The drawing of power supply has an error: 400V rectified and 400V stabilized are connected by mistake.
As you may see, output stage has local parallel feedback around output tubes. It requires driver with stable output resistance.
Thank you Jonny!
All disciplines we learned in TIASUR were taught around optimization backbone. I feel no good when I see some spoilers that are not functional. One man said, "I like you amps, they look like military Jeep, nothing odd". I hope, I have no overlooked spoilers in my designs.
Since tubes in driver are directly coupled I used DC from tail of driver LTP as feedback to screen grid of the 1'st tube to stabilize working points for all of them. The amp has nested feedback: global feedback goes from 8 Ohm secondary to cathode of the 1'st input pentode.
Optocouplers in screen grids of output tubes sense sharp grow of current before clipping. One of them fires up bright red LED, another one is used as an input signal attenuator, so when you see flashing red light on front panel that means compression instead of clipping.
