Thanks Kay... I don't how to answer your question,but, I only know how to follow schematic. Yes, I only needs a fix voltage. Presently, It's operating at 259@2.3mA and sounding good!?? Perhaps more current could be better? However, by doing so, the plate voltage drops further.
Perhaps an LR8? Regulating high voltage (up to 450V) has become as easy as a LM317!
Available at Farnell https://sg.element14.com/microchip/lr8k4-g/linear-volt-reg-0-02a-440v-to/dp/2448524
Available at Farnell https://sg.element14.com/microchip/lr8k4-g/linear-volt-reg-0-02a-440v-to/dp/2448524
I dont know how precise the voltage has to be, because the EL34 is less sensitive to varying G2 voltage than for instance sweep tubes.
Those mosfet followers are pretty much rock solid, the only drift you will get in those kind of setups is due to the temperature coefficient of the Zener diodes. And some very minor drift due to shifting threshold voltage of the mosfet
Mosfets make great source followers due to their high transconductance, in general operation the mosfet VGS is biased slightly above the treshold voltage, this threshold voltage will vary a few 100 mV due to temperature effects, but nothing to be concerned about.
However for this application, i don't think there is much to be gained by switching from a zener stack to a mosfet source follower fed by a zener stack.
Another option is to use a 0B2 and 0A2 in series to stabilise the Vg2 for the driver, and force a constant current of about 20mA through the shunt regulator with a 10M45S regulator IC. But this means turning your amplifier into a Christmas tree.
Please keep in mind that most LR8 are available in TO92, those packages will heat up somewhat in this position, and cause temperature drift in the regulator IC.
You can use the LR8 to provide base current for a pass transistor, like a TIP50 to provide the screen grid voltage. By utilizing a TIP50 you can remove most of the heat from the LR8 and get better stability.