Quote:
Originally Posted by N2IXK
Some modern high power tubes (including the 8974 mentioned earlier) incorporate a built-in titanium ion sputter pump to maintain vacuum, even though they are sealed off. ...
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Wonder if tubes used in spacecraft, like geostationary satellites, might be opened to the vacuum of space, to get rid of any gasses that were left when the tube was made on the ground. Probably not, as the mechanism needed to do that would be extra cost and weight. But imagine tubes not needing envelopes...