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I am seeing issues with the glass in JJ power tubes, more and more. As a vacuum tube is being evacuated (having the air pumped out) traditionally the tubes had a gas flame applied to the glass to help all the air get evacuated. This is a dual process where the extreme heat cooks off moisture and drives out all of the air and other impurities, and then the pinch is closed off and the glass needs to cool at a controlled rate to properly anneal the glass. Traditionally glass items are annealed in a lehr, but with vacuum tubes the annealing happens while the tube is having the air sucked out. JJ has been cooling their tubes MUCH TOO FAST, creating stresses within the glass that will break the glass when the tube is heated from being in 100% normal use within design parameters. So JJ is definitely NOT burning any of their tubes in at all on a test jig, or the simply do not care if a sample batch shows a whole batch is faulty.
