81

Rectifier — DHT — UX4

Known Equivalents & Substitutes

CV2546UX281VT-812935A

Similar Tubes (Not Drop-In Compatible)

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.
GE Essential Characteristics lists: Half-Wave High-Vacuum Rectifier, 4B base, 7.5V/1.25A filament.
MIL-HDBK-213A lists other_number as 2935A.
Sams substitution data lists UV216 and UX216B as substitutes for the 81.
Half-wave single tube: AC plate voltage 700V RMS max, DC output current 85 mA max.
Full-wave two tubes: AC plate voltage 700V RMS max, DC output current 170 mA max, Vdrop 91V at 170 mA.
The 4B base is similar to UX4 but uses a bayonet locking mechanism rather than the push-in arrangement of the 4C base.
Primarily used in transmitter power supplies and higher-voltage receiver applications of the late 1920s and 1930s.

Pin Layout — UX4

1Anode (Plate)

2Filament

3No Connection

4Filament

References

https://frank.pocnet.net/sheets/127/8/81.pdf