[Penthode]

The filament voltage question is a good one.

In the early twenties, 5.0v @ 1A was standard for such tubes as the UV-201. In 1922 thorium filaments provided better emission facilitating lower filment current. The UV-201A in 1922 filament ran at 5.0v @ 0.25 amp. Note 5.0v was chosen to allow a 6 volt lead-acid battery terminal voltage to drop with the difference taken up with a series rheostat. In 1922 Carbon-Zinc dry cell tubes became available such as the WD-11 1.1v @0.25A and UV-199 3.0v @ 60mA.

In 1925, the UX201A was introduced. Characteristics the same as the UV-201-A only a different base. In 1925 a power output tube to drive bigger loudspeakers became available: the UX210 and in 1927 the UX250. Both filaments were 7.5v @ 1.25A.

1927 is when US manufacturers introduced the first of the indirectly heated tubes to run on AC. In the US 2.5 volts @ 1.75A. The UX226 in 1927 was directly heated with a center tap resistor providing the emitter return. It's filament ran at 1.5v @ 1.05A. The UX226 must have about the shortage use in design of any US tube as it was only used for just over a year before becoming obsolete!

So it looked like 2.5V was to become the US standard. In the late 20's and early 1930's a wide range of pentodes, tetrodes and triodes were introduced with 2.5 volt filaments. In 1931-32 however a few 6.3v indirectly heated tubes began to appear intended solely for car radios. Note that each lead acid cell is 2.0v and a car battery has three in series. This means the battery yields 6.0v and the extra 0.3v was to account for the slightly higher terminal voltage as the car battery is constantly being charged by the car generator.

Manufacturers in 1933-34 began moving away from 2.5v filaments in preference to 6.3v for household sets. I think this was to reduce proliferation of tube types which was beginning to be a problem. Also it reduced the amount of copper required in the filament wiring of a set! The current drop from 1.0A to 1.75A for the 2.5 volt tubes to only 0.3A for the 6.3 volt tubes was a benefit in saved wiring cost. (2.5v was originally chosen as less hum induced into circuits by the lower filament voltage.)

In the UK and other countries there was a different story. The UK for example used 4.0v AC from the late 1920's until about 1939 when the UK proceeded with US tubes at the beginning of WW2.