February 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.
Even with the domination of LED,
plasma, and LCD displays, there are still a whole lot of cathode ray tubes (CRTs) on
the job. Hobbyists workbenches are filled with them for sure, but design and manufacturing
facilities still have huge inventories of test equipment with CRTs, and a lot of computer
equipment on the production line with CRTs sitting in racks. LED, LCD, and plasma displays
all have their own claims to genius on the part of their designers, but cathode ray tube
designers - and the designers of the driver circuits - deserve special recognition. Consider
the physics and materials involved: glass, phosphor, magnetics, thermonics, electrostatics
and electrodynamics, relativity (electrons traveling at relativistic speeds gain mass,
requiring stronger deflection fields). This article from the February 1955 edition of
Popular Electronics provides a look into the CRT from a layman's perspective.
How the Cathode-Ray Tube Works
This analysis of both electrostatic and magnetic type tubes provides better
understanding of TV picture tube operation
Fig. 1. (top) - The larger of these electrostatic cathode-ray tubes
is made of glass and measures 5 inches. The smaller is a 1 inch metal tube.
Fig. 2. (bottom) - Construction details of an electrostatic type cathode-ray
To many, the cathode-ray tube is symbolic of the entire television industry. There
is little question that if it were not for this tube, the industrial giant of modern
television could never have come into being. However, while it is true that a cathode-ray
tube is used in every TV receiver and that thousands of others are used in TV broadcast
stations, its use is not limited to this field alone. These tubes are used in radar,
in medical electronic apparatus, in test equipment, in atomic research, in industrial
electronic equipment, in fact, in all phases of present-day industry and science. To
understand the operation of the cathode-ray tube, then, is to be in a better position
to understand the entire field of modern industrial technology.
Fundamentally, the cathode-ray tube (or CRT) is nothing more than a special type of
indicating device. Just as a loudspeaker converts electrical signals into sound vibrations,
and a meter changes electrical energy into the mechanical movement of a needle pointer,
the CRT is an instrument used to change electrical signals into patterns of light.
Cathode-ray tubes are made in numerous sizes and shapes and for many special purposes.
However, most cathode-ray tubes can generally be divided into two broad classes -"electrostatic"
tubes and "electromagnetic" tubes. This classification is based on the means used for
deflecting (moving) the electron beam which "paints" the light pattern on a fluorescent
screen. Each class of tube has certain advantages as well as certain limitations, and
these advantages and limitations have resulted in each type of tube being used in specific
Electrostatic tubes use an electrostatic field for controlling the electron beam.
This field is built up between a pair of electrodes called "deflection plates" by the
application of moderate a.c. and d.c. voltages. Electrostatic tubes are generally made
in small sizes, with screens from one inch to about ten inches in diameter. They have
a good frequency response and are widely used in cathode-ray oscilloscopes, medical electronic
equipment, industrial equipment, and in some types of radar work.
Electromagnetic tubes use a magnetic field for controlling the stream of electrons.
Two pairs of coils are used for building up this magnetic field. The coils are external
to the tube proper and are generally mounted in a single assembly called a "deflection
yoke." Electromagnetic tubes are made with screen sizes from five inches to thirty inches
in diameter. Although they have a comparatively narrow frequency response, these tubes
do permit the formation of large size, sharply focused, bright images and are used in
tremendous quantities in television receivers and as indicators in radar systems. Virtually
all modern TV receivers employ electromagnetic tubes.
Fig. 3 - Photographic illustration of the electron gun assembly of
Fig. 4 - In an electromagnetic type of tube, deflection coils are
placed on the outside.
Fiq. 5 - The photograph shows a typical deflection yoke of a modem
Fig. 6 - This industrial multi-gun cathode-ray tube has five electron
gun and deflection assemblies.
Since most people find it easier to understand the operation of the electrostatic
cathode-ray tube, we shall discuss this type first. Once a clear understanding of the
electrostatic tube has been acquired, the electromagnetic tube is easy to master. The
basic construction of an electrostatic CRT is shown in simplified form in Fig. 2.
An a.c. voltage is applied to the filament of the tube, heating it to a bright glowing
red. The filament, in turn, heats the cathode which is placed close to it. The cathode
is covered with materials which "boil out" electrons when heated, and these electrons
gather in a cloud close to the cathode.
A high positive voltage is placed on the accelerator anode and this voltage attracts
the negatively charged electrons, causing them to move in a stream toward the front of
the tube. The electron stream passes through a narrow hole in a cylindrical shaped electrode
called the grid. Although not shaped like a conventional grid, it is given this name
after the grid in an amplifier type vacuum tube because it serves the same function...
it controls the number of electrons which can pass through.
