[Tube-Swap] Why 6.3V heater?

[Pete Rossi]

Related to this subject...

In my 1959 tube manual, I see a lot of tubes listed with 12V filaments
and a maximum plate voltage of 12 volts.   There are notes stating they
were intended for automotive use where you run the filaments and plate
from the 12 V battery.

Did these tubes ever really catch on?   I suspect they were being 
developed right about the time the transistor came out and I guess
that these 12V tubes never really came into use.. or they were used
only for a very short time.   Did they work?  Did they work well?

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Pete Rossi - WA3NNA
pete@pjrnet.com