The U18/20 is a medium power full wave rectifier. To protect the cathode from damage the rectifier needs to be fed from a supply with an effective series resistance of 180 Ohms. The reservoir capacitor may have a maximum value of 16 μF.
The side of one anode. The flat side faces out for maximum radiation away from the centre.
The GEC label indicates a valve for professional use - still made by M-OV the same as Marconi and Osram valves.
The top mica: the tension springs hold the oxide coated ribbon filaments taut.
The classic envelope is 50 mm in diameter and, excluding the B4 base pins, is 110 mm tall.
References: Data-sheet, 1040 & 1043. Type U18/20 was first introduced in 1939. See also1939 adverts.
