The CV31 is a directly heated full wave rectifier and essentially the U20 from M-OV.
The maximum reservoir capacitor is 4 µF and the minimum series resistance specified is 150 Ohms.
The cathode is an oxide coating on a thin ribbon filament that has a single hairpin fold at the top and is held in place in a slot in the top mica. The anode is blackened to form a better radiator.
Each anode is formed from a single sheet that is folded to form a narrow rectangle with rounded side folds. The two ends of the anode are bent out at 90 degrees to the cavity and joined by notches and lugs from the sheet metal. The fixing rods pass between the plates in a round channel. The top of the electrode assembly is firmly held in the top mica that is in tern firmly held by the dome of the envelope.
The GEC logo and the inner surface of one of the anodes.
The classic envelope is 45 mm in diameter and, excluding the B4 base pins, is 110 mm tall.