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The output tubes (Type 43 valve) used in the British "Chain Home" radar stations were designed to be opened up and repaired, rather than replaced. They were never actually sealed off, but continuously pumped down during operation to maintain a hard vacuum.
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. Conventional flash getters are inadequate for tubes with very large evacuated volumes.
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