The PL84 frame output pentode was designed for use in television receivers. 110° deflection picture tubes required considerable power to the deflection coils and the PL84 was designed for this service.
The PL84 is based on the EL84 audio output valve but designed for lower HT voltages. The data-sheet gives values for both single ended and push-pull amplifiers.
This exhibit is branded Mullard but made by Philips in Holland.
The side seam of the anode. above the top mica can be seen the copper supports of the control grid with heat radiating fins.
Here the cathode can be seen as a rectangular tube.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 69 mm tall.
References: Data-sheet & 1040. Type PL84 was first introduced in 1956. See also1956 adverts.