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One of the problems with this design

-Is the direct-coupled fully differential circuit.

FWIW Victor had a set of our amps in front of him and were the inspiration for the VK-60, which was the first amplifier BAT made after he and Steven Bednarski (a customer of ours) went into business together.

Unbeknownst to Victor (who to his credit did not copy our circuit although he did use the Circlotron) some years earlier (about 1985) we had tried and rejected the driver circuit he used in the VK-60 (this driver topology was also used by Tenor a few years later). The problem with it is accurately described in JKT's post next to this one (and I believe his explanation for your problem is also accurate- you need matched tubes in the driver for this to really work right). A balancing pot could be used in the cathode circuit of the first gain stage; the problem we found is the pot causes as many problems as it solves. We never got this circuit to sound right and by all accounts I have ever heard neither did Victor (nor Tenor for that matter).

IMO, you are better off having an additional set of coupling caps between the first and second stages to avoid the offsets (and distortions) that otherwise occur. Easier said than done of course- that would require a bit of doing!




Edits: 02/10/17 02/10/17

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