The UY21 half wave rectifier dates from the early days of all glass construction in the early 1940s. The device was designed to operate as part of a series heater chain in universal sets. The 100 mA current allowed a few valves to run from 240 Volts, eg a domestic radio.
The maximum reservoir capacitor is 60 µF and the minimum series resistance has to be 175 Ohms. The design is quite efficient compared to octal valves of the same period.
The face view with one of the side flanges facing the camera.
End elevation, looking along the long axis of the anode. The coated cylindrical cathode can be seen through the centre hole.
The wide glass tube envelope is 27 mm in diameter and, excluding the B8B base pins and spigot is 78 mm tall.
References: Data-sheet, 3002 & 1040. Type UY21 was first introduced in 1941. See also1941 adverts.