The NU7 is an interesting rectifier as the mercury can be clearly seen on the inside of the envelope. The heater is a thick coil of ribbon and the anode is a curved plate.
The civilian prototype of the NU7 is the M-OV Type GU1, introduced in the mid 1930s. The GU1 was very popular as a rectifier for cinema amplifiers (etc) but it had a short life and was discontinued in favour of the GU50. The Navy, as ever, wanted a continuing supply of spare valves long after everyone else had converted to the superior type.
Our exhibit was probably supplied by Ediswan Mazda, probably in the late 1940s.
The glass rod support can be seen at the bottom of the picture. The filament is connected at both ends and in the centre.
The right hand and centre filament supports are clearer in this image.
The filament running at 4.0 V and drawing 2.5 A. The valve had not been used for a long time and so the filament was run with no anode volts for 15 minutes as required for mercury vapour rectifiers. The printed identification is clearly seen. The Admiralty Pattern is number 3822, and the serial number is given as E.
Running with anode volts the blue glow fills the bulb. With too low an anode voltage the ionisation appears as a glow between filament and curved anode only.
The balloon envelope is 42 mm in diameter and excluding, the B4 base pins, is 94 mm tall.
References: Datasheet and private communication. Type NU7 was first introduced in 1930.