Directly heated triode — filament is the cathode. No separate heater-cathode insulation; Vhk_max is not applicable.
Grid voltage is measured from the mid-point of the AC-operated filament.
The coated filament should be operated at exactly 1.5V via a step-down transformer. Designed specifically for AC filament operation (unlike the earlier UX-201A which used thoriated tungsten).
Original 1920s production used globe-shaped envelopes (UX-226); later production (1930s onward) used ST-14 bulbs. Both are electrically identical.
High plate resistance (7.3 kOhm) limits its usefulness as a driver for power tubes — a step-down interstage transformer is recommended for low output impedance.
No maximum plate dissipation rating is specified in the original RCA datasheet. At the maximum rated operating point (180V, 6.2mA), plate dissipation is approximately 1.1W.
Pin Layout — UX4
1Filament
2Plate (Anode)
3Grid
4Filament
Socket-Compatible Tubes ⚠ Not electrically compatible