The PL81A was designed for use in portable television receivers and has a more rugged construction than the PL81 line output valve. Portable sets would also operate without a mains transformer to save weight and cost.
As a line output valve in television service, the peak anode surge voltage is 7,000 Volts. The anode dissipation is 8 Watts.
The construction is as a beam tetrode and the bright beam plate can be seen within the anode. Both inner grids have copper supports and heat sink fins at the top of the supports.
The beam plate is more easily seen in this image.
The anode connects to the top cap and is not connected to the lower mica. The grid winding can be seen to pass flat across the cathode.
The thin glass tube envelope is 21 mm in diameter and, excluding the B9A base pins, is 73 mm tall.