The Cossor 53KU is an indirectly heated full wave rectifier with a considerable rating. The reservoir capacitor should not exceed 16μF and the valve needs a minimum effective series resistance of 75 Ohms to protect it from excessive peak currents.
The construction is typical with two separate valve sections in the envelope. The cathode is tied to one side of the heater and this requires a well insulated independent winding on the mains transformer. In fact it was normal to rate rectifier valves at 5.0 Volts when other valves ran at 6.3 Volts to ensure that the rectifier was powered separately.
One anode seen close-up. The surface is blackened to enhance thermal radiation and either side of the tubular cathode the anode plates are flat extensions to increase surface area and thus radiating capacity.
The classic envelope is 44 mm in diameter, and excluding the IO base pins is 126 mm tall.
References: Datasheet & 1040 Type 53KU was first introduced in 1958.