Hi Bob,
> My copy of Bill Orr's Radio Handbook shows one version of a grid
> over-current protection circuit designed, as I recall, by W8ZR.
> Some years ago, a fellow (might have been ZR) was selling printed-circuit
> boards for his own flavor of circuit.
> What's the latest wisdom on creating a suitable circuit? Any source
> of PC boards or other short cuts to getting the job done?
You might contact Ten Tec and see what they did to the later units.
Otherwise, Ameritron can sell a timer-overload board that is easily
modified for any rig. It responds within a few milliseconds of the
overload, much faster than you can see a meter move.
When I was testing a PA with 3CX800's, I had no overload system
installed. I had a few "woops" that lasted only long enough to see
the grid meter get near the pin for a few brief instants. I ruined a
new pair of 3CX800's in one day, after about five hits on the grids
with a few hundred ma for a very brief period. When I cut open the
tubes, they had gold "melt balls".
Adding the overload circuit is a must with any low dissipation grid,
since a 100 watt exciter can easily and quickly trash a "few watt
rated grid".
73, Tom W8JI
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