As a precursor to this discussion I should mention why I picked the bias point I did for the 12AT7 stage. The output stage requires 100V peak to peak drive, and I knew I had to use a 47K grid resistor on the output stage to stay within the output tube specs when using fixed bias. I wanted the 12AT7 stage to operate in a (visually) linear portion of the grid curves. The bias point picked was 3V cathode, 225V across the tube, and 4 mA quiescent current. Swinging 100V peak to peak from that Q point into a 47K load is running the tube near its limit on the negative side of the signal swing, leaving a mere 1 mA conduction headroom on the negative most excursion before the tube goes into cutoff. But I knew this and picked that bias point anyway because I favored linear swing over anything else.
When adding plate to plate feedback, the plate is loaded down even further at AC, so much so that keeping the bias point the same would reduce drive signal ability down to 80V peak to peak--not enough to drive the output stage to full power. To address this, the 12AT7's bias point would need to be moved. If it were moved up to 7.5mA quiescent current, -2V cathode, and 225V across the tube, that would do it, but it's swinging close to 0V grid now at the positive peak signal excursion, as well as moving the signal swing into a more non linear portion of the grid curves. Also running at that Q point puts the plate's static dissipation at 67% of design center max, so tube life might be shortened a bit as compared to the original bias point I selected.
The feedback calculation with plate to plate feedback is a little more complicated now so I didn't run through that math to determine how much feedback would be applied given a 100K feedback resistor. Very rough estimating says maybe 10 dB feedback would be applied.
So...I don't know but it seems the 12AT7 would not be the best choice in this amp for Schade style feedback. But, not having built it, I'm just theorizing.
I can see how this type of feedback would be useful with lesser requirements on the output stage, such as with EL84 or 7591 type output stages, and/or with a stouter frontend tube.
All that being said, it's still pretty easy to try it, and look only at small signal analysis. It would still be instructive to learn how this type of feedback affects HF performance.
When adding plate to plate feedback, the plate is loaded down even further at AC, so much so that keeping the bias point the same would reduce drive signal ability down to 80V peak to peak--not enough to drive the output stage to full power. To address this, the 12AT7's bias point would need to be moved. If it were moved up to 7.5mA quiescent current, -2V cathode, and 225V across the tube, that would do it, but it's swinging close to 0V grid now at the positive peak signal excursion, as well as moving the signal swing into a more non linear portion of the grid curves. Also running at that Q point puts the plate's static dissipation at 67% of design center max, so tube life might be shortened a bit as compared to the original bias point I selected.
The feedback calculation with plate to plate feedback is a little more complicated now so I didn't run through that math to determine how much feedback would be applied given a 100K feedback resistor. Very rough estimating says maybe 10 dB feedback would be applied.
So...I don't know but it seems the 12AT7 would not be the best choice in this amp for Schade style feedback. But, not having built it, I'm just theorizing.
I can see how this type of feedback would be useful with lesser requirements on the output stage, such as with EL84 or 7591 type output stages, and/or with a stouter frontend tube.
All that being said, it's still pretty easy to try it, and look only at small signal analysis. It would still be instructive to learn how this type of feedback affects HF performance.
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