EC8010 said:I saw a circuit similar to this recently on AA that described itself as PP. You seem to have a conventional input stage with an output valve buffered by a source follower. The op-amp and MOSFET are a bias servo(?) to balance the currents in the output transformer and allow use of a PP transformer? It's hard to tell exactly what's going on without values.
EC8010 said:Ah, so you have a concertina phase splitter. I wondered about that earlier, but it could just have been a feedback path, depending on values. Have you considered the balance of the capacitive loading on each output?
kenpeter said:The FET in the lower "shoulder" is not running as anti-current
mirror slaved (in reverse) to the tube "shoulder". It is servo'd
to balance DC well enough, but the AC gate drive drive through
the op-amp is a unity gain (because of your NFB capacitor.)
Gate tracks the cathode of your Concertina splitter, but not the
anti-current of the other shoulder.
Nothing here forces your FET to have a similar voltage gain to
the tube, nor any sort of anti-tube SEPP curvature... The way
you got it driven is sort of Concertina driven Pentode-ish FET.
Which would probably cancel a lot of SE in the real Pentode of
the other shoulder...
I do like your A2 superdiven grid for the output tube...
I am uncertain about automatic baising. I don't think
there is anything wrong with it, just don't understand
your circuit yet. I probably need to stare at it for awhile...
This might be a fun one to tie grid #1 to the cathode,
and drive screen grid #2, just to see what happens...
No sense leaving any good rules unbroken!
smoking-amp said:Looks like C207 in Version 2.0 is just 10 pF for HF stability. So the R218 current pickoff is allowing the Op Amp to program the Mosfet current linear to its + input.
I believe Anatoliy is not trying to program accurate anti-triode current here (in the Mosfet), but just lighten the plate loading on the triode side. So resistors are used to set the VCCS gain here, which could even be set to greater than -1*triode AC current. This would allow the triode to operate at more nearly constant current with more constant Mu as a result.
I think the design goal here is more along "why not do SET better than SET can" instead of just equaling SET. Clearly the triode characteristic will be modified some, even with nominal -1 current gain, since the non-linearities of the triode will be altered. Whether this will still sound like a SET, or some sort of triode/pentode(Mosfet) P-P remains to be seen.
smoking-amp said:What! Not putting the big tubes in front with a see thru acrylic panel?
I have decided on a name for my emulated "SET" amplifier now:
Hodja Nasreddin's Magic Tubeless SET
Since the SET sound will be so convincing, no one will believe that there are really no tubes in it. So the emulation tubes will be partially hidden, but not so well that someone cannot discover their "hidden" presence, thus "confirming" there suspicion that it is really a tube design in disguise. Only fitting for an amplifier with many levels of revelation. (Pentode emulation of triode, and Mosfet P-P, not SE, breaks all the rules!)