Long time ago I breadboarded the 1624 valve and liked the sound. The 807 is famous and I never ended up with a permanent system with this valve. Yet, I like it. And if you have them at hand, use them. Even for guitar amps are amazing.
This is what I used. A 1:4 SUT with the LL7903 driving a PMOS – any of your choice will do. The output would look something on these lines:
Which was very pleasing to my ears as far as I could remember. Now, look at the driving signals:
You can see that the grid will swing about 20V. but the source of the MOSFET will go from nearly -180V to -40V so about 140-150V swing.
I never tried this, but if you’re seeking for lowest distortion profile, you can implement this, not cheap but possible:
The depletion FET (IXCP10M90S in simulation but can be anything you like that can stand 1kV or so) needs a 700V source voltage to allow full swing of anode voltage albeit not loading the anode with the feedback loop. Clever arrangement but expensive, or audiophile-stupid level proof.
If you do this, you would improve the distortion of this stage to this:
And yes down from 1.1% to 0.35% for 4W output. Worth looking into.
Happy Sunday.