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Old 03-02-2015, 02:16 PM
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Final anode (ultor) seal problems with Pyrex

There have been several reports of problems with the glass/metal seal for the fnal anode (ultor) connection in Pyrex CRTs. The EMI 6/6 that I had rebuilt in 1987 didn't have any problems here. Though there may be a long term problem which we haven't seen. For some years the set hasn't been readily accessible as it's in the stores of the National Media Museum, Bradford UK.

A recent correspondence with Darryl hock set me thinking. Why have recent attempts at Pyrex rebuild had problems with the final anode connection? An unusual aspect of the 1987 rebuild was that the CRT was pumped cold. 2 reasons:

1: The oven wasn't big enough
2: The guys doing the rebuild felt there was too much risk of implosion if the CRT was baked in the normal way.

I don't know if a HF heating coil was used to outgas the electrode assembly. I assume it was.

Of course this anecdote doesn't amount to evidence but it might just be important. Cold pumping has its risks, mainly from gas molecules adhering to the Pyrex and wrecking the vacuum. I know that oversize getters were fitted because of this risk.
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Old 03-15-2015, 11:12 PM
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In the book "Tube: The Invention of Television" the author wrote that early tubes which Zworykin witnessed at a contemporary lab in France had tubes that were pumped down to a very high vacuum whilst working in the lab.

I would assume that these tubes were cold pumped as it has been described that they continued to pump the tubes at the time to maintain the high vacuum.
If the gettering was sufficient, and any metal inside the tube heated, one would imagine that any outgassing would be controlled.
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Old 03-27-2015, 03:38 AM
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Further tiny bit of evidence. The pinch was completely replaced with a modern soft glass one. Hence the need for a graded seal. So the final anode was the only original pyrex to metal seal that remained.

This is a bit of a long shot, but if the pinch from an old pyrex CRT is saved, it could be attached to a vessel and pumped. If this is done in an oven and leaks are found that's reasonable proof that pyrex to metal seals don't like being re-heated. If leaks aren't found then it's simply inconclusive.
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