June 1963 Popular Electronics
of Contents]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 (if any) 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
and Part 3.
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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 below left)
Designed for forced air-cooling, this Amperex high-power
transmitting tube has a finned radiator fitted over the plate.
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
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
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 flow.
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 of electrons.
Details of the ignitron are shown above.
symbol for this industrial tube.
Basic phototube symbols. At left: the standard version. At right:
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
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 of both.
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 (or phosphor).
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
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 picture.
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 way.
(to be continued)
Posted March 31, 2014