The pre-war UK Navy NU20 is a directly heated full wave rectifier, with inverted v's of coated tape as filaments.
The feint outline of the M-OV etched ellipse. The centre contains the commercial Type designation U50 but sadly it can not be read.
The familiar pattern of folded and stitched anodes containing the filament.
Looking through the top of the valve the filament ribbons can be seen and they are held in tension by the spring wires fixed to the top mica. The tight fit of the valve to the envelope is achieved by the use of four clips around the periphery of the mica disc.
The maximum reservoir capacitance is 32 µF and minimum series resistance is 100 Ω.
The classic envelope is 45 mm in diameter, and excluding the IO base pins is 107 mm tall.
