June 1963 Popular Electronics
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
What is capable of putting out more power, a solid state amplifier
or a vacuum tube amplifier? The simple answer is that given a large
enough array of power combiners, a solid state power amplifier can
theoretically put out as much power as a tube amp, but the complexity
is much, much greater. A single vacuum tube can output RF power
levels in the megawatt range, but even the highest power GaN (gallium
nitride) semiconductor devices do not reach the kilowatt realm.
That is the reason there are still so many vacuum tube transmitters
- the tubes themselves - still being produced today. Equivalent
size solid state transmitters are generally much larger because
of the huge number of individual power amplifier modules and massive
power combiners needed. The great advantage of solid state PA (SSPA)
systems is the elimination of a single point failure. Output power
is degraded gracefully rather than catastrophically and maintenance
is vastly simpler since most individual SSPA modules are hot-pluggable,
meaning the system continues to operate in a crippled state while
failed parts are replaced. There is no new technology on the horizon
that will change the equation, but don't be surprised if some form
of graphene surfaces as the new wonder power amplifier material.
Here you can read
Part 2 and
Part 3. See all articles from
The Tube Family Tree, Part 2
"second generation" of vacuum tubes, offsprings of the first simple
types, found new jobs to be done
By Louis E. Garner, Jr.
In the early days of radio, essentially the same tubes were used
for both transmitters and receivers. Even today, although transmitting
tubes are considered a distinct class, there is a considerable overlap
between higher power receiving and lower power transmitting tube
types - in construction, in design, and in electrical characteristics.
Hams, for example, frequently use receiver power tubes, such as
the 6L6, in their radio transmitters. The low-power transmitting
tube does not differ appreciably in appearance, size or power-handling
capacity from the tubes used as horizontal deflection amplifiers
in television receivers.
There is also a good correlation between transmitting and receiving
tubes as far as generic types are concerned. Both classes can be
divided into such groupings as diodes, triodes, tetrodes, pentodes,
and beam power tubes. Both filamentary and indirectly heated cathodes
are used in each class. The tube electrodes have the same designations
- plate, grid, cathode, and so on - in both. And the same general
characteristic terms are used in describing both.
When we turn to specifics, on the other hand, we find that there
is a considerable difference between transmitting and receiving
tubes. Transmitting types, in general, are constructed of sturdier
materials, and, as a result, are larger, heavier, and more expensive
than their receiving type counterparts.
Two extreme examples may be helpful.
This is a typical transmitting tube, by Westinghouse. Plate
is made of graphite.
Components of a high-power Federal Telephone & Radio
transmitting tube. Electrodes are heavy and special insulation
is used to withstand high voltages and heat; heavy-duty
terminals take care of high currents. Tubes of this general
type may be forced air-cooled or water-cooled. (above and
Designed for forced air-cooling, this Amperex high-power
transmitting tube has a finned radiator fitted over the
Multiplier-type phototube. General Electric version.
The 6AQ5 is a typical beam power receiving tube, while the RCA
2039 is a high-power shielded-grid beam triode transmitting tube.
The basic specifications of the 6AQ5 include: filament voltage,
6.3 volts; filament current, 0.45 amps; peak positive-pulse plate
voltage, 1100.0 volts; peak plate current, 0.115 amps; average plate
current, 0.040 amps; and plate dissipation, 10.0 watts. The same
basic specifications of the 2039 are: filament voltage, 7.3 volts;
filament current, 1140.0 amps; peak positive-pulse plate voltage,
40,000.0 volts; peak plate current, 92.0 amps; average plate current,
5.7 amps; and plate dissipation, 150,000.0 watts. These comparative
specifications emphasize the primary difference between transmitting
and receiving tubes: their power-handling capacity.
To obtain high powers, very high voltages and currents are required.
This means that the tube's electrodes must be very heavy in order
to handle the large currents without melting, and widely separated
to prevent arcing at the high voltages. (Arcing can destroy a tube.)
