[AMPS] Porcelain Standoffs

[George Clute W7LFD]

Thanks Jim,
I suspect you've hit the mark after considering the very high anode voltage
present in the circuit. The tiny carbon pits after a few hours of use
provides a clue to what is happening. I started hanging and suspending wire
around the cabinet until reaching the blocking capacitors.  Teflon would
certainly be a better choice with very high DC voltage.

Again thanks for your thoughts.  My wife's kiln may be the solution, however
I'm not sure it is wise to continue using that method of stringing high
voltage, and I suggest those that rely on porcelain to handle the high
voltage be very careful.  I've destroyed 4 to 6 inch stuff mounted in the RF
deck.

Geo W7LFD
Jim Writes:
I've heard of this but never seen it.    Take a look at a porcelain
standoff.  You'll see it's glazed on the body, but not on the ends.  The
bare porcelain is slightly porous and absorbs water and other impurities
from the air.  Over time -- remember your stand-off is likely WW2 surplus
-- enough stuff gets in that (1) the insulator becomes slightly
electrically conductive and (2) at some point after that, resistive heating
in the insulator is great enough to cause the impurities to dissipate
faster than the tiny pores in the porcelain will allow.  And it explodes. I
suspect that mechanical and RF heating play a factor too.

The vaguely remembered reference suggested refiring the insulators to drive
off the impurities.  I don't remember the particulars, but I think a small
(explosion-proof) vacuum chamber that could be heated to about 160 F at a
couple inches absolute for several hours (or days) would do the trick.
This is something that could be made with commonly available junk.

A glass or teflon (or delrin in a low-ambient temp environment) standoff
would probably avoid the problem.

Comments?

Jim N6OTQ



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