🔗 Magic Eye Tube

A magic eye tube or tuning indicator, in technical literature called an electron-ray indicator tube, is a vacuum tube which gives a visual indication of the amplitude of an electronic signal, such as an audio output, radio-frequency signal strength, or other functions. The magic eye (also called a cat's eye, or tuning eye in North America) is a specific type of such a tube with a circular display similar to the EM34 illustrated. Its first broad application was as a tuning indicator in radio receivers, to give an indication of the relative strength of the received radio signal, to show when a radio station was properly tuned in.

The magic eye tube was the first in a line of development of cathode ray type tuning indicators developed as a cheaper alternative to the needle movement meters. It was not until the 1960s that needle meters were made economically enough in Japan to displace indicator tubes. Tuning indicator tubes were used in vacuum tube receivers from around 1936 to 1980 before vacuum tubes were replaced by transistors in radios. An earlier tuning aid which the magic eye replaced was the "tuneon" neon lamp.