Magic eye (radio)
Magic eye is the colloquial term for a tuning indicator tube , a special electron tube that shows the strength of a signal according to the principle of the bar graph display as a light bar or sector and is often built into older radios and televisions .
Also, tape recorders , audio amplifiers and other equipment had a magical eye as a level indicator .
In receiving devices equipped with electron tubes from the mid-20th century to the 1970s, both the "magic eye" and the later emerging "magic tape" show how precisely the device is set to the transmission frequency of the transmitter to be received. The display tube takes on the function of a tuning display , for which the control voltage of the receiver is usually used directly. The reason for using this (simple) display is that control devices such as phase-locked loops for automatic setting, balancing and continuous readjustment of the receiving frequency were available for receiving devices such as radios at that time , but these were technically so complex and expensive that only luxury devices could use them were equipped.
Forms and Applications
Depending on the design of the display, a distinction is made between magic fan , magic ribbon and magic balance . Originally the magic eye had a round fluorescent screen, which explains the term "eye"; however, the functional principle is always the same, only the type of representation is different.
In the case of tape recorders, the level of the recording was set based on the deflection of such a display tube (usually tape or fan) . With older amplifiers (e.g. so-called orchestral amplifiers), such displays could be used to identify the signal level or to determine an overload. Today, VU meters ( moving coil measuring units ) or LED bar displays are used for this.
If a transmitter is poorly tuned or the signal is low (e.g. in the case of a bar-shaped display, i.e. magic band), the non-luminous area between the luminous bars is extended over the entire width. The cleaner the reception or the higher the signal, the narrower the dark area between the bars becomes and disappears completely with a strong transmitter or when the signal is reached. When overloaded, the bright areas on both sides overlap and lead to a strip of double light intensity, which serves as a clear indicator that the level should be reduced accordingly.
Semicircular "eyes" (i.e. magic fans, e.g. EM80, EM81 or EM85) show a fanning out when reception is poor, while only the center appears bright when reception is better.
Completely round displays are so-called real magic eyes : they show four circular sectors in a cross shape , the angle of which changes (e.g. EM35). With weak control, an X similar to a St. Andrew's cross is visible, with full control a wide cross or almost a full circle. There are variants (e.g. EM11, EM34) that have two light angles of different sensitivity. So the receiver with the more sensitive light angle can be perfectly tuned to weak transmitters, while with the less sensitive light angle it can be tuned to strong transmitters (the more sensitive part appears to be fully controlled). Technically, this was implemented with different steepnesses of the amplifier triodes integrated for each variable light angle.
There are round magic eyes in which there are only two circular sector-shaped luminous surfaces, the angle of which increases when fully controlled (e.g. EM34), as well as designs with only one angle (e.g. 6E5).
- Different versions of the magic eye
Various tubes, EM84, EM80, UM11
The eye of an EM11
The light fan of an EM80
The "magic ribbon" of an EM84
(often also installed horizontally)
A pair of EM84s in operation, on the left in a fully controlled state
functionality
Voting indicator tubes are electron tubes whose electron current is directed onto a fluorescent layer (usually made of pure zinc orthosilicate ( willemite ) or a mixture of willemite and zinc oxide ). This always shone in green or blue-green light and was applied to the correspondingly shaped anode or inside on the glass bulb.
In newer designs, the luminescent layer is no longer applied to a metallic support inside the tube, but directly to the inner glass surface of the tube bulb. The luminescent layer was no longer made of pure zinc orthosilicate, but of willemite with an admixture of zinc oxide. Pure willemite loses its luminosity relatively quickly due to the electron bombardment, while the zinc oxide even gains luminosity due to this treatment. This phosphor mixture shows a blue-green cathodoluminescence , while pure willemite here emits a deep green .
The directional deflection takes place with control electrodes (webs), which form the electron stream emanating from the cathode into bundles or just push them apart (gap formation, e.g. in magic ribbons).
In order for the electrons to flow off the crystalline, non-conductive luminescent layer better, a graphite layer ( Aquadag ) was often applied under the luminescent layer. For tubes with a luminous layer on the glass, transparent conductive oxide layers are used.
However, there were also tuning displays based on special glow lamps in which the cathode coverage could be changed with a control electrode like a bar graph .
Examples
| Type | Idle state | Controlled |
|---|---|---|
| EM71 |
 |
 |
| EM80 |
 |
 |
| EM85 |
 |
 |
Well-known types of the E series (the E means 6.3 V heating voltage):
- Eye: EM4, EM11, EM34 , EM35
- Compartments: EM71, EM80, EM85
- Volume: EM83 , EM84 , EM87, EMM801, EMM803
The types EM83 / 84/87/800 (magic tape) and the EM80 (compartments) are still available today as new parts, but the types EM34 and EM35 are very rare and therefore expensive.
Many types of the corresponding U-version (series heater 100 mA) are still available, for example the UM 80/81/84. The UM 11, which was often used in the past, has become quite rare.
literature
- Werner Espe: Materials science of high vacuum technology . tape 1 : metals . VEB Deutscher Verlag der Wissenschaften, Berlin 1961.
- Werner Espe: Materials science of high vacuum technology . tape 3 : auxiliary materials . VEB Deutscher Verlag der Wissenschaften, Berlin 1957.
Web links
- Japanese Antique Vacuum Tube Radios
- Magic eyes - the display tubes
- Magic eyes / voting indicators - history and technology
Individual evidence
- ↑ Werner Espe: Materials science of high vacuum technology. Volume III, pp. 190ff.