late edit:
The differential parallel feed also keeps DC out of the OT, a significant advantage for a toroidal OT. But the two parafeed CCS's make for very poor power efficiency. I would not use this for an output stage.
The differential parallel feed also keeps DC out of the OT, a significant advantage for a toroidal OT. But the two parafeed CCS's make for very poor power efficiency. I would not use this for an output stage.
I see another issue with the differential parallel feed circuit. The tube with higher gm is allowed to swing higher voltage into an effectively higher Zprimary. The higher plate swing is what lowers its current (triode plate feedback) to match the other tube. The high gm tube will therefore saturate easily. And the low gm tube has an effectively lower load Z, which may cause it to distort more. The conventional differential stage alters the drive signal amplitudes to the two tubes via the cathode tail voltage swing to equalize currents (and so helps sum the gms to a more constant level). Looks to me like the conventional CCS tailed amp is still the best.
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