Since I am very satisfied with 12AX7 gain stage supported by MOSFET, I was curious how to reuse this design elsewhere.
Here it is, a push-pull amplifier with some non standard feedback mechanism. I am tired of millions schematics with low quality feedbacks, like UltraLinear or even global feedback (where signal from output of amplifier comes back again to the front..) Those mechanisms are old (outdated). Signal which was at the output of the amplifier, and comes again to his input, has often some phase shifts.
This can start awful oscillation, distortion profile is not always improved by these old fashioned feedbacks.
If you carefully use semiconductors in tube amp, end result can be much better. MOSFET does two things here; gives very high impedance to input tube, and low drive impedance (both not possible with anode resistors!) Gains like these are impossible with anode resistors, and will not work here! (..and cathode unbypassed ECC803 sounds lovely too) It´s possible to have >85 gain for ECC803 (with MOSFET), and >95 on 12AX7WB.
Because gain is so high, we can expect the impedance provided by MOSFET to preamp tubes is really huge = ultra low distortion!
Two stage PP amplifier with this feedback system is possible. Datasheets for 6L6GC PP mentions "70V Peak AF grid-to-grid voltage" for full output.
Even if you used small gain input tubes, just wire input transformes in 1:2. I recommend tweak the fb ratio, 10-40% must be enough in all cases. Due to high sensitivity of tetrodes, 20% or less can be enough. The grid choke is optional, improves DC bias stability, and does change the sound too. Since choke impedance rises with frequency; it will make feedback stronger at treble frequencies. In bass range, its impedance is small, pulling Schade feedback ratio closer to minimum %. (therefore damping factor will be smaller too).
If my expectations are right, amplifier with choke bias, should have beefier bass and negligible distortion at treble. Treble and mids is where ears are most sensitive to distortion. Smaller DF will make bass feel strong. (virtually making your woofers like higher Qts..)
Input sensitivity is about 0,8Vrms for max power out. (85x input gain+40%FB)
Sice this is voltage feedback taken directly from primary windings; higher impedance transformer will improve distortion profile. (then you can use less feedback ratio %) EL34 or 12E1 (CV345) are even better here.