DM70

Apart from being perhaps the smallest "magic eye" and designed for only 25mA @ 1.4V operation the DM70 (or DM71) is also very simple yet reliable and long lived.

Though intended for portable battery tube radio and battery tube tape recorders it was also used Table Top mains radio such as Spezialsuper 53 SH830GW (likely a 1952 or 1953 Model).

In a sense it's more the predecessor of the Vacuum Florescent Display (VFD) of the 1960s till today than the 1959 DM160 which was purely a VFD indicator.

Here is internal construction with getter, support rods and glass removed:

DM70  or DM71 approximate construction
(dimensions approximate)

Due to the filament closer at the dot and the slot wider at the top, at the most negative grid voltage before "cut off" the dot of the ! and top of the ! are both lit. As the voltage of grid changes toward zero the electrons are less repelled and the bar grows downwards. In fact this is an opposite characteristic to most "magic eyes" that are using an electron beam deflected by a control rod to create a shadow. In most, this rod is internally or externally connected to a Triode, which is also usually internal using the same cathode.

Other feature are the fact that the grid is more like G2 in position, it's much closer to Anode than Filament, more like modern VFD. Tubes during 1940s moved the grid ever closer  to the filament or cathode.

So

DM70 / DM71 : 0V on grid = maximum size bar

Typical Magic Eye: 0V on grid = minimum size bar = maximum size shadow.

The published graphs show grid up to 3V positive for about 45V or 50V HT compared to full size bar at 0V for about 75V to 85V  HT. Similarly just before "cut-off" dot and top of bar only is about g= -7V at 45V HT and perhaps -15V at maximum HT.

So what if we reduce HT to +18V? (anode voltage)

Actually the display is still quite bright, but the operation point of grid has moved even more positive with about +9V needed for full bar and 0V close to minimum. At g =+18V the image is brighter and defocused. Thus above a particular value of grid volts, depending on HT, the device is only variable brightness.

What about 9V HT?

At this point it's dim. Also there is no bar control noticeable. Instead it acts like a VFD, A DM160 or iv-15 with the positive grid voltage varying brightness of the whole bar. The DM160 was developed a few years later (1959) with a grid like anode. But later VFDs all used the DM70 type construction, simply with a level filament and mesh grid even closer to anode.

The DM71 is simply a DM70 with the leads factory cut for a sub-miniature B8D connector released first in 1953.

See also

History of tuning indicators

Vintage Indicators