Post by Peter Wieck
Actually, it is a bit more detailed than this. Telegraph systems used Wet Cells (Copper/Zinc/Cupric Sulphide/Acid) to drive their systems. These cells are stable, long-lived, have very limited self-discharge issues and use (relatively) safe and low-cost materials. Along comes telephony, largely using the same technology. And from that long-distance and ultimately repeaters, finally tube-based repeaters. Western Electric, the equipment-maker for Bell adapted their tubes to prevailing available currents - a Gravity Cell of the chemistry they used is a nominal 1.25 V. Which falls nicely into the 2.5/5 pattern.
Now, radio comes along - and the need to bring radio to 'remote locations', using reliable battery power-supplies - not exactly possible with multi-gallon, loose-top wet-cells, and more in the realm of those 'new-fangled' *sealed* lead-acid batteries (the technology was invented in 1859). We owe the transition from 2.5/5/7.5... to the Japanese, who were the first to use 6.3V, in/around 1935/6. The rest is history.
But, any decent tube tester has filament voltages from 0.6 - 117 including Gravity and lead-acid multiples.
Not meant to be a lecture - but, sometimes historical details add to the fascination with the hobby.
Melrose Park, PA
That's curious. John Stokes in his "70 years of Radio Tubes and Valves"
says the first claimed 6.3 V tubes were the National Union NY64, NY65,
NY67 and NY68 announced in May, 1931.
Arcturus, Ken Rad and Raytheon marketed the types 236, 237, 238, 239 in
July 1931. Sylvania and RCA announced their 6,3v tubes around the same
time. At least according to Stokes.
Anyway, I'm not trying to start an argument or anything. I don't have
any first hand knowledge of this one way or the other.