Mullard describe the PD500 as a shunt stabiliser triode for colour television receivers. This exhibit has lost the Mullard name but the lettering warning of X-rays is identical to our other Mullard PD500.
Regulation of the EHT voltage it makes the picture stable in both geometry and intensity. Stabilised HT voltages had been used on professional display equipment such as oscilloscopes for many years.
The same rectifier but with a 6.3 Volt heater is the ED500.
The dissipation is high because of the very high anode voltage and the fact that with low picture intensity most of the EHT is dissipated in the regulator. Under such conditions, where the intensity had been turned down, the anode could glow cherry red.
The triode section lies horizontally in the envelope. Here the coiled heater can be seen to be insulated and inserted within a helical insulated guide in the cathode tube. This extra separation improves the heater-cathode breakdown voltage.
The cup shaped screen with the triode section below. The anode is the long tube seen on the left and connecting to the top cap.
The gold plated grid wires with the white oxide cathode below.
The wide glass tube envelope is 38 mm in diameter and, excluding the B9D base pins, is 104 mm tall.
References: Datasheet & private communication. Type PD500 was first introduced in 1966.