The PL509 was one of the last TV line timebase output pentode valves to be designed and this exhibit carries the later Mullard logo and was made in Great Britain.
The PL519 replaced the PL509 and was pin compatible. These valves were designed for service in colour television receivers employing shadow mask tubes. The EL509 differs only in heater requirements.
The normal anode voltage of 160 Volts is designed to be exceeded by pulses of up to 8,000 Volts and 1.4 Amps.
Radio amateurs also used such valves, known in America as 'sweep tubes' for short-wave transmitters, sometimes with the valves in parallel to increase the power available.
This valve has seen service as the discolouration of the glass indicates.
A closer view: the beam plate can be seen just within the anode. The two inner grids have the same pitch and are aligned.
The wide glass tube envelope is 38 mm in diameter and excluding the B9D base pins, is 113 mm tall.
Reference: Data-sheet. Type PL509 was first introduced in 1969. See also1969 adverts.