The DM70 was quite a departure from previous tuning indicator/magic eye/ tune-on designs and was designed specifically with the portable dry battery receiver in mind but additionally was specified as an inexpensive device for the mains receiver too, not to mention as the reading indicator for a range of Negretti & Zambra high precision aneroid barometers made for the Royal Navy and Royal Air Force - in fact if you take a look at our product listings, you'll see a range of these barometers for sale!!!.
The DM70 has a subminiture envelope and is constructed as a wire ended triode with the control grid being a metal plate shaped shaped like an exclamation mark. The varying grid width gives this valve a vari-mu character. The anode is a plate coated with an electroluminescent phosphor and it is located behind the grid. The filament, rated at 1.4V @ 25mA is a single wire located in front of the grid. The configuration is shown in fig 2(b) in the photograph below.
The operation of this fascinating device is rather elegant which I will attempt to describe. With the grid at zero potential, electrons from the cathode travel through the 'exclamation mark' aperture of the grid to impinge on the anode hence causing it to fluoresce. VIewing from the filament side, the fluorescence is visible through the 'exclamation mark'. As the grid voltage becomes more negative, the anode current will decrease and hence the fluorescence will fade.
The fade commences as shown in photograph fig 2(a) above at area 'B' until only areas 'A & C' are luminous. At an even higher grid voltage, the electron flow will stop completely and the fluorescence wil disappear completely. The idea was that the grid would be fed from a resistive line in the AVC line of a receiver so that when the receiver was accurately tuned, the grid voltage would be at a maximum and conversely the fluorescent area would be at it's minimum so lo and behold, tuning indication was provided, simples as the meercats say!