Special insulation must be used where the electrodes are mounted
to withstand a combination of high heat and tremendous voltages.
And, of course, heavy-duty terminals are needed to handle the currents.
Finally, all of the above construction factors must be taken into
account and balanced against the tube's designed operating frequency
(which may require close spacing) and desired electrical characteristics.
While maximum electrical ratings, amplification factor, mutual
conductance, and similar characteristics are all important, the
transmitting tube's most important single characteristic is probably
its rated maximum plate dissipation. Specified in watts (or kilowatts),
this is directly proportional to the amount of power that the tube
can handle and hence the r.f. power it can deliver.
In practice, the tube's actual plate dissipation is the difference
between its d.c. plate input power (plate voltage multiplied by
average plate current) and its r.f. output power. For example, if
a Class C r.f. power amplifier is 70% efficient and has a d.c. input
of 10 kw. (5000 volts at 2 amperes, say), it will deliver 7 kilowatts
r.f. (approximately) and will have a plate dissipation of 3 kw.
With plate dissipations running into the kilowatt range for some
types of tubes, it is obvious that a means must be provided for
removing the heat generated if the tube is to be kept from melting.
While lower power transmitting tubes are invariably convection air-cooled,
higher power types are either forced air-cooled or water-cooled.
Medium-power tubes are often provided with radiating fins, while
high-power types are equipped with water jackets. The cooling device,
whether a radiating fin system or a water jacket, may be either
an integral part of the tube or a separate accessory.
Industrial Tubes. Except for a few special types,
industrial electron tubes correspond in most ways to their receiving
and transmitting tube counterparts. Low-power receiving types are
used in industrial controls, alarm circuits, counters, protection
devices, and similar equipment, while transmitting types are found
in high-voltage and high-current power supplies, welders, and induction
and dielectric heaters.
In general, industrial receiving type tubes, while basically
similar to ordinary receiving tubes, are usually of sturdier construction
and designed for continuous operation under rigorous physical conditions.
Industrial tubes, as a rule, must have extremely long filament life,
for equipment shutdowns - even for short periods - can be extremely
costly to a manufacturer. In addition, the tubes must be able to
withstand extremes in temperature, shock, and vibration.
Gas-filled tubes are used extensively throughout industry. Thyratrons
and cold-cathode tubes are utilized for motor, electromagnet, and
solenoid control, while mercury vapor rectifier tubes are employed
in heavy-duty d.c. power supplies for electroplating, electrolysis,
and similar work.
There is one type of electron tube that is used in many industrial
applications but which is not, however, found in communications
equipment: the ignitron. Used for high-capacity switching and in
heavy-duty d.c. power supplies for welding, motor control, and certain
electro-chemical processes, the ignitron is basically a special
type of cold-cathode tube in which mercury vapor is produced by
a controlled electric arc. In one sense, it is a type of rectifier.
Some types are capable of handling voltages as high as 20,000 volts
and conducting currents as great as 35,000 amperes for short periods.
In its basic form, the ignitron consists of an evacuated metal
envelope (which may be double-jacketed for water cooling), a pool
of mercury which serves as a cathode, a heavy metal anode, and a
special ignitor of rough-surfaced material which resists "wetting"
by the mercury but which projects into the pool of liquid metal.
In operation, the tube will not conduct until "fired" by current
applied to its ignitor electrode. A moderate-current pulse here
creates high-current densities at the rough points of contact with
the mercury pool, establishing a hot arc which vaporizes the mercury,
filling the tube with vapor and allowing conduction to take place
between the cathode and anode. Afterwards, the anode-cathode current
is sufficient to keep the arc established and to maintain current
Phototubes. When light falls on certain metals
and metallic compounds, such as cesium, cesium oxide, potassium,
and zinc, electrons are emitted from the material's surface. This
photoemissive effect was first noticed, although not fully understood,
by Heinrich Hertz in 1887. Like many early discoveries, this one
eventually led to the development of the phototube: a light-sensitive
electron tube with an electrical output proportional to the amount
of light falling on its sensitized surface.
