In 1925 (Stokes page 17) RCA replaces the UV200 with the UX200.
Tyne: In May 1926 RCA announces the UX200A as a new detector tube, replacing the UX200. The UX200A contains caesium vapor isntead of argon gas of the UV200 and UX200 to get a much more stable operation and increase of sensitivity. The UX200A has a thoriated filament but at that time, 1926, was already outdated and only being used as replacement for UX200 (EE).
