[MrFixr55]

Hi MZip,

These "1.4V" tubes were designed to run on a 1.5V battery; no resistor is needed. The "A" batteries were not as big as a "#6" Dry Cell (the big round thing used in crank telephones, Model T ignitions, 1920's radios like the Radiola III, etc., but were likely 2 "F" cells in parallel. (A 6V lantern battery used 4 F cells in series.) I don't think that the voltage dropped to 1.4V under load.

Like Arran says, if you design a power supply for the filaments, do put a 1.8 - 2V Zener across the output, anode to negative, cathode to positive to prevent tube damage in case of severe overvoltage. This could be done in the radio itself. Do note that unless protected by a 0.5A fuse between the output of the LM317, the load and Zener to protect overcurrent and permanent shorting of the Zener. However, the tubes are protected.

There has to be a minimum of 3.5V difference between input and output for stability of the LM317. This means that you should use a 6V supply because a 5V supply is at that 3.5V limit.

You can either breadboard the supply on "perfboard" or buy a complete regulator assembly from Amazon, eBay or (gulp) AliExpress or (Gulp Gulp Gulp) Temu.

I have not had a problem using switch mode power supplies for filaments on the 2V tubes that were used from about 1930-1938 in battery sets. I used a 3V Wall Aart power supply and the correct amount of resistance to drop the voltage to 2V depending on the total current of the filaments. However, when trying to use one for B supply, I ran into very much switching noise. It may be about shielding of the supply or whether or the output is rectified by full wave or half wave rectification (The unit that I used had half-wave rectification.)