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RE: One of the problems with this design

Posted by Eric Chan on February 11, 2017 at 08:47:58:
Hi Ralph and JKT,

>> who to his credit did not copy our circuit although he did use the Circlotron) some years earlier

I think that all BAT 6c33c amp use typical plate load OPT coupled rather than Circlotron circuitry. and VK75se is completely no global negative feedback from 1st stage to final(To my understanding, Circlotron utilize output tube cathode to connect opt, correct me if mistaken)

A quick look on VK75se, there are two differential stages, 1st to 2nd are direct couple, then 2nd stage(paralleling 3 section 6SN7) cap couple to 6c33c grid, 6c33c plate load to OPT, quite a typical circuit that I always find similar schematic in MJ magazines.

As far as I know for the direct couple, the head room allow the 1st stage to swing on the 2nd stage grid is limited on the bias voltage difference applied on the 2nd stage grid to cathode. For the headroom allowed to swing on 2nd stage grid(both grids), the conservative 0.7 of amplification factor of 2nd stage x grid swing headroom seem more than enough to get second stage to produce 80-90 V p-p to drive the 6c33c into max power OP.

for sure I will give it a check to see the grid voltage of both 2nd stage grid voltages being equal.

One thing I do not understand is why you would say the balance pot used for differential stage cathode is problematic

Also if there is slight difference in tube match for differential stage, would it potentially produce unwanted IM distortion significantly?