The U81 is a full wave rectifier with anode ratings of 500-0-500 Volts and a maximum rectifier current of 150 mA. The maximum reservoir capacitor is 16μF. The series resistance must be 100Ω or greater.
This M-OV product carries the GEC brand that was used for valves for the professional market.
The GEC paper label.
The anodes are stitched together and the width is chosen to give adequate area for radiating away the heat generated in use.
The two rectifier sections seen from the side.
The wide glass tube envelope is 32 mm in diameter, and excluding the B8B base pins is 70 mm tall.
Reference: 1040. Type U81 was first introduced in 1950.