My 6BK4C glows green sometimes. CTC-10C
The original 6BK4 was regular glass. I have an original dud from 1957. Around 1972 or so they started making these and 3A3C with leaded glass. X-ray problems, they said.
Early 3A3's look just like a 1B3, and I used to have people that checked their own tubes constantly burn out their HV rectifier because someone in the past sold them a 1B3 when they needed a 3A3. 3A3A was a bit larger, I believe.
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