Rich Measures wrote:
>
>>
>>It has always appeared to me that normal well documented arcs
>>and sparks in tubes have been used to sell nichrome.
>>
>Tom -- My position has always been that unless gas is subsequently found
>with a high-pot., an arc is not logical. Tom's answer is that the gas
>disappeared because of gettering. Is gettering very likely while the
>tube is being removed from the amplifier for high-pot. testing?
>
Yes. If there is a small gas release, the gettering action (which
depends on the gas molecules colliding with the metal surfaces) would
take place on a timescale of seconds. That's way too slow to prevent an
arc, but fast enough to clean up the vacuum before you can get the tube
to a hi-pot tester.
>
>>Terry's amplifier very well could have been the victim of just such a
>>common effect, but since it is not nearly as well publicized outside
>>engineering circles as parasites it is overlooked.
>>
>Barnacles that disappear?
>'
Microscopic 'whiskers' may be a better term. They can start an arc by
field emission, but are burnt off by the high current density.
However, I'm still uneasy about Terry's experience, because that seemed
to be reproducible on demand, which is not characteristic of a random
event like an arc.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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