There was probably something deposited on the glass. I have seen this with NOS tubes many times. If I were you I would pull the tubes and wash them carefully so that the glass is squeaky clean. Use only the slightest amount of dish soap. Avoid getting water on the base.
Dry the tubes carefully before reinserting them in the amp!
Its a good idea to clean the envelopes in this way at least once every year. Dust and fingerprints (and smelly rubber contaminants) will shorten the life of the tube due to heat. The tubes will get rid of their heat most effectively if the envelopes are clean.
Dry the tubes carefully before reinserting them in the amp!
Its a good idea to clean the envelopes in this way at least once every year. Dust and fingerprints (and smelly rubber contaminants) will shorten the life of the tube due to heat. The tubes will get rid of their heat most effectively if the envelopes are clean.