Same UX4 base, full-wave high-vacuum rectifier — different filament voltage (5V vs 7.5V), higher current per tube (125 mA vs 85 mA) but requires only one tube
Electrical Specifications
Absolute Maximum
Anode Voltage (max)AC Plate Voltage (max RMS)700V
Rectifier
Voltage Drop (typical)Voltage Drop (at 170 mA DC, full-wave pair)91V
DC Output Current (max)DC Output Current (max)85mA
Heater
Heater VoltageFilament Voltage7.5V
Heater CurrentFilament Current1.25A
Notes
Tung-Sol datasheet (1939) confirms: half-wave HIGH VACUUM rectifier, 7.5V/1.25A coated filament, 4B base (medium 4-pin bayonet base).
NOTE: MIL-HDBK-213A incorrectly classifies the 81 as a mercury vapor rectifier. The Tung-Sol datasheet, GE Essential Characteristics, and radiomuseum all confirm it is a high-vacuum type.
Raytheon data confirms: 7.5V/1.25A filament, 4D-SM4B basing, half-wave rectifier.