A business partner of mine offered the comments reproduced below
relative to his historical conversations with Eimac research
engineers about tube arcs:
>Most of the ceramic tubes with the multiple cathode strips generate
>large electric fields, large enough to cause metal migration of the copper
>anode material to form minute "dendrites" which, when sufficient length
>has developed (may only be < .1") will be a focal point for an arc.
>Naturally, right after a tube is newly made, other points due to machining
>may also be present. The subtle "ping" resulting from an arc of this
>nature is due to mechanical "flexing" or displacement of the anode material
>during the process and NOT the arc itself... other components may chime
>in as appropriate.
>When I designed for commercial use, I designed the power supply to
>have a max of 4 joules of energy that wasn't controlled such that
>if an arc occured, only 4 joules was available to the anode, also giving
>time to shut off the power supply and crowbar the remaining power supply
>energy.
>A fault current-limiting resistor of 50 ohms
>works but you will notice that the resistor doesn't always survive
>the event (not rated for that type of service, nor would
>a ham pay for the extra cost of one that was!)
>
>Note that, yes, gassiness will help propagate an arc, but any "sharp"
>protrusion will be the primary starting point... needs a really good
>electric field build up typically.
73 de Bob, K0KR
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