Phototubes are used extensively in both industrial and commercial
applications - burglar alarms, automatic door openers, electronic
counters, doorway annunciators, safety equipment for industrial
machines, sound motion picture projectors, etc.
The phototube is a special type of cold-cathode diode. The cathode
is generally a semicircular metal plate coated with photo emissive
metallic compounds, the plate a small rod or wire. In operation,
light falling on the cathode causes electrons to be emitted. If
a positive voltage is applied to the plate (or anode), these free
electrons migrate to it, producing a minute output current.
Like human eyes, phototubes differ in their response to light.
While their current output is directly proportional to light intensity,
the current may vary considerably with identical light levels in
different colors. Depending on the types of photoemissive compounds
used, phototubes may be made more sensitive to infrared, ultraviolet,
or to the whole spectrum of visible light. Except for physical construction
and type of lead connections, the chief differences between phototubes
are found in their spectral responses.
Sometimes, a small amount of selected gas will be introduced
in a phototube. The gas ionizes and reduces the tube's internal
cathode-anode resistance, permitting it to deliver a greater current
output for a given cathode illumination. Gas phototubes have a higher
sensitivity than high-vacuum types but are more easily damaged by
excessive voltages and are somewhat less stable.
Photomultipliers. Unfortunately, the current
output of standard phototubes is extremely small - on the order
of a microampere or less at typical illumination levels. This fact
has led to the development of a class of special phototubes called
photomultipliers. Used in scintillation counters, automatic light
dimmers, and in similar applications, photomultipliers make use
of the principle of secondary emission (which we discussed in Part
1) to increase their current output.
The photomultiplier consists of a photoemissive cathode, a series
of secondary anodes called dynodes) and the output anode or plate.
Depending on tube type and physical design, the dynodes may be arranged
in a circle around the cathode, or in parallel lines behind the
cathode, which is tilted at a small angle.
In electrostatic cathode-ray tubes, the electron
deflected and focused by the internal elements.
Cathode-Ray Tubes. By definition, a cathode-ray
tube (CRT) is a device which utilizes cathode "rays," i.e., "rays"
emitted by the device's cathode. Cathode rays are, of course, streams
Details of the ignitron are shown above.
symbol for this industrial tube.
Basic phototube symbols. At left: the standard version.
At right: a photomultiplier.
A typical phototube, by General Electric./td>
Although often considered a relatively modern invention, cathode-ray
tubes are, historically, even older than more familiar electron
tubes. Various types of cathode-ray discharge and display tubes
were used extensively in physics laboratories and schoolrooms before
the turn of the century, and as early as 1897 Karl Braun developed
a cathode-ray display tube very similar to modern television tubes.
TThe "heart" of most present-day CRT's is the electron gun. The
gun is made up of a filament, an indirectly heated cathode, a disc-shaped
control grid, and disc- or cylindrical-shaped focusing and accelerating
grids (or anodes). Its purpose is to produce a sharp stream of accelerated
electrons. The number of electrons in the stream (and hence its
intensity, as well as the brightness of the spot it produces when
it strikes a screen) is controlled by the voltage applied to the
control grid. The beam's sharpness of focus is determined by the
voltage relationships between the focus and accelerating anodes.
In electromagnet CRTs, the beam is focused
and deflected by br>magnetic fields set u around the neck.
Display Tubes. Direct descendants of the early
Braun tube, display CRT's are used extensively in TV receivers and
monitors, oscilloscopes, radar equipment, and in a variety of test
and research instruments. As the name implies, these tubes serve
to display electrical phenomena on a fluorescent screen, either
as a line, pattern, or reproduced picture.
In general, display tubes are made up of an electron gun assembly,
a means for focusing (if not contained within the gun itself) and
deflecting the electron beam, and a fluorescent screen. Manufactured
in sizes ranging from tiny units with a 1" -diameter screen to giant
picture tubes with 30" screens, they are usually funnel-shaped.
