August 1963 Popular Electronics
People old and young enjoy waxing nostalgic about and learning some of the history
of early electronics. Popular Electronics was published from October 1954 through April 1985. All copyrights
are hereby acknowledged. See all articles from
Magnetron, photomultiplier, traveling wave, compactron, klystron,
backward wave, pencil, lighthouse, cathode ray, indicator, nuvistor,
acorn, peanut, T-R, electrostatic, cat's-eye, orithon, and loctal,
are just a few of the many types of vacuum tubes that have been
and in some cases still are in use in various types of electronic
equipment. Some you have heard of, others you probably have not.
All are discussed in a series of three articles published in Popular
Electronics. This is part 3, which includes operational descriptions
of klystrons, magnetrons (in your microwave
oven), and traveling wave tubes (radar &
satellite communication), all of which are still designed
into new products today.
Here is a great webpage for
magnetron operational theory that includes animations.
Here you can read
Part 2 and
Part 3. See all articles from
The Tube Family Tree
By Louis E. Garner, Jr.
amplify weak signals, but they also do many other vital electronic
jobs, as this final part shows
What about the future of the vacuum tube? Will designers continue
to develop tubes based on new principles, and improve tubes employing
already-known ones? The answer to this question is probably "yes,"
and a good look at the types discussed in this final portion of
the tube "family tree" should convince anyone that the end is certainly
not in sight. The family history of the cathode-ray tube alone ably
illustrates how present-day tubes are built on past developments
and discoveries. The first ancestor of the CRT was in actual operation
in 1897, nine years before De Forest's triode put amplification
into the hands of electronic researchers. Even so, practical television
had to await development of more sophisticated CRT's, and particularly
that much more unprecedented invention, the camera tube.
Camera Tubes. Used in television cameras, these
tubes produce a video signal corresponding to the light image of
a picture or scene which is to be televised, recorded on tape, or
transmitted over a wired installation. In general, camera tubes
consist of an electron gun assembly, a means for deflecting the
electron beam in a regular pattern. and a photosensitive target
assembly of some type which can convert light patterns into electrical
charges or signals when struck by the electron beam. The electron
guns and deflection techniques used with camera tubes are basically
like those used with display-type CRT's.
The iconoscope was, for a number of years, the most practical
type of camera tube. The heart of this device is a sensitized target
on which the scene to be televised is focused by a lens system.
The target is of sandwich-like construction and consists of a conductive
coating or signal plate, a layer of insulation, usually mica, and
a mosaic pattern of tiny photoemissive globules. Each globule acts
somewhat like a miniature photocell.
When light strikes a globule, it emits electrons to a greater
or lesser extent depending on the intensity of the light. These
electrons are picked up by a collector ring in front of the photo-mosaic,
leaving a greater or lesser positive charge on each globule. The
globules and the insulated signal plate behind them thus act as
a capacitor. As the beam from the iconoscope's electron gun strikes
each globule, it restores the electrons lost by photoemission, and,
in effect, discharges the capacitor, developing an output signal
at the signal plate. The signal corresponds to the image of the
scene being televised.
The image orthicon is one of the most complex, precise vacuum
tubes in common use.
The Image Orthicon. Today, the most sensitive
and widely used type of camera tube is the image orthicon. It is
made up of three principal parts: an image section; a scanning section;
and an electron multiplier. The image section of the tube contains
a semitransparent photocathode on the inside of the glass faceplate,
an accelerating grid (No.6), and a target which consists of a thin
glass disc with a fine mesh collector screen positioned very close
to it on the photocathode (front) side. Focusing is accomplished
by means of a magnetic field produced by an external coil and by
the proper selection of photocathode and accelerating grid voltages.
In operation, the scene to be televised is focused on the photocathode,
which emits electrons proportional to the intensity of the light
striking each of its areas. The electron streams are focused on
the target and cause secondary electrons to be emitted by the glass
disc. These secondary electrons are collected by the adjacent wire
mesh, leaving the photocathode side of the disc with a pattern of
positive charges corresponding exactly to the image being televised.
Since the glass is quite thin, a similar charge pattern is set up
on its opposite side.
The glass is scanned by a low-velocity electron beam produced
by an electron gun and deflected by external electromagnetic coils.
