Magic eye tube
Encyclopedia
A magic eye tube gives a visual indication for audio output, radio-frequency signal strength, or other functions. It is also called a cat's eye, or tuning eye tube. These tubes were used by radio receivers
from around 1936 onwards, replacing the earlier "Tuneon" neon lamp
type tuning indicators, as a tuning aid.
(who spent most of the 1930s improving the lifetime of cathode ray tube
s, and ultimately formed the DuMont Television Network
), it is a miniature cathode ray tube, usually with a built-in triode signal amplifier. It usually glows bright green, and the unlit segment on the display narrows as the voltage on a control grid
increases. It is used in a circuit that drives the grid with a voltage that changes with signal strength; as the tuning knob is turned, the eye display becomes narrowest when a station is tuned in correctly.
These devices were basically made in two forms – an end-viewed type usually with an octal or side-contact base, and a smaller side-viewed noval B9A based all-glass type. The round cone-shaped fluorescent screen together with the black cap that shielded the red light from the cathode/heater assembly was what prompted the contemporary advertisers to coin the term "Magic Eye", a term still used.
There was a 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 6.3V heater supply via a 220-ohm resistor. One or two small reel-to-reel tape recorders also used the DM70/DM71 to indicate recording level, including a transistorised model with the valve lit from the bias-oscillator voltage.
(AGC) action tends to increase the audio volume of a mistuned station, so that the volume varies relatively little as the tuning knob is turned. The tuning eye was driven by the AGC voltage rather than the audio signal.
When, in the early 1950s, FM radio sets were made available on the UK market, many different types of magic eye were made available, with differing displays, but they all worked the same way. Some eyes had a separate small display to light up indicating a stereo
signal on FM.
The device consists of a valve with two electrode assemblies, a triode amplifier and a display section consisting of a conical-shaped target anode coated with zinc-silicate or similar. The display section's anode was usually directly connected to the receiver's full HT+ voltage, whilst the triode-anode was (usually internally) connected to a control electrode mounted between cathode and the target-anode, and externally connected to HT+ via a high-value resistor, 1 megohm typically.
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 150-200V 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 AGC line in an AM superhet, or the limiter stage or FM detector in an FM receiver (see circuit extract shown). As a station is tuned in the triode-grid becomes more negative with respect to the common cathode.
s.
A magic eye tube is essentially an inexpensive uncalibrated and not necessarily linear voltage indicator, and can be used wherever an indication of voltage is needed, saving the cost of a more accurate calibrated meter
.
The functionality of a magic eye can be achieved with modern semiconductor
circuitry and optoelectronic displays. The 100-volt-plus voltages required by magic eyes are not present in modern devices, so the magic eye is now obsolete.
Receiver (radio)
A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...
from around 1936 onwards, replacing the earlier "Tuneon" neon lamp
Neon lamp
A neon lamp is a miniature gas discharge lamp that typically contains neon gas at a low pressure in a glass capsule. Only a thin region adjacent to the electrodes glows in these lamps, which distinguishes them from the much longer and brighter neon tubes used for signage...
type tuning indicators, as a tuning aid.
History
Invented in 1937 by Dr. Allen B. DuMontAllen B. DuMont
Allen Balcom DuMont also spelled Du Mont, was an American scientist and inventor best known for improvements to the cathode ray tube in 1931 for use in television receivers. Seven years later he manufactured and sold the first commercially practical television set to the public...
(who spent most of the 1930s improving the lifetime of cathode ray tube
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
s, and ultimately formed the DuMont Television Network
DuMont Television Network
The DuMont Television Network, also known as the DuMont Network, DuMont, Du Mont, or Dumont was one of the world's pioneer commercial television networks, rivalling NBC for the distinction of being first overall. It began operation in the United States in 1946. It was owned by DuMont...
), it is a miniature cathode ray tube, usually with a built-in triode signal amplifier. It usually glows bright green, and the unlit segment on the display narrows as the voltage on a control grid
Control grid
The control grid is an electrode used in thermionic valves used to modulate the flow of electrons in the cathode to anode or plate circuit.- Operation :...
increases. It is used in a circuit that drives the grid with a voltage that changes with signal strength; as the tuning knob is turned, the eye display becomes narrowest when a station is tuned in correctly.
These devices were basically made in two forms – an end-viewed type usually with an octal or side-contact base, and a smaller side-viewed noval B9A based all-glass type. The round cone-shaped fluorescent screen together with the black cap that shielded the red light from the cathode/heater assembly was what prompted the contemporary advertisers to coin the term "Magic Eye", a term still used.
There was a 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 6.3V heater supply via a 220-ohm resistor. One or two small reel-to-reel tape recorders also used the DM70/DM71 to indicate recording level, including a transistorised model with the valve lit from the bias-oscillator voltage.
Use in radios
The purpose of tuning eyes in radio sets is to help tune a station in; the visual aid of the tube makes variations in signal strength more obvious than by ear alone, because the automatic gain controlAutomatic gain control
Automatic gain control is an adaptive system found in many electronic devices. The average output signal level is fed back to adjust the gain to an appropriate level for a range of input signal levels...
(AGC) action tends to increase the audio volume of a mistuned station, so that the volume varies relatively little as the tuning knob is turned. The tuning eye was driven by the AGC voltage rather than the audio signal.
When, in the early 1950s, FM radio sets were made available on the UK market, many different types of magic eye were made available, with differing displays, but they all worked the same way. Some eyes had a separate small display to light up indicating a stereo
STEREO
STEREO is a solar observation mission. Two nearly identical spacecraft were launched into orbits that cause them to respectively pull farther ahead of and fall gradually behind the Earth...
signal on FM.
The device consists of a valve with two electrode assemblies, a triode amplifier and a display section consisting of a conical-shaped target anode coated with zinc-silicate or similar. The display section's anode was usually directly connected to the receiver's full HT+ voltage, whilst the triode-anode was (usually internally) connected to a control electrode mounted between cathode and the target-anode, and externally connected to HT+ via a high-value resistor, 1 megohm typically.
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 150-200V 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 AGC line in an AM superhet, or the limiter stage or FM detector in an FM receiver (see circuit extract shown). As a station is tuned in the triode-grid becomes more negative with respect to the common cathode.
Other applications
Magic eye tubes were used as the recording level indicator for tape recorderTape recorder
An audio tape recorder, tape deck, reel-to-reel tape deck, cassette deck or tape machine is an audio storage device that records and plays back sounds, including articulated voices, usually using magnetic tape, either wound on a reel or in a cassette, for storage...
s.
A magic eye tube is essentially an inexpensive uncalibrated and not necessarily linear voltage indicator, and can be used wherever an indication of voltage is needed, saving the cost of a more accurate calibrated meter
Voltmeter
A voltmeter is an instrument used for measuring electrical potential difference between two points in an electric circuit. Analog voltmeters move a pointer across a scale in proportion to the voltage of the circuit; digital voltmeters give a numerical display of voltage by use of an analog to...
.
The functionality of a magic eye can be achieved with modern semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
circuitry and optoelectronic displays. The 100-volt-plus voltages required by magic eyes are not present in modern devices, so the magic eye is now obsolete.
External links
- A more technical description
- Magic eye tubes in action and solid-state magic eye recreation
- Wealth of information on the magic eye
- Precise dB measurements with eye tubes