A magic eye tube or a tuning indicator , in a technical literature called electron-beam indicator tube , is a vacuum tube that provides a visual indication of the amplitude electronic signals, such as audio output, radio frequency signal strength, or other functions. The magical eye (also called the cat's eye, or tuning eye in North America) is a particular type of tube like that with a circular look similar to EM34 illustrated. The first major application is as a tuning indicator on the radio receiver, to give an indication of the relative strength of the received radio signal, to indicate when the radio station is set up properly.
The magic eye tube is the first in the line of development of a cathode ray tuning indicator developed as a cheaper alternative to metered needle movement. It was not until the 1960s that the meter needle was made economically enough in Japan to replace the indicator tube. The tuning indicator tube was used in the receiver vacuum tube from about 1936 to 1980 before the vacuum tube was replaced by a transistor on the radio. The previous tuning aid that the magic eye replaced was a fluorescent "tuneon" lamp.
Video Magic eye tube
Histori
The magical eye tube (or valve) for radio receiver tuning was invented in 1932 by Allen B. DuMont (who spent most of the 1930s improving the lifetime of cathode ray tubes, eventually forming the DuMont Television Network).
The RCA 6E5 of 1935 was the first commercial tube.
This device is basically made in two forms. The previous type is the final view (see EM34), usually with octal base or side contact. Subsequent developments feature a smaller view of the BZA noval side-view based on an all-glass type with either a fan type or a band view (see EM84). The final-seen version has a circular conical fluorescent screen along with a shaded black cover of red light from the cathode/heater assembly. This design encourages contemporary advertisers to coin the term magic eye, a term still in use.
There is also a sub-miniature version with wire ends (Mullard DM70/DM71, Mazda 1M1/1M3, GEC/Marconi Y25) intended for battery operation, used in one ready-made AM/FM receiver with push-pull output, as well a small quantity of AM/FM power receiver, which ignites the valve from the 6.3V heater supply through a 220 ohm resistor or from the output audio valve cathode bias. Some reel-to-reel cassette recorders also use DM70/DM71 to indicate the recording level, including the transistorized model with the flashing valve of the bias-oscillator voltage.
The function of the magic eye can be achieved with modern semiconductor circuitry and optoelectronic display. The high voltage (100 Volts or more) required by these tubes is not present in modern devices, so the magic eye tube is now obsolete.
Maps Magic eye tube
Operation
The magic eye tube is a miniature cathode ray tube, usually with a built-in triode signal amplifier. It usually shines bright green, (sometimes yellow on some very old types, eg EM4) and the glowing end grows to meet in the middle as the voltage on the control increases. This is used in a circuit that drives the grid with a voltage that changes with signal strength; when the tuning button is rotated, the gap in the eye becomes narrowed when the station is set properly.
Internally, this device is a vacuum tube consisting of two plank electrode assemblies, one creating a triode amplifier and the other a display portion comprising a conical target anode coated with a silicate material or the like. The anode of the display is usually directly connected to a positive high voltage (HT) receiver, whereas the triode anode is usually (internally) connected to the control electrode mounted between the cathode and the target-anode, and connected externally to the HT positive via a high-value resistor, usually 1 megaohm.
When the receiver is switched on but not tuned to the station, the target-anode shines green because the electron is striking, with the exception of the area by the internal control-electrode. These electrodes are typically 150-200V negative with respect to the target anode, repelling electrons from the target in this region, causing the dark sector to appear on the screen.
The grid-control section of the triode amplifier is connected to the point where the negative control voltage depends on the signal strength available, e. g. AGC line in AM superheterodyne receiver, or phase limiter or FM detector in FM receiver. As stations tuned in triode-grid become more negative with respect to common cathode.
Use on radio
The purpose of the magic eye tube on the radio set is to help with an accurate setting to the station; the tube makes the peak of the signal strength more clear by producing a visual indication, which is better than using the ears only. Eyes are useful because automatic gain control measures (AGC) tend to increase the audio volume of the wrong station ruler, so the volume is relatively small because the tuning button is rotated. The tuning eye is driven by an AGC voltage rather than an audio signal.
When, in the early 1950s, FM radio sets were available in the UK market, many different types of magic eye tubes were available, with different looks, but all worked in the same way. Some have separate small screens to illuminate the stereo signal on the FM.
The British Leak Company used the EM84 indicator as a very precise tuning indicator in their Troughline FM tuner series, by mixing AGC voltages from two limiter valve grilles on the grid-sensing indicator. This means that an accurate adjustment is shown by a fully open sharp image, while the off-tune indicator produces a partially closed image.
Other apps
The magic eye tube is used as an indicator of recording level for the cassette recorder, and it is also possible to use it (in a specially adapted circuit) as a rough frequency comparison tool as a simpler alternative to Lissajous numbers.
A magic eye tube is essentially an uncalibrated (and not necessarily linear) voltage indicator, and can be used wherever voltage indications are needed, saving more accurate meter cost.
At least one capacitance bridge design uses this type of tube to show that the bridge is balanced.
The magic eye tube is also used on the cover of My Morning Jacket 2011 album Circuital . The tube is shown as almost completely lit.
See also
- S meter
- VU meter
References
External links
- More technical description
- Gallery tube-eye by SM5CBW, some with animation
- The magic eye tube acts and a magical spy sport
- Wealth of information about the magical eye
- Right dB measurement with eye tube
- How DM70 operates
- Demonstration and video explanation
- Tunograph, first introduced in 1933 (data sheet)
Source of the article : Wikipedia
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