Eimac's recommendation for fault protection
http://www.cpii.com/docs/library/9/AB17%20Fault%20Protection.pdf
suggests fragile tubes (I guess 8877?) need the energy in a flashover to be
limited to 4 Joules. Eimac's figure of 50V across the arc and Jim's 2ms for
the HT fuse to open with a 3KV PSU and 50 ohm glitch resistor gives 60A
fault current and 50V*60A*2ms = 6J, so a bit too much. However the 3-500Z
being discussed in this thread is very probably more robust than an 8877.
Jim: how did you measure the 2ms fuse opening time? I can't see it is all
that easy with a multi-kilovolt supply dumping many tens of amps through
the fuse and glitch resitor!
I wonder if any US readers use a crowbar circuit as described in the Eimac
paper to really cut down on the energy a valve has to endure during an arc?
There is the outline of a circuit using a string of thyristors (SCRs) in
figure 2 here
http://www.qsl.net/oe5jfl/flashover.htm
although the circuit does not offer protection during the first half
second, until the gate trigger reservoir capacitor has charged up. It also
may not keep the thyristors triggered for 20ms or more, until after the
primary-side relay/breaker/contactor has dropped out. A better approach
might be to use transformer coupled gate triggering, driven from the low
voltage DC supply that is already provided for the control logic. Some kind
of pulse train circuit (NE555?) would be needed to keep the thyristors
triggered during the "follow on" phase.
73, Alan G3XAQ
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