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 1,
The Tube Family Tree, Part 2
The "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
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
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
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 beam is
deflected and focused by the internal elements.
Details of the ignitron are shown above.
Schematic symbol for this industrial tube.
Basic phototube symbols. At left: the standard version. At right:
A typical phototube, by General Electric./td>
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.
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 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 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 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
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)
PostedNovember 30, 2021(original 3/31/2014)