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Considerations in the design of broadcast transmitters

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This thesis, having been approved by the
special Faculty Committee, is accepted by
the Committee on Graduate Study o f the
University o f Wyoming,
in partial fulfillm ent o f the requirements
fo r the degree o f.c f.^ ^ i^ .^ f..
Chairman o f the Committee on Graduate Study.
1 certity this student received a Master
o f Science Degree at the University o f
Tammy AagartK Registrar
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Cons ide rat ions
in the Design of
Broadcast Transmitters.
Floyd Willard Wickenkamp
Thesis submitted to the Department of
Electrical Engineering and the Committee
on Graduate Study at the University of
Wyoming, in partial fulfillment of the
requirements for the degree of Electrical
□ F THt
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UMI Number: EP24885
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List of Charts and Illustrations............................... ii
Chapter One.
Discussion of available e q u i p m e n t ..................
Chapter Two.
A Study
of Tube Costs in the Low and High PowerFields........
Tubes for low power transmitters....................
Tubes for high power transmitters
A comparison of tube c o s t s ..........................
Chapter Three.
A Discussion of Two and one Half Kilowatt Transmitters....... . . 8.
1. Water-cooled tubes, their advantages and faults
. . . . S.
2. Life records of representative tubes............
3. Tube complement costs for various t u b e s .............
4. Additional cost for auxiliary equipment. . . . . . . .
3. Plate power requirements ............................
6. Ease of handling various tube t y p e s ................... 13.
7. Filament power requirements.......................... 14.
8. Circuit c o m p a r i s o n s .................................... 16.
Chapter Four.
A Practical Design Problem . . . . . ........................
1. Reasons for
choice ofcircuit in practical problem . . .
2. Operation conditions for radio frequencycircuit . . . .
3. Conditions of operation for audio circuit. ...........
Charts and Illustrations.............................. . . . .
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-List of Charts and Illustrations-
Block diagram
of 2.5 kilowattclass "B" modulated transmitter ..
Characteristics of
RCA802 vacuumt u b e .......................
Characteristics of
RCA805 vacuumt u b e .......................
Characteristics of
RCA807 vacuumtube
Characteristics of
RGAS10 vacuumt u b e ........................
Characteristics of
BCABl^ vacuumt u b e ........................
Characteristics of
RCA2&5 vacuumt u b e ........................
RCA.1603 vacuum t u b e ......
Amperex 220C vacuum
t u b e .........
Amperex 228A vacuum
t u b e .........
Amperex 892 vacuum t u b e ...............
Characteristics of Eimac 750TH vacuum t u b e ............. .
Power ratings
of vacuum tubes for plate modulation......
Power ratings
of vacuum tubes for linear amplifier u s e ..
Power ratings
of vacuum tubes for grid bias modulation use . . .
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-Ackno wledgement b -
The author wishes to take this opportunity
to express his appreciation of the courtesies
extended, through the prodigality with which the
following companies furnished information on their
Amperex Electric Company.
Eitel-McCullogh, Incorporated.
Federal Telegraph Company.
EGA Manufacturing Company.
Lapp Ceramic Manufacturing Company.
Thanks also should go to Eadio Stations
KDFN and WLW for information concerning tube
F. W.
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Chapter One.
Primary Discussion.
In designing a broadcast transmitter, the matters of first im­
portance are original cost, upkeep, reliability, and approval by the
Federal Communications Commission.
Of all the above, with the exception of the Commission's approval,
the primary interest is reliability. Broadcast transmitters, as well as
their tube complement, must be designed for long trouble-free life,
since the loss of even a few minutes on the air may mean loss of
station prestige plus loss of revenue in case of disruption of a
commercial program. Hence, it is usually desireable to purchase the
best available tubes and parts.
Since, over a period of years, the greater part of the equipment
and upkeep cost will be in the tubes, it is the purpose of this paper
to endeavor to show how this cost may be kept as low as possible,
consistent with the guarantee of reliability and stability.
In the case of the smaller transmitters, up to and including 1000
watts, there is little question involved as to the use of air-cooled
as opposed to water-cooled tubes. The many new types of air-cooled tubes
made available recently by the Radio Corporation of America, Heintz and
Kaufman, ltd., and Eitel McCullough, Inc., among others, has made
possible very economical design.
On the other extreme, above five kilowatts, there is no question
but that water-cooled tubes are the only possible solution. At the
present time, the largest air-cooled tubes are capable of a class "C"
output under conditions of modulation of
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However, it is in the so-called medium power, regionally allocated
class that there is a question as to which type of cooling is the
In the past, since most of the design problems have been kept
closely connected with the larger equipment manufacturers, there has
been little incentive to try to reduce cost to the purchaser.
of the expense connected with building transmitters, there has been
very little competition for the three or four large companies in the
Since the first consideration of the manufacturer is to make a
profit, it has been common practice to keep the older style, and usually
more costly methods, to arrive at the ultimate setup.
This does not
mean that the manufacturers are deliberately keeping the cost up; it is
merely a matter of habit, and the fact that their products have been
used and have proved their reliability for years.
In the past year or
two, however, the newer items, especially tubes, have been proving
their worth in many of the commercial and police transmitters through­
out the nation.
The writer has had some experience also with the new
air-cooled tantalum plate tubes in broadcast service and in high
frequency service in an amateur transmitter and found them to be
While the original tubes in this group were not as long-
lived as the older types, continued improvements have resulted in a
series of tubes that compare favorably with tubes of the older
In connection with this paper, much of the work has necessarily
been research through tube charts and manuals, with the exoeption of
the writer’s own experience gained as chief engineer of broadcasting
station KDFN.
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Chapter Two.
A Study of Tube Costs in the Low and High Power Fields.
As stated in chapter one, the most controversial issues so far
as the type of cooling is concerned, are in the medium class field.
It might be interesting, however, to make a short survey of the more
usual lineups in the other two classes, viz., the low-power and the
high-power classifications.
In the low-power class, that is, 100 and 250 watts, the usual
powers for which the Federal Communications Commission issues licenses
for local stations, the tube costs are almost low enough to be
left out of the picture in comparison with the total cost.
In fact,
it is usual to use the same type tubes in the more recent transmitters
for the final R. F. stage and for the class ”B" modulator stage in
both the 100 and 250 watt classifications, merely adding one tube to
the final radio frequency stage when increasing power.
Since the
cost is low, no attempt is made to use class "B" linear finals to
reduce the number of tubes necessary.
There are many types of tubes suitable for use in the final
R. F. stage which are approved by the Commission, among which are
the followingi (l)
Radio Corporation of America, RCA805, RCA810.
Heintz and Kaufman, HK254.
Halted Electronics, 905.
Taylor, Tl25.
(l25 watts per tube)
(l25 