you old enough to remember, or have you ever heard about the "cat's
eye" on old tube radios that was used for fine tuning stations? The
"eye" was generated by a special type of electron-ray vacuum tube like
this 6E5 from RCA. A fluorescent disk at the top of the tube was caused
to glow depending on a control voltage. The electron-ray tube had to
be mounted horizontally in the chassis so that the "eye" was visible
from the front of the chassis. This article from the January 1955 edition
of Popular Electronics also describes how the electron-ray tube can
be used as a voltmeter.
January 1955 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
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The Tuning Eye - How It Works
By E. Bukstein
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.
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.
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.
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
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
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. END
Fig. 4. (A) The ray-control electrode is connected
internally to the plate of the triode. The voltage drop
resistor R makes the ray-control electrode
negative with respect
to target. (B) When the
electron-ray tube is used as a tuning indicator.
grid is connected to the a.v.c, line. When station is
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
the amount of voltage applied to the grid the
tube can be used as a voltmeter.