My feeling about cryo’d power tubes is the stress from extreme temperatures weakens or makes brittle, the center alignment plastic guide.
Cryogenic Treatment of Tubes, and why you should not.
Came across this paper while ago on cryo treating tubes and thought iI'd share it here. There is some other great information on tubes on the site as well.
Cryogenic Treatment of Tubes: An Engineer’s Perspective - Effectrode