The Collins 618T-x HF transceiver, 400w PEP, built from about 1962-78, was universally equipped in overwater airliners and military. It had tube finals, 4CX250A(?), and remained in service for many more years. In about 1995, the shop at Continental airlines noticed a drastic drop in service life of new tubes. They then looked closely at a couple of the tubes: both said Eimac, with same model number and date code, but one was rough finish silver, and the other nickel. They called the FAA, who called the FBI...
Succeeding generations of airline HF are solid state.
My younger son is in the A-V business, and I laugh every time I see a 6L6 tube. Those were obsolete when I was a ham, half a hundred years ago.
GB