The U52 is directly heated full wave rectifier with a rating of 500 Volts on the anodes and a PIV of 1,400 V. The U52 has a maximum reservoir capacitor rating of 16 µF and a minimum series resistance of 180 Ohms.
The remains of the Marconi brand label. Etched on the glass is a second Type lozenge with the designation of 5U4G indicating complete equivalence with U52.
The anodes are folded to shape and then stitched together.
The top mica showing that it is an interference fit to the shoulder of the bulb. The tension springs hold the oxide coated ribbon filaments.
The classic envelope is 50 mm in diameter, and excluding the IO base pins, is 112 mm tall.
References: Data-sheet, 1040 & 1043. Type U52 was first introduced in 1938.