What are you trying to do?


TR1 looks like a current limiter for the FET. Why are you doing that?

The source voltage looks like 425 volts, the series resistor 10k 2W,
the shunt regulator threshold 340-350 volts.

that's about 75 volts differential, so the 10k would have about 7.5 mA
of current. If the screen loaded more than 7mA it would fall out of
regulation.

The FET dissipation would be 2.5 watts assuming no negative screen
current.

What holds the screen to ground when the overload opens?

Why is the VDR on the screen side of the screen current meter?

Maximum screen dissipation would be around 21 mA when the screen was at
about 200 volts. Maximum screen current about 40mA.

73 Tom