Magic eye tube

[3] An earlier tuning aid which the magic eye replaced was the "tuneon" neon lamp.

The end-viewed version had a round cone-shaped fluorescent screen together with the black cap that shielded the red light from the cathode/heater assembly.

There was also a sub-miniature version with wire ends (Mullard DM70/DM71, Mazda 1M1/1M3, GEC/Marconi Y25) intended for battery operation, used in one Ever Ready AM/FM battery receiver with push-pull output, as well as a small number of AM/FM mains receivers, which lit the valve from the 6.3 V heater supply via a 220 ohm resistor or from the audio output valve's cathode bias.

The function of a magic eye can be achieved with modern semiconductor circuitry and optoelectronic displays.

When the receiver is switched on but not tuned to a station, the target-anode glows green due to electrons striking it, with the exception of the area by the internal control-electrode.

This electrode is typically 150–200 V negative with respect to the target-anode, repelling electrons from the target in this region, causing a dark sector to appear on the display.

The control-grid of the triode-amplifier section is connected to a point where a negative control voltage dependent on signal strength is available, e.g. the automatic gain control (AGC) line in an AM superheterodyne receiver, or the limiter stage or FM detector in an FM receiver.

Also, a type 6AB5 aka 6N5 tube with lower plate voltage was introduced for series filament radios.

Several other "eye tubes" were introduced in U.S. radios and also used in test equipment and audio gear, including the octal-based types 6AF6GT, 6AD6GT and 1629.