Same UX5 base and pinout, higher mu (13.8 vs 9) and different heater (2.5V/1.0A vs 2.5V/1.75A). Sams lists as substitute.
Electrical Specifications
Absolute Maximum
Anode Voltage (max)Maximum Plate Voltage275V
Cathode Current (max)Maximum DC Plate Current5mA
Heater-to-Cathode Voltage (max)Maximum Heater-Cathode Voltage90V
Typical Operating
Anode Voltage (typical)Plate Voltage250V
Grid 1 Bias Voltage
Heater
Heater VoltageHeater Voltage2.5V
Heater CurrentHeater Current1.75A
Notes
Indirectly heated cathode — the 5th pin (cathode) is separate from the heater, unlike the directly heated type 26.
Maximum heater-to-cathode voltage is 90V. Exceeding this damages the heater-cathode insulation.
Maximum DC plate current limited to 5.0 mA per the Tung-Sol datasheet. No explicit maximum plate dissipation rating is given in any manufacturer datasheet consulted. At the maximum rated operating point (250V, 5.2 mA, Vg = -21V), plate dissipation is approximately 1.2W.
Original 1927 production (UY-227) used globe-shaped S-14 envelopes; production changed to ST-12 bulbs around 1931-32. Both are electrically identical.
Grid resistor for detector service: minimum 0.5 MΩ, maximum 0.5 MΩ (biased); grid leak detector uses return to cathode with 250 pF grid condenser.
Pin Layout — UX5
1Heater
2Anode (Plate)
3Control Grid
4Cathode
5
Socket-Compatible Tubes ⚠ Not electrically compatible