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 Mullard advert gives details of the construction. The mica discs are coated to improve insulation, the cathode is made from nickel alloy and the anode is a copper laminate.
This Mullard rectifier is specified as having a peak inverse voltage rating of 4,500 Volts, and a peak current rating of 450 mA.
The stitched anode plates connect via the base pins whilst the cathode is taken to the top cap.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 73 mm tall.
References: Data-sheet, 3002 & 1040. Type PY81 was first introduced in 1951.