Well, that's good to know! Somehow I'd gotten the impression that the old IRF840 or similar old MOSFETs would at least theoretically cause some likelihood of distortion rising with frequency, getting to be a (admittedly minor) problem as you up to 15kHz or so. But if that's not a concern even with a 12AX7, then there you go. Use any old MOSFET with a high enough Vds and there you go. Good thing, too. I've collected a bunch of IRF820s and IRF840s from surplus sales. I was using them for power supplies, but I guess they're good for source followers too.