The screen itself may be round, square, or rectangular. The envelopes
or "funnels" are made either of metal or glass, or a combination
Most display tubes are identified by a combination numeral-letter
type number. The first number indicates the nominal size of the
tube's screen, the first letter (or letters) the particular tube,
and the last letter and numeral the type of fluorescent material
Phosphors. Typically, a type 5BP1 tube has a
5" screen with a type "P1" phosphor. Similarly, a type 20DP4 has
a nominal 20" screen and a "P4" phosphor. Cathode-ray tubes used
as TV picture tubes generally have rectangular screens and their
size designation refers to a diagonal measurement across the face
of the tube. In some cases, TV picture tubes are called kinescopes.
An arbitrary system is used for identifying the various phosphors
used. A type P1 phosphor, for example, has green fluorescence and
medium persistence; you'll find this type in most oscilloscope tubes.
Type P4 phosphors have white fluorescence and medium persistence,
and are employed primarily in television tubes. Type P5 phosphors
have a bluish-white fluorescence and very short persistence; tubes
with this type of phosphor are used for high-speed photography of
electrical phenomena having a short time duration. The P11 phosphor
is similar to the P5 type, but has a slightly longer persistence.
Types P7 and P14 are both two-layer phosphors. The P7 type has
a long persistence, first emitting a bluish light, then a greenish-yellow.
The P14 type has medium persistence, first emitting a bluish, then
an orange light which persists for over a minute. These two types
of phosphor are useful in instruments employed to observe low-speed
recurrent and non-recurrent phenomena. The last type of phosphor,
P15, has a very short persistence in the near ultra-violet region,
emitting a visible blue-green light afterwards; its principal application
is in flying-spot scanner tubes.
Electrostatic and Electromagnetic. Electrostatic
CRT's are those which employ electrostatic fields to move the electron
beam obtained from the gun assembly. Electron beams may also be
deflected by magnetic as well as electrostatic fields, however.
Most TV picture tubes are electromagnetic types.
In many cases, the beam may be focused as well as deflected by
magnetic fields, with a permanent magnet or electromagnetic coil
placed around the tube's neck near the gun assembly. In some tubes,
the electron gun is aimed at an angle, rather than straight towards
the center of the screen, so that gas ions (in the cathode beam)
which may be produced are sent to one side and do not strike the
screen (where they could cause a damaging "burn"). Where this technique
is used to "trap" ions, a separate ion trap magnet restores the
lighter electron beam to its straight-line path before deflection.
Some CRT's combine the basic operating features of both electrostatic
and electromagnetic types. Electrostatic focusing may be employed,
for example, by using a suitable electron gun, with electromagnetic
means used for deflecting the beam.
Cathode-ray tubes designed for color television receivers are
basically similar to the tubes described above, except that several
electron guns are employed and a special screen is used which fluoresces
in the three primary colors: blue, green, and red. The screen itself
is made up in a repetitive triangular pattern of small phosphor
dots and protected by a mask, aligned so that each of the electron
guns excites only its particular phosphor (blue, green, or red).
Flying-Spot Scanner. The flying-spot scanner
is a special type of display tube, similar to more conventional
CRT's except for its phosphor. In general, it is used in conjunction
with picture transparencies (such as motion picture film or slides)
and a phototube to produce a sequential electrical signal (or video
signal) which can be televised or used to reproduce the original
In operation, a raster, or rectangular light pattern of fixed
intensity, is formed on the flying-spot scanner's fluorescent screen
as the spot of light produced by the electron beam "flies" across
the screen. This moving spot of light is transmitted through the
transparent film to the phototube, where it develops a varying electrical
signal, dependent on the film emulsion density at each spot and
hence on picture content. The video signal obtained from the phototube
is similar to that produced by a TV camera and is used in the same
(to be continued)
Posted March 31, 2014