> 2- I would direct couple the diff-amp to the cathode followers.
> Using the bootstrapped input configuration of the cathode followers reduces the load on the diff-amp plates and that is good, but I would want to look at (or rather, listen to) that configuration and compare it to other non-bootstapped cathode follower input topologies.
Looks good to me, except with that whopping B+ there is some danger of heater-cathode breakdown. And overload will be different, probably nicer with the R-C coupling.
> 3- I wouldn't use 12AX7 for the cathode followers (use 12AT7 or 12AU7).
For small load and cathode current, 12AX7 gives higher Gm and lower output Z than AU or AT. Also 12AX7 is in-stock everywhere there are Fender guitar amps. But for even 10K loading, I think the 12AX7 will run out of poop to drive the load.
> 5- With 1M grid resistors you do not need 10 mfd coupling caps.
> 6- Use 63v rated coupling caps on the input not 400+ volt caps.
Looks like someone had an excess of 10uFd 415V caps.
> The total gain for this circuit will be appox +40 dB
Only get that high if you bootstrapped the 270K resistors.
> will not be time/age stable.
Will vary a dB or two. Good enough for rock and roll, where the musicians vary +/-9dB depending on mood, drugs, and line-voltage.
> The gain is also static.
Low, but if you need less you gotta get a pad.
> The HF response probably won't be all that great either.
There is no Miller-effect and the 12AX7's plate resistance will be much less than that 270K plate load resistor. Depending a LOT on layout (and how physically large those coupling caps are) it could be over 50KHz or only 10KHz. Still good enough for rock and roll.
> The gain is too low for certain mics, distortion will be fairly high, output impedance is fairly high
Zout is around 1K-2K. Gain is essentially 80 or 37dB. I doubt distortion is an issue (if load impedance is high): If output voltage is much above 1V to 10V your next stage will overload, so you'd need an input pad. Working with 395V B+ and push-pull, THD will be far below 1%. Good enough for rock and roll.
> noise performance running directly into the grid of a triode.
Input noise will be several microvolts. Compared to 0.2uV for a dynamic mike in a silent room, this sucks. But my AKG 414s output 1uV of noise, equivalent to a pretty quiet 14dB SPL. With those hot mikes and 3uV of tube noise we are at 24dB SPL, which isn't that bad, lower than many rooms, and insignificant in the face of 104dB SPL guitar amps with 40dB SPL hum/hiss. Good enough for rock and roll.
If I remember the origin of this plan: it WAS a rock-band tool, using available parts in former-USSR lands, and was said to be very successful in that use. That may be a happy accident of band, mikes, and plan, but isn't an uncommon need.