The NU3 is a directly heated full wave rectifier to a design specified by the Admiralty - AP7403. The design coincided with the design for the U14 and both Type designations are present on this exhibit.
Initially the U14 was designed to meet all HT requirements up to a maximum of 120 mA at 500 V, using a capacitor-input filter. However, some valves performed better than others on final factory test so the best ones were graded and marked U14 whilst the also-rans were marked U12.
This U14 was designed to work with a maximum reservoir capacitor of 32 µF and to have an effective series resistance of 100 Ω. The filament is a thin inverted V ribbon in each of the two anode cavities.
The two anodes are stitched together each formed from a thin sheet of nickel.
The M-OV ellipse containing U14.
The filaments are kept under tension by the top springs.
The oxide coated filament tapes held in the spring hooks.
The classic envelope is 44 mm in diameter and, excluding the B4 base pins, is 96 mm tall.
References: Data-sheet & private communication. Type NU3 was first introduced in 1931. See also1931 adverts.
