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In Reply to: RE: It can, but posted by Eric Chan on February 10, 2017 at 03:24:38
-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 02/10/17Follow Ups:
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?
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)
The VK-60 uses an output transformer in a Circlotron output circuit. It is not a 'typical plate load OPT'.
Let me give an example to clarify what I was saying. Suppose the driver tube (2nd stage) required a grid bias voltage of -6V at its operating point. As a first approximation this would limit the incoming signal to 6V peak to peak. If we're talking about a diff. pair, as an example let's say that the grids are offset by 3V. Then this condition would cut that 6V envelope in half and incoming signals greater than 3V p-p would distort. This is what I meant when I wrote that it would reduce headroom.
An imbalance in plate currents in the 1st stage diff. amp will result in different plate voltages (depending on the DCR of the plate loads) for the pair. Since the 1st and 2nd stages are direct coupled this difference will be expressed on the grids of the second stage.
If this proves to be the problem you will need to replace the input tube with one that has matched sections as measured in circuit.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
I have fully differential mono-block power amps where the 1st and 2nd stages are direct coupled. Voltage offset in the output of the 1st stage is avoided by the use of a CT plate choke and a reasonably balanced 6SN7 (necessary for the choke) for the first stage. Low DCR of the choke and small imbalance current take care of that.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
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