2 wrote:
>>
>I have seen 'em go kaput in several years without any glyptol in
>evidence. Apparently, sometimes there are defects in the ceramic-metal
>seals.
>
The whole glass-to-metal seal business is a black art, and ceramic-to-
metal is even worse.
A glass-to-metal seal starts off with a weird glass that is capable of
sticking to a very fine feather edge of copper, ie has a similar
expansion coefficient. However, the glass is not suitable for the whole
body of the part, so typically it goes through 2 or 3 transition rings
of other glass with carefully graded expansion coefficients. Even so,
the heating/cooling processes leave stress in the seal, so it's no
wonder it sometimes cracks.
The "glyp it and ship it" parts were probably ones that didn't have
through cracks... at least not when they left the factory.
Ceramic-metal seals are worse because they depend on metalization of the
ceramic, followed by a vacuum brazing operation on to the main metal
part. This has to leave stresses in the ceramic, but you can't inspect
the seal visually anything like as well as glass. Waiting to see if it
will hold a vacuum will hold up the throughput on the pumping lines, so
this time it's "weld it and ship it".
When you think of the millions of glass-metal seals that used to be out
there in pin-based radio/TV tubes, the wonder is that most of them work
as well as they do...
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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