Tubes were used in the radios of World War II aircraft at flight levels of 25 thousand feet or more - the same types now sold as NOS. Also, these were the VT or military spec tubes which were built for the altitudes, vibration and temperature extremes of unpressurized flight. I don't see a problem with good 'ol USA NOS tubes or even Russian military. Chinese tubes tubes, though, I wouldn't trust for many other reasons.
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