This beam is made to decelerate and to approach the glass vertically
at essentially zero velocity by the potentials applied to the decelerator
grid (No.5) and the field mesh. Some of the electrons are deposited
on the glass to neutralize its positive charge while others are
repelled to form a return beam. The return electron beam, then,
is modulated by the positive charge pattern on the target and hence
in accordance with the original light image.
On coming back to the electron gun area, the return beam passes
through a five-stage electron multiplier similar to that employed
in photomultiplier tubes, developing an output video signal. The
dynodes in the multiplier section may amplify the modulated beam
by 500 times or more, with the result that the image orthicon is
basically more sensitive than the human eye in picking up faint
The monoscope is a special type of camera tube. Its basic principle
of operation is similar to that of other CRT's, for it incorporates
essentially the same type of electron gun and deflection systems.
However, it is fitted with a permanently installed fixed pattern
- such as a TV test pattern - and develops only a repetitive video
Special CRTs. In addition to the cathode-ray
tubes we've discussed, there are a number of special types which
depend on electron beams for their operation. Among these are a
variety of discharge and demonstration tubes used for classroom
study and laboratory experiments, but by far the most common type
is the X-ray tube.
The light-house design raises the upper frequency limit
for conventional negative-grid vacuum-tube amplifiers.
High transconductance, close element spacing, and very low
lead inductance are the design factors responsible for the
good performance of the light-house tube at frequencies
up to 3000 mc.
The basic X-ray tube consists of two principal electrodes: an
electron source (cathode) , and a target anode. The anode is of
dense metal and set at an angle with respect to the electron source.
In operation, extremely high voltages are applied to the two electrodes,
accelerating the electron stream to tremendous velocities. On striking
the target anode, the electron beam excites the metal atoms, causing
them to emit ultra-short electromagnetic radiation - X rays. Since
the target is set at an angle, the X rays are radiated out through
the side of the tube's glass envelope, where they can be photographed
and used to trace in outline the interior make-up of solid matter.
UHF Tubes. Conventional receiving and transmitting
electron tubes cannot be used effectively at ultra-high and super-high
radio frequencies, that is, from five hundred to tens of thousands
of megacycles. At these frequencies, short lead lengths begin to
have considerable inductive reactance and act like. coils or even
r.f. chokes, minute inter-electrode capacities become short circuits,
and even the time required for an electron to move from a cathode
to a plate may represent several cycles of the frequency to be handled.
When even higher frequencies are considered, familiar tuned circuits
cannot be used and are replaced by resonant cavities - essentially
hollow, metal-enclosed spaces which behave like tuned circuits.
Tube manufacturers have designed a number of special tubes for
use at extremely high frequencies. In general, these tubes have
close electrode spacing to reduce electron transit time and, often,
disc-shaped electrodes to reduce terminal lead inductance. Interelectrode
capacities are minimized by keeping electrode supports small and
shaping them for maximum spacing with respect to other tube elements.
Due to their construction, many UHF tubes take on strange and unusual
shapes, and are often named after their physical appearance. Thus,
one firm may offer long, slim "pencil" tubes, while others produce
stepped tower-like "light-house" tubes, and so on. Quite frequently,
the tubes are manufactured with resonant cavities as an integral
part of their structure.
One pencil-type triode oscillator tube, the RCA 7533, is made
with two built-in resonant cavities, one between grid and cathode
and another between grid and plate. The tube looks very much like
a small can. Designed for use as an oscillator in the 1660-1700
mc. band, the 7533 has a plate dissipation rating of 3.6 watts and
can deliver approximately 500 milliwatts.
Another interesting UHF tube is the RCA 7457, a beam power type
which can be used at frequencies up to 2000 mc. With a maximum plate
dissipation rating of 115 watts, it can handle input powers as high
as 180 watts up to 1215 mc. Used as a class C amplifier with 900
volts on its plate, it can deliver approximately 40 watts at 1215
mc. It is designed for forced-air cooling and has a built-in finned
radiator. In general, the 7457 is used with external cavities, coaxial-cylinder,
or parallel line circuits.
