, or the OT tap, will adjust the total feedback level.  Just by checking the speaker output voltage both with and without feedback (for a lowish signal level), you can calculate the -dB feedback level that is inherent - I'd suggest you not exceed 6dB feedback, and even then without suitable measurement tools you could still end up with quirks that you may incorrectly attribute to other aspects of amp operation.

I suggest you confirm the common cathode bias voltage and the idle anode voltages for that PI, and confirm that both plates generally hit the rails and cut-off symmetrically (as an initial setup) - although without a scope of some kind (even a soundcard with scope software and a bit of input signal management) you are starting to get in to deep water for your capabilities to modify circuitry or use proven circuitry but with different key parts.  Unless you can confirm all sections of the amp are operating ok, your decisions to modify any part of the amp could easily be misled by your assumptions.

I also see that you have modified the input circuit - you may want to check that 470pF, as it could make some significant change to your guitar pickup/pot setting response.
ELS122:
yes i calculated that i should use a 1.8nF capacitor to have the same effect. but i already found out that the problem is the power supply, for testing purposes i connected the output of the pi on the push pull el84 amp to the grids of the 6p3s's and it worked amazing. it couldnt push the 6p3s's very hard tho since the pi was connected to 340V while the ouput plate voltage was 480V. havent drawn any load lines but it seems pretty clear that the reason why i wasnt getting decent output power was the much higher b+ and cause of that it needed more drive voltage to utilize most out of the tubes. while testing this i just connected the feedback resistor to ground to bias the pi. and it was suprisingly stable with no NFBL.
anyway, i think using zeener diodes to regulate the voltage to the preamp stages will fix it mostly
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