The electron stream next passes through a group of two or three cylindrical electrodes
which have different d.c. voltages applied to them. The accelerating anode may be one
of this group. The d.c. voltages applied as well as the shape and size of these electrodes
set up an electrostatic field which narrows the electron stream and focuses it into a
sharp beam. Because of this action, one of the electrodes may be termed the focusing
Next, the sharply focused electron beam passes through a pair of flat electrodes arranged
in a horizontal plane with respect to the tube. If a d.c. voltage is applied to these
plates, the electron beam will be attracted toward the more positive plate and repelled
from the negative plate, bending either up or down, depending on how the d.c. voltage
is applied. If an a.c. voltage is applied, the beam will move up and down alternately.
Since the beam moves in a vertical direction, these electrodes are called the vertical
defection plates. The horizontal defection plates are a similar pair of electrodes, but
arranged at right angles to the vertical plates, and serve to move the beam either to
the left or right.
After passing through the deflection plates, the electron beam goes on to strike the
front of the tube, which has been covered with chemicals which glow when struck by the
electrons. This action is termed "fluorescence" and, therefore, the film of chemicals
on the face of the CRT is called a fluorescent screen. The choice of chemicals used in
making the fluorescent screen determines the color of the glow and how long the glow
continues after the electron beam strikes the screen. This latter characteristic is called
the persistence of the screen.
Most of the cathode-ray tubes used in oscilloscopes have a medium persistence green
screen. Zinc orthosilicate is frequently used for such screens. Television receivers
generally employ a medium persistence white screen. Various mixtures may be used to produce
a white screen, including a combination of zinc sulphide and zinc beryllium silicate.
For high speed photography, a CRT using a short persistence blue screen is desirable.
Such screens may be made from calcium tungstate.
Since the purpose of a cathode-ray tube is to obtain a pattern of light on a screen,
the fluorescent screen is often considered one of the most important parts of the tube.
This is shown by the fact that a tube's size is given in terms of its screen diameter.
A "seven inch tube" has a screen with a diameter of approximately seven inches.
In a cathode-ray tube, the assembly of electrodes which produces the stream of electrons,
not including the deflection plates, is called the "electron gun." The electron gun of
a typical CRT is shown in Fig. 3.
An electromagnetic CRT is somewhat simpler. in construction than an electrostatic
tube since it does not have deflection plates. However, it still has an electron gun
assembly, although the focusing anodes may be missing. The beam of electrons sent out
by the electron gun is deflected by a magnetic field set up, in turn, by two pairs of
curved coils mounted around the neck of the tube close to the bulge of the "funnel."
See Fig. 4. The coil assembly, or deflection yoke, of a typical television receiver is
shown in Fig. 5. Note how the two sets of coils are mounted at right angles to each other.
Electromagnetic cathode-ray tubes which do not have focusing electrodes in their electron
gun assembly employ an external magnet to produce a magnetic focusing field. Such magnets
may be either electromagnets or permanent magnets, or a combination of both. Thus, electromagnetic
tubes may be subdivided into two smaller classes; those employing electrostatic focusing
and magnetic deflection and those employing both magnetic focusing and magnetic deflection.
The shape of the light pattern formed on the screen of a CRT depends on the type of
electrical signals applied to the deflection elements (whether coils or electrostatic
plates). If steady d.c. voltages are applied, a dot will appear on the screen, with its
exact position determined by the relative sizes of the applied deflection voltages. If
a.c, signals are applied, a line or pattern will be formed, with its shape and size determined
by the electrical waveforms and amplitudes of the a.c. signals. In a television receiver,
the light pattern forms a raster made up of a series of horizontal lines.
Varying the voltage applied to the grid electrode in the electron gun assembly will
change the number of electrons that can pass through and strike the screen and hence
the instantaneous brightness of the glow. In a television receiver, the video signal
is applied to the grid-cathode circuit of the CRT and changes the evenly glowing raster
into a pattern of light and dark segments which, in turn, makes up the picture.
The vast majority of present day cathode-ray tubes employ a single electron gun assembly
and a single set of deflection elements. However, tubes have been made with a number
of electron guns, including some of the tubes designed for color television. Cathode-ray
tubes have also been made with several complete electron guns plus deflection elements.
Such tubes arc virtually several independent cathode-ray tubes with a single screen.
One such tube is shown in Fig. 6.
Special purpose cathode-ray tubes, color television picture tubes, TV camera tubes,
and other types of cathode-ray tubes will be discussed in other issues of POPULAR ELECTRONICS.
Posted December 6 (original August 17, 2011)