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Back when I worked for RCA Sarnoff Labs in the 1980's, they had epoxy like stuff called "Torr-Seal" which I think was meant to seal vacuum systems. It dried hard. Don't know if it could take expansion of metal or glass parts though. And even if you put this stuff on the joints, there may still be enough air in air pockets under this stuff that would make the tube gassy if the original seals failed.
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