I'm finishing repairs on a radio that uses locktal tubes. I replaced one that went bad adeveloped a crack on side. Today I went and put new tube in and another one went bad same way! Keep these tubes out of cold storage above 40F. Tubes ran a long time while hot, the glass these are made of are of minimal thickness and I don't think can handle thermal stress of cold.

It may have just been coincidence. Locktals were developed for military purposes and were used in car radios. Think of WWII military aircraft, unheated, and flying in below 0F weather, crew wearing shearling-lined bomber jackets. I think they would have tested locktals under such freezer circumstances before releasing them to production.