>
>Thanks for all the ideas that came in on this. An objection I had to
>installing a HV fuse was that in case a flashover occured right when
>that rare DX came in, the down time becomes intolerable. A scheme that
>allows fast recovery to get back on the air quickly is the only way to
>go. I find it a scary thought to replace a HV fuse in a hurry!
>
A correctly sized "glitch" resistor does not need to be replaced after an
"event". A HV fuse may not prevent gold-sputtering damage to the grid of
the tube from VHF parasites. {see photos in September 1990, QST}
>I am making my project enclosure completely air tight, and the only air
>inlets are via fine air filters in order to lessen the chances for
>external arcs on the tube anodes due to foreign particles passing thru.
>
>Dave Kirby mentioned that a proper HV fuse in the DC output will reduce
>the amount of energy dumped by the HV power supply to the flashover.
Limiting peak current discharge current is quite probably more important,
and a fuse isn't fast enuff.
>There is a slow realiziation on my part to recognize this. Yes indeed,
>saving on the cost of a HV fuse could be false economy! This fuse
>lowers the chances of damaging an expensive PA tube in a flashover.
If a tube flashes over it has a gas problem. Gas does not vanish unless
the tube is equipped with a built-in ion pump.
>Hey Dave, that is a serious PA you are building! Please tell us more about
>it sometime.
>
>Peter Chadwick's experience with the #40 string of wire for a fuse
>sounds interesting, relying on gravity to break up the arc. A similar
>idea was suggested by Mark Wilson in the ARRL handbook although I dont
>see any evidence that Mark actually tried it.
>
HV fuse elements are typically surrounded by fine silica sand, contained
by a sturdy insulated tube. The sand quenches the metal-vapour arc as
the fuse attempts to interrupt the flow of current.
>So I think I will appraise the cost of my PA tubes and then decide which
>method of protection to use.
>
If one has a lifetime free-supply of tubes, one can safely leave out the
"glitch" resistor.
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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