The PL81 was designed for universal mains 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 although the main purpose of the PL81 was as a television line output valve or a series pass element in stabilised power supplies.
As a line output valve in television service, the peak anode surge voltage is 7,000 Volts. The anode dissipation is 8 Watts.
The anode is basically a rectangular shape with slightly rounded ends. Both control and screen grids are wound on copper rods and fitted with heat radiating fins. There is no suppressor grid as the design is in fact a beam tetrode and the beam plates can be seen between the top of the anode and the top mica.
The beam plate can be seen through the gaps in the anode as well as above the anode. Of special interest are the ceramic spacers between the beam plates and the anode. Note the fins on the grids.
The thin glass tube envelope is 22 mm in diameter and, excluding the B9A base pins, is 73 mm tall.
References: Datasheet & 1040. Type PL81 was first introduced in 1951.