Mic Kaczmarczik's TubeInformation - The Blue Guitar

After that, people wanted radios in their cars. Here, the batteries
were kept at full charge most of the time, so you could count on
a full 6 volts. Evidently, somebody measured a bunch of car batteries,
because they decided on 6.3V as the average. According to Tomer, this
was the first effort at standardization. Later, the current draw
of heaters were standardized to increments of 300mA so that they could
be used in series. Subsequently, there were also tube types with heater
voltages that ran in multiples of 6.3. This allowed series and parallel
combinations to be built up in TV's and radios.
Tomer also says, in a different chapter, that running heaters 10% over
their rated voltage will reduce the heater life by 50%. Likewise,
a 10% decrease in heater voltage is likely to double the lifetime. Of
course cathode emission will also be reduced, so performance might
suffer depending on the application.
-- Dave Cigna
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