The CT-201A triode is specified for a variety of uses according to the data sheet. Like the CT-199 the 201 and 201A are classic early 1920's valves initially introduced by the then newly formed RCA. The 201 was a bright emitter and the 201A introduced later has a thoriated tungsten filament. The initial valves had brass base shells but around 1925 the Bakelite base was introduced.
The locating pin for the UV bayonet base can be seen but the inside of the valve cannot.
The short base pins.
The balloon envelope is 42 mm in diameter, and excluding the UV base pins is 102 mm tall.
References: Data-sheet & 1043. Type CT-201A was first introduced in 1925. See also1925 adverts.
