The PL81 was designed for universal mains television sets without a mains transformer and with the heaters on a series chain. The parameters given are for a pair in push pull audio service.
As a line output valve in television service, the peak anode surge voltage is 7,000 Volts. The anode dissipation is 8 Watts.
The PL81 is also specified for use as a series pass element in voltage regulators.
This generic PL81 was marked as new. The side ceramic strips are still white.
The beam plates can be seen inside the anode.
The ceramic insulators are fixed to the anode with spot welded tape the fits into the notches in the ceramic.
The thin glass tube envelope is 21 mm in diameter and, excluding the B9A base pins, is 74 mm tall.
References: Data-sheet & 1040. Type PL81 was first introduced in 1951. See also1951 adverts.
