The PY81 was specifically built as an efficiency diode for television line scan and the design dates to the early 1950s. 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.
The anode sheets are folded outwards at the ends to offer a greater radiating surface.
The anode is formed from two sheets stitched together as shown. This metal component would be made on an automatic machine fed with thin sheet Nickel.
The thin glass tube envelope is 21 mm in diameter and, excluding the B9A base pins, is 71 mm tall.
References: Datasheet, 3002 & 1040. Type PY81 was first introduced in 1951.