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 Osram paper label.
The two rectifier sections supported by the pinch and the tight fitting top mica sheet.
The filament is an oxide coated ribbon. Filament tension is maintained by the springs secured to the top mica.
The classic envelope is 49 mm in diameter and, excluding the B4 base pins, is 113 mm tall.
References: Data-sheet, 1040 & 1043. Type U18/20 was first introduced in 1939.