The tuning eye tube, also known as a magic eye, was a vacuum tube that indicated, visually, the effects of a controlling voltage. Its most common application was in radio receivers, to indicate accurate tuning of a desired station.
The tuning eye resembled an ordinary radio tube. It was designed to be viewed from its top, wherein was a small dish-shaped element (the target), with a smaller light shield in the center of the target. A ray-control electrode was placed near the target, and this electrode was connected to the plate of the amplifier triode also located inside the tube. The tube was installed such that the top of the tube was visible on the front panel of the radio.
In operation, the tuning eye’s target was illuminated with a green glow extending around the entire target, except for a wedge-shaped shadow. The width of this shadow was controlled by the ray-control electrode, and therefore varied with the effects of the controlling voltage. This presented a visual display of how well the listener had tuned the radio, and made accurate tuning fast and easy.
The first tuning eye tube was type 6E5, introduced by RCA in 1935. It was standard equipment on RCA’s higher-end receivers, and with a special parts kit, could be retrofitted to existing receivers. The tuning eye lent an air of class to the receiver, and quickly gained wide acceptance with consumers. Soon, tuning eyes began to appear in other manufacturers’ radios.
Within the next few years, other models of tuning eye tubes followed the release of the 6E5:
- Type 6G5, released by RCA in 1936 and replaced by the 6U5/6G5 in 1937. This type was able to handle weaker signals more efficiently than the 6E5, and was the most widely used of the various tuning eyes.
- Type 6H5, released by Raytheon in 1936. Rarely used by manufacturers, rarely seen today, and replaced by the 6U5/6G5.
- Type 6T5, released in 1937 by Arcturus, Raytheon and Sylvania. This type was electrically identical to the 6U5/6G5 and was used extensively by Zenith. It differed from the other models in that its display was in the form of a ring of light on the target, rather than the wedge-shaped display. The ring contracted or expanded in accordance with the controlling voltage.
- Types 6N5 and 6AB5/6N5, released in 1937 by RCA and electrically identical to the 6U5/6G5. This type featured a lower heater current, making it useful in battery-operated sets.
- Types 2E5 (released 1935 by RCA) and 2G5 (released 1936 by Tung-Sol), electrically identical to the 6E5 and 2G5 respectively. These types were intended for retrofit in radios using the 2.5-volt heater series tubes.
Some tuning eye tubes were designed for special applications. Types 6AD6G and 6AF6G were similar in appearance to the 6E5, but required external amplifier tubes for its control voltages. These tubes contained two separate ray-control electrodes and could simultaneously indicate the effects of two voltages. Type 6AL7GT had a square screen that resembled regular cathode-ray tubes. It was used in AM/FM receivers and provided an excellent display in both AM and FM mode.
The tuning eye was used in radio receivers well into the 1960s. It found its way into other devices as well: as a level indicator in tape decks and other recorders, in measuring instruments, and even in a few television receivers. As the vacuum tube era came to a close, the tuning eye’s function was gradually replaced by panel meters and light-emitting diode displays.
RCA Receiving Tube Manual, 1940 edition
Rheinschild, Philip A., “Tuning Eye Tubes in 1930s Radio Sets”, Philbert’s Place. <http://home.pacbell.net/philbert/tuning_eye/tun_eye.htm> 1 January 2003