Magic eye tube
A vacuum tube that visually indicates signal strength for tuning.
A magic eye tube, also known as a tuning indicator or electron-ray indicator tube, is a vacuum tube that provides a visual indication of the amplitude of an electronic signal, such as audio output or radio-frequency signal strength. Its first broad application was as a tuning indicator in radio receivers, showing when a radio station was properly tuned in by displaying the relative strength of the received signal. The magic eye tube was the first in a line of development of cathode ray type tuning indicators, developed as a cheaper alternative to needle movement meters.
- Inventor
- Allen B. DuMont
- Year of invention
- 1932
- First commercial tube
- RCA 6E5
- Year of first commercial tube
- 1935
- Period of use
- around 1936 to 1980
Lore & Background
The magic eye tube was invented in 1932 by Allen B. DuMont, who spent most of the 1930s improving the lifetime of cathode ray tubes. The first commercial tube was the RCA 6E5 from 1935. Earlier types were end-viewed, such as the EM34, usually with an octal or side-contact base. Later developments featured a smaller side-viewed noval B9A based all-glass type with either a fan type display or a band display, such as the EM84. The end-viewed version had a round cone-shaped fluorescent screen together with a black cap that shielded the red light from the cathode/heater assembly, prompting advertisers to coin the term 'magic eye.' There was also a sub-miniature version with wire ends, 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. Some reel-to-reel tape recorders also used the DM70/DM71 to indicate recording level.
Reader's Guide
The magic eye tube was significant as a tuning aid in vacuum tube receivers from around 1936 to 1980, before vacuum tubes were replaced by transistors in radios. It replaced an earlier tuning aid called the 'tuneon' neon lamp. The tube made peaks in signal strength more obvious by producing a visual indication, which was better than using the ear alone, especially because automatic gain control action tends to increase the audio volume of a mistuned station. The tuning eye was driven by the AGC voltage rather than the audio signal. In the early 1950s, when FM radio sets were made available on the UK market, many different types of magic eye tubes with differing displays were used, all working the same way. The British Leak company used an EM84 indicator as a very precise tuning-indicator in their Troughline FM tuner series. It was not until the 1960s that needle meters were made inexpensively enough in Japan to displace indicator tubes. The function of a magic eye can be achieved with modern semiconductor circuitry and optoelectronic displays, but the high voltages required by these tubes are no longer in modern devices, so the magic eye tube is obsolete.
Did You Know?
- The EM34 is an example of an end-viewed magic eye tube with an octal or side-contact base.
- The British Leak company used an EM84 indicator for precise tuning in their Troughline FM tuner series.
More in Radio Electronics, Part 2 1-24
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced