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The U381 is a direct equivalent of the UY85. This is made obvious by the ledgend on the glass envelope itself. This half wave rectifier was designed for use across the European mains, as the 100mA heater suggests. The reservoir capacitor specified is 100 µF and the minimum effective series resistance in the circuit needed to be only 100 Ohms.

The probable use was in black and white domestic television receivers or radio receivers, to supply the main HT rail. Colour receivers would have many more valves and use a 300 mA heater chain.

The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 57 mm tall.

References: Data-sheet & ¹⁰⁴⁰. Type U381 was first introduced in 1954. See also 1954 adverts.

Pin Connections

Absolute Maximum Operating Conditions^¶

Thanks to Frank Philipse for supplying the above PDF datasheet.