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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

On Cathode Feedback

Greetings, Friends. Now that the Holidays have begun, it'll be all theory and no bench for a while, and I wanted to deepen my understanding of Feedback. I've been looking at some Single-Ended amp designs over at Single-Ended.com, and noticed several of the designs, especially in the EL84 section, use feedback applied to the (-) end of the power tube's cathode bypass resistor, instead of the preamp tube's cathode.

EL84single-Amp.png


I believe the Elekit 300b amp also features Cathode Feedback.

Morgan Jones doesn't seem to be a fan. Are special OPT windings required? I have a pair of Edcor GXSE15-5K OPTs I'd like to use for a SE EL84 amp, they are meant for guitar amps and only rated down to 40Hz, probably don't have as much bandwidth as the fancy Tango OPTs. Would they work well in an amp with cathode feedback? Should I stick to the more traditional loop to V1's cathode? How does one reckon the resistor value needed in such a feedback loop? Any other ideas?

Thanks!
 
But couldn't you say the output stage and OPT is the part of the amp where feedback is being applied, so some of the deficiencies of the OPT are being addressed?

My understanding is that the drive requirements are increased with CFB because the cathode swings more relative to the grid. The input level would have to be adjusted for the same volume, so potentially the ECC85/6AQ8 will be operating with different characteristics?