I have built a valve/tube tester. To provide the various screen and plate voltages I'm doing this:
The required voltage is selected by a rotary switch down the stack of zeners. The momentary button is pressed to test the tube. The MOSFET is a 500V device that can handle more than 7 amps. It has protection zeners built in but I've added an external one to the gate anyway. Datasheet here R2 represents the ESR of my power supply. At 150mA its already sagged to 260v but thats fine; the tubes I'm testing draw less than that.
It works as expected. There are two instances, one for screen volts, the other for plate. But every so often I find the MOSFET has failed in one or the other, shorted source to drain.
There is sometimes a 68nF capacitor 'downstream' (depends on configuration), the MOSFETS are mounted on an aluminium heatsink but never get even warm to touch (they only run for a few seconds at a time) and the device itself seems to be well protected and rugged to boot.
What have I missed? Whats the most likely way I'm destroying these MOSFETS and what can I do to mitigate this?