I also posted this reply on the Antique Radio forum...
Did you mean UX-201? Specifications, apart from filament current to achieve emissivity is similar, although the interelectrode capacitance is slightly different. the 1/4 amp UX-201-A as I recall, is so designated becuase it uses an alkaline-earth coated- filament versus the non-coated tungstun filament. The non-coated tube will draw considerably higher current in order to achieve equivalent emission to the 'A' type tube. Even as early as 1932, RCA was manufacturing alkaline-earth coated filaments. They often abreviated the typed designation to, simply, the 01-A. This was also done to distinguish RCA tubes from other manufacture type numbers that did not employ the alkaline coating.
From RC-10, published 1932
Characteristics: UX-201-A (alkaline-coated filament, 3 electrode) TRIODE
Filament Voltage (D.C.) 5.0 volts
Filamanet Current 0.25 amps
Plate Voltage 90 typ. 135 max volts
Grid voltage -4.5 to -9 volts
Plate current 2.5 to 3.0 milliamps
Plate resistance 11000 to 10000 ohms
Amplification Factor 8
Mutual conductance 725 to 800 Micromhos
Grid-Plate capacitance 8.1 uuf
Grid-filament capacitance 3.1 uuf (3.5 uuf for type 01)
Plate-filament capacitance 2.2 uuf (2.4 uuf for type 01)
Bulb type S-14
Base Medium 4-pin
Note: tube must operate in VERTICAL position. Tube is suceptable to microphonic disturbances. Filament is designed to operate from a 6 volt battery and a fixed or variable resistor must be employed to reduce this to 5 volts. The tube is designed for use an interstage audio or rf amplifier or as a detector.
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