[AMPS] Zener screen supplies

[Steve Thompson]

A different aspect to consider - what happens when there is anode-screen
flashover. I inherited a PSU (with 10x33V/5W zener screen supply) from a
friend. A dud tube caused a flashover and lots of damage. After some
investigation, I came to the conclusion that 375V VDR clamps on the screen
didn't protect anything, as their clamping voltage at 10s Amps is around
600V+. The arc current went through the screen meter and shunt and down the
zeners, taking some of them and permanently damaging the meter and shunt.
Yes, there was a suitable 50 ohm resistor in the plate supply.

One advantage of an 'electronic' screen supply is that you can build in a
current sink limit and make the VDRs work for a living.

Steve


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