> if you're talking about using a cathode follower
Well, no. Cathode followers are kind of "too-new" for a classic gitar-amp. And as you say, watch the heater insulation.
Use a tube with a Mu of 10 or 20 with a plate winding of 10K-20K. FLowing plate current through the winding is most efficient, but DC-rated iron is harder to find, so an R-C coupled deal may be easiest.
A plate-loaded triode in 10K:600 can live on around 3mA-5mA, much less than any 600:600 output like a simple CF or Dave's WCF. Even 3mA is more than the whole existing circuit, but less than a CF/WCF and 600:600 iron needs.
> the output impedance of the Alembic is high enough to start causing common-mode distortion in the opamp.
Quite true at the point-oh distortion level. I'm not sure I would care in a guitar amp. Or at least: if it was convenient, I'd try it and see rather than reject it on theoretical grounds.
> One possibility might be the old LM310 buffer amp. ...into a 1:1 transformer
IIRC, not a heap of output current. Sure it will drive 10K loads, but it may be less happy seeing a "600 ohm" winding, even un-loaded. Of course if I had a 310 and a 600:600 on hand, I'd give it a whirl.
> The curves aren't curved.
Ah, 12AX7 has very high Mu and thus the cathode curves are nearly straight for reasonable currents. Go to extremes with low-Mu tubes and they curve more.
It does seem like there should be a rough-cut formula like Rl + Rk*Mu, but I can't work it today.
> The white cathode can be made with any dual triodes yes?
As you and Dave agree: you "can" build one with any twin triode (very-dissimilar dual triodes would be an odd pick). But in many many cases, a WCF is not the best use of a twin triode.
Oh, and tubes with very low Mu, like 6080 Mu=2, make crummy WCFs. And any time you need power output, high-Mu tubes are poor choices. Stay in the Mu=5-30 range, except some of the supercharged late TV tuner tubes do make lovely WCFs.
