The PL509 was one of the last TV line timebase output pentode valves to be designed. 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. Line output valves operated as switches and linearity of response was not a design requirement.
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.
As is expected the copper control grid supports are topped with radiating fins.
The main heat generating region is at the centre of the working face and that is where the additional fins are place.
The wide glass tube envelope is 38 mm in diameter and excluding the B9D base pins, is 116 mm tall.
Reference: Data-sheet. Type PL509 was first introduced in 1969.