The GE GL-6299 is a co-planar triode suitable for use as an amplifier
at frequencies as high as 3000 mc. Designed for use in receivers,
it has an extremely low noise rating. As a rule, it is used with
external cavities or coaxial circuits. Of ceramic construction,
the GL-6299 generally resembles the "light-house" tube of a few
The UHF tubes we've just examined, as well as many similar types,
operate on the same principles as more conventional electron tubes,
except for their frequency of operation and the types of tuned circuits
with which they are used. In addition, however, there is a group
of high-frequency tubes which operate on entirely different principles:
magnetrons, klystrons, traveling-wave tubes, and related types.
We'll examine these next.
Magnetron operation depends upon interaction between the
electron stream and a strong, constant magnetic field.
For efficient magnetron operation, a definite relationship
between plate voltage and magnetic field strength in the
interaction space must always be maintained.
The multi-cavity magnetron has advantages that are important
in practical radar applications.
The Magnetron. Although used extensively since
World War II as high-power oscillators in radar transmitters and
other types of ultra-high frequency equipment, the magnetron is
basically a diode. In its common form, it consists of a coaxial
cathode and a circular anode (or plate) which may, or may not, be
split into two or more segments. This assembly is placed between
the poles of a powerful permanent or electromagnet and aligned so
that the magnetic field is coaxial with the cathode and plate.
In operation, a high positive voltage is applied to the magnetron's
plate. If it were not for the magnetic field, the electrons emitted
by the cathode would travel in a straight, radial line directly
to the plate. The magnetic field, however, forces the electrons
to travel in a spiral or circular path; and if the field is made
strong enough, most of the electrons swing in complete circles,
returning to the cathode. These high-speed electrons, whizzing by
the plate structure, induce high-frequency currents. To obtain oscillation,
then, a proper balance between anode voltage and magnetic field
strength is needed, for the electron resonance must approximate
that of the resonant cavity formed by the plate structure.
A split-anode magnetron can be made to oscillate at frequencies
much below the natural electron resonant frequency by connecting
the segments to a tuned circuit, such as a tuned line. In higher
frequency types, the tuned circuit may be little more than a heavy
bar of metal connecting the segments together to form a simple closed
loop. Split-anode magnetrons need not be limited to two segments;
four, six, eight, or more segments may be used.
A different type of magnetron employs a solid anode in which
small resonant cavities have been formed. The high-speed electrons
moving past the cavity openings shock the cavities into oscillation.
The action is somewhat analogous to what happens when a person blows
sharply across the open end of a small closed tube to produce a
Commercially available magnetrons operate at frequencies from
a few hundred to as high as 30,000 mc. and can deliver peak output
powers ranging up to 2000 kw. (2 megawatts!) when used as pulse
generators, or hundreds of watts when used as c.w. sources.
The Klystron. In one sense a special type of
cathode-ray tube, for it utilizes an electron gun and a stream of
electrons for its operation, the klystron can be used as an ultra-high
frequency oscillator or amplifier. When first invented, the device
was originally dubbed a rhumbatron, for the electrons were said
to be made to "dance the rhumba" within the tube, since they were
The components of the basic klystron tube include an electron
gun assembly, a pair of closely spaced grids called a "buncher,"
another pair of grids called a "catcher," and an anode or plate
called, in this case, a "collector," since it receives the electron
stream sent down the tube by the gun assembly. There is a narrow
"drift space" between the buncher and catcher grid assemblies.
In operation, the electron beam is aimed down the tube by the
electron gun, and an r.f. voltage is applied to the buncher grid.
As the electrons approach the buncher and pass through it, they
are alternately slowed and speeded up, that is, velocity-modulated.
To visualize how this occurs, consider that the first buncher grid
is momentarily negative and the second positive. Those electrons
which are approaching the first grid are repelled and slowed down.
Those which are between the first and second grids are repelled
by the first and attracted to the second and hence speeded up. Those
which have passed the second grid are attracted "backwards" and
hence slowed down. On the next r.f. half-cycle, when the first grid
is positive and the second negative, the action is reversed.
Thus, the net result is that the electron stream is separated into
tiny bunches corresponding to the applied r.f. frequency. As the
velocity-modulated stream moves along the drift space, the faster
moving electrons in each bunch (or bundle, if you prefer) overtake
the slower moving ones and the bunch, in one sense, becomes "stronger,"
for a greater number of electrons are compacted together. When these
bunches pass the catcher grid assembly, they give up most of their
energy, shock-exciting the tuned circuit into oscillation. Afterwards,
the spent electrons are accumulated by the positive-charged collector.
In practice, klystrons are operated at such high frequencies
that resonant cavities, rather than conventional tuned circuits,
are used to tune the buncher and catcher grids. The electron stream
is generally focused by a strong permanent magnet or electromagnet
placed on the outside of the tube. A tunable klystron can be assembled
by using a bellows-like arrangement for the cavities, permitting
the cavity size to be reduced (to increase frequency) or expanded
(to reduce frequency).
Bunching of groups of electrons as they move through tube
is the basic principle of the klystron.
Klystron plate voltage must be accurate and have good regulation
Since the output signal is much greater than the input signal
applied to the buncher, due to the electron concentration which
takes place in the drift space, the klystron may be used as an amplifier.
It can also be used as an oscillator by coupling the catcher cavity
back to the buncher.
While the two-cavity klystron is basic, it is not the only type
produced. A single-cavity type, called a reflex klystron, uses the
same cavity as both a buncher and catcher; here, a negative voltage
is applied to the collector, repelling the electron stream back
on itself so that it passes the double-grid assembly both "coming"
and "going." More recently, a three-cavity electrostatically focused
klystron has been developed.
Commercially available klystrons operate at frequencies from
a few hundred to over 120,000 mc. (120 gigacycles), delivering output
powers from less than a milliwatt (for receiver applications) to
many watts (for transmitters).
Traveling-Wave Tubes. Utilizing some of the
basic operating principles of both magnetrons and klystrons, traveling-wave
tubes (or, simply TWT's) may be used both as amplifiers and oscillators.
Like the magnetron, these tubes depend on the interaction between
moving electrons and a magnetic field, and, like the klystron, they
employ the principle of velocity-modulation.
The traveling-wave magnetron is one type. Consider the multi-cavity
magnetron in the drawing on page 55. Suppose the circular anode
were split at one point and "straightened out." The result would
be a multi-cavity anode similar to the tube shown on this page (top).
To this we add a plane electrode to serve as a cathode plate, an
electron gun, and a collector, plus a focusing magnetic field (not
shown) to keep the electron stream projected by the gun from actually
touching either the anode or cathode. The anode and plane cathode
form a wave guide. If an r.f. signal is introduced at one end, it
will travel to the other end. Now, if the velocity of the electron
stream is adjusted to match the phase velocity (speed at which a
constant phase progresses) of the electromagnetic wave moving down
the tuned wave guide, the electron stream will be velocity-modulated
and will transfer some of its energy to the traveling wave. The
result, then, is that the output wave collected at the far end is
stronger than the input signal, thus fulfilling the basic condition
A different type of TWT consists of an electron gun, a wire helix,
and a collector. A tube connects the input and output wave guides
at each end of the helix. In operation, a stream of electrons is
sent down the axis of the helix and the input signal is fed in.
The helix, acting as a coiled transmission line, transmits the input
signal to its far end at an axial velocity determined by the ratio
of the pitch to the circumference of the helix. If the electron
stream velocity matches the traveling wave's axial velocity, there
is an interaction between the wave and electrons, transferring energy
from the electron stream to the wave and thus amplifying it. Since
the currents induced in the helix are displacement currents, the
electron stream need not actually touch the helix and hence a strong
magnetic field is generally used to focus the electron stream and
to keep it from diverging over its relatively long path.
The traveling-wave magnetron combines functional principles of
traveling-wave tube and a magnetron.
Traveling-wave tube provides broadband amplification in the 3000
to 50,000 mc. frequency range.
Wave motion in the backward-wave oscillator is in the opposite
direction to wave motion in other traveling-wave tubes, but the
principle is the same.
The backward-wave oscillator tube (or BWO) operates on general
principles similar to those employed in conventional TWT's except
that the traveling wave moves in a direction opposite to that of
the electron stream (hence the name). In one sense, the tube serves
to supply its own "input" signal. Sometimes, a strong transverse
magnetic field is used to bend the electron stream in a circular
path and thus to reduce the overall size of the tube.
Modified versions of magnetrons, klystrons, TWT's and BWO's are
made by a number of manufacturers under special trade names, such
as "Amplitron" and "Stabilotron."
Special-Purpose Tubes. In addition to the basic
electron tube types we've discussed, the tube "family tree" has
one branch which is literally "loaded" with twigs. These are the
special-purpose tubes - those types designed for one or more specific
functions and, therefore, of limited general application. A prime
example is the electronic flash tube used in photographic equipment,
essentially a gas-filled triode with a trigger electrode. A mere
description of all the various special-purpose tubes would fill
a book, so we'll just examine a few representative types.
The voltage-regulator (or VR) tube is a diode filled with an
inert gas having a specific ionizing potential, such as neon, argon
or krypton. In operation, this tube acts like an open circuit until
sufficient voltage is applied to ionize its gas. At this point,
it "fires" and maintains a constant voltage drop, drawing a greater
or lesser current (within its rated limits) as the applied voltage
varies. The purpose is to hold the output or resultant voltage constant.
The gas-filled regulator has a constant voltage drop between
anode and cathode.
The gas-filled radiation detector tube depends on ionization
of gas by high-velocity atomic particles.
Used in radiation detectors, the Geiger counter tube is also
a gas-filled diode. Generally, the tube is a thin metal shell with
a coaxial wire or rod-like electrode. In use, a high d.c. voltage
is applied to the two electrodes. If an alpha or beta particle or
a gamma ray enters the tube, the gas is ionized momentarily, permitting
conduction to take place and delivering a pulse of current. Each
time another radioactive particle enters, the tube delivers another
pulse. These pulses can be amplified and used to drive a loudspeaker
or headphones or, if preferred, fed to an electronic counting circuit.
The number of pulses in a given period of time (that is, the pulse
rate) is proportional to the number of radioactive particles or
rays which enter the tube, and hence to the intensity of the radioactivity
Nixie indicators are gas-filled tubes having cathodes that glow
when passing current.
There are a variety of indicator tubes, with the simple neon
bulb being a prime example. Another type is the Nixie tube. A cold-cathode
gas-filled type, this tube is equipped with a number of cathodes,
each shaped to represent a numeral from 0 to 9. When an ionizing
voltage is applied between one of the cathodes and the common plate,
that cathode glows, rendering the numeral visible. Nixies are used
as read-out devices in computers and counters.
The famous tuning-eye tube is another type of indicator tube.
In its basic form, it consists of a cathode, control electrode,
and fluorescent screen to which a positive voltage is applied. The
electrons leaving the cathode are attracted to the fluorescent target,
causing it to glow. When a negative voltage is applied to the control
electrode, it repels these electrons, leaving a "shadow" on the
target, with the shadow area proportional to the amplitude of the
applied d.c. voltage. Sometimes, a tuning-eye type indicator tube
and triode are combined in one envelope.
Used in computers, counters and similar equipment, the beam-switching
tube is made up of a cathode, beam forming and holding spades, shield
grids, switching grids, output (target) electrodes, and rod-type
permanent magnets. In operation, the electrons emitted by the cathode
are formed into a narrow beam by a combination of magnetic and electrostatic
fields. This beam is held in a fixed position by the potentials
applied to the various electrodes, but can be switched from one
target electrode to another, in rotation, by applying suitable voltages
to the switching grids.
In the beam-switching type of indicator tube, deflector electrodes
direct the electron beam to the desired target anode. Such tubes
may have 20 or more anodes.
Other special-purpose tubes include ionization and thermocouple-type
vacuum-gauge tubes; tubes in which one of the control elements is
mechanically linked to an external pressure button so that the tube
can be used as a mechano-electronic transducer; types with built-in
fixed resistors used as ballast tubes; special T-R (transmit-receive)
tubes to prevent the application of transmitter power to a receiver
when both units share the same antenna system; and many, many other
The Future. Electron tube manufacturers
are constantly seeking ways to improve their tubes and to develop
new types to meet the needs of equipment designers and manufacturers.
Great efforts are being expended in the development of UHF and microwave
tubes. Several firms are working on tubes requiring low operating
voltages which will be competitive with the transistor. One firm
has developed a tube without a filament - it's designed to be used
in an environment so hot that the cathode requires no additional
heating. And an envelope-less tube has been developed for use in
the vacuum of outer space.
How will the tube "family tree" branch out in the future? Even
an educated guess is likely to be wrong. Only two things are certain:
There will be many new types of tubes introduced in the next several
years, and the "tree" will keep right on growing!
Posted April 10, 2014