> 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.
.....and you can win the lotto by buying only one ticket!
Think about the problem a little bit.
The screen has to rise to probably a thousand volts or more to give
any transient protection time and voltage to act.
You better be very careful what you are doing, and not be foolish
enough to assume current limiting will protect anything.
On the other hand, you could also protect a zener supply just as
easily. It is not an advantage at all to use a more complex circuit.
73, Tom W8JI
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