During the 1920's, there was no standard numbering system for vacuum tubes. Although the "201A" was the standard used in most battery operated radios for RF & AF amplification. The suffix "A" indicated a thoriated filament which drew less current than it's predecessor the 201 which had a 1 amp, tungsten filament. The 201 with it's 5 volt tungsten filament lit up like a light bulb whereas the "A" version with it's 1/4 amp filament produced a dull, warm orange glow.
Many manufacturers had their own prefix designation for the "201A". For the RCA "Radiotron", it was UX-201A, Cunningham was CX-301A, Montgomery Wards; GX-201A and there were many other variations by brand name and some brands had their own numbers.
The UX, CX, DX etc, simply indicated standard 4 pin, plug-in base. This "UX" applied to other 4 pin tubes as well such as UX171A, UX-210, UX280 and any 4 pin plug-in tube. The UV prefix was for the bayonet version (stubby, short pins) that required push & twist to lock in place. The tube sockets in many radios would accommodate either UX or UV based tubes. In the early 1930's tube numbers were mostly standardized, the prefixes were dropped and nearly all of the 1920's tubes were now designated with two digits. The famous 201A was now an "01A", UX120, was now a 20, the 280 became an 80, 210 a 10 and so on.
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