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Magic eye / Tuning eye
The Tuning Eye - How it works (By E Bukstein)
Popular Electronics - January 1955
The electron-ray tube, or as it is more familiarly known, the "tuning eye," is a voltage indicator which, in many applications, replaces the far less rugged and far more costly meter movement. As shown in the drawing of Fig. 1, the plate of the tuning eye tube is circular and is known as the "target." This portion of the tube structure is coated with a fluorescent chemical which glows a vivid green color when it is bombarded by electrons from the cathode. Viewed from the top of the tube (which is usually mounted in a horizontal position so that the top, in reality, becomes the front) the target appears as a ring of green light. The dark disk in the center is a shield to block light from the cathode.
The thin, vertical wire parallel to the cathode is known as the "ray-control electrode." If this electrode is made negative with respect to the target, it will repel some of the electrons. In this way, that portion of the target which does not receive electrons will not glow and will appear as a dark area or a shadow. The more negative the ray-control electrode is made with respect to the target, the wider this shadow becomes. This action is demonstrated diagrammatically in Fig. 2.
Fig. 1. The target of the electron-ray tube is coated with a fluorescent chemical which glows when bombarded by electrons from the cathode. Photo courtesy of RCA.
Fig. 2. (A) The top view represents the top view of the tube (for simplicity, only target, cathode, and ray-control electrode are shown). The dotted arrows indicate electron paths from cathode to target. Ray-control electrode is negative and therefore repels electrons. That part of the target which receives no electrons does not glow. The appearance of the shadow is shown in the lower drawing. (B) Here the ray-control electrode is less negative than in (A). Electrons are not repelled as much and shadow is narrower. (C) When ray-control electrode is at same potential as target, electrons are not repelled, there is no shadow.
Fig. 3. The electron-ray tube, in many applications, replaces a more delicate meter.
Most electron-ray tubes contain a triode amplifier, housed within the same glass envelope. The plate of the amplifier is internally connected to the ray-control electrode as shown in the diagram of Fig. 4A. The plate current of this triode flows through resistor R and produces a voltage drop of the polarity indicated on the diagram. This drop makes the ray-control electrode negative with respect to the target and therefore produces a shadow. The greater the triode plate current, the greater the voltage drop across resistor R and the wider the shadow becomes. If the triode is biased to cut-off, there will be no drop across resistor R and thus no shadow on the target.
When the electron-ray tube is to be used as a tuning indicator for a broadcast-type receiver, its grid is connected to the a.v.c. (automatic volume control) line as shown in Fig. 4B. When a station is properly tuned in on the broadcast receiver, the a.v.c. voltage will be at its maximum negative value. This negative voltage will then cut off the triode section of the tuning eye tube and there will be no shadow. However, if the receiver is mistuned or is "off station" to one side or the other, the a.v.c. voltage will be reduced. With less negative voltage on its grid, the triode will now draw plate current and produce a voltage drop across resistor R. Under these conditions, a shadow will appear on the target portion of the tube.
Since the width of the shadow appearing on the target of the tube depends on the amount of voltage that is applied to the tube's grid, the electron-ray tube can be used as a simple and rugged voltmeter.
An arrangement of this type is shown in Fig. 5. The type 6E5 electron-ray tube, which is operated with a 1 megohm plate resistor and a 125 volt power supply, will give a zero shadow with -4 volts grid bias. At zero bias, the shadow angle will be 90 degrees. The variable resistor in the cathode circuit serves to bias the tube to cut-off and thus functions as the zero adjustment. The voltage divider and tap switch in the grid circuit provide the three ranges.
The electron-ray tube will serve as a convenient indicator for alignment purposes in receivers which incorporate it. One type of indicator often used for alignment is a d.c. voltmeter to measure the a.v.c. voltage. Only a relative indication of this voltage is needed. The electron-ray tube as connected in a receiver gives just that.
(A) The ray-control electrode is connected internally to the plate of the triode. The voltage drop across resistor R makes the ray-control electrode negative with respect to target.
(B) When the electron-ray tube is used as a tuning indicator, its grid is connected to the a.v.c, line. When station is properly tuned in. a.v.c, voltage biases the grid to cut-off and the shadow on the eye tube disappears.
Fig. 5. Since the width of the shadow depends on the amount of voltage applied to the grid the electron-ray tube can be used as a voltmeter.