The PY81 was specifically built as an efficiency diode for television line scan and the design dates to the early 1950s and here we have the UK made Brimar version. Considerable energy is fed to the line deflection coils and this has to dissipate during flyback. The efficiency diode channels this energy back to the supply rail smoothing capacitors for use on the next line scan.
The rectifier is specified as having a peak inverse voltage rating of 4,500 Volts, and a peak current rating of 450 mA. No voltage drop is quoted so the dissipation can not be given.
At the top of the picture the heater coil can be seen emerging from the cathode tube. It emerges with the insulated coating present but this tapers to bare metal for the connection.
The stitched anode from the face..
The thin glass tube envelope is 21 mm in diameter and, excluding the B9A base pins, is 71 mm tall.
References: Data-sheet, 3002 & 1040. Type PY81 was first introduced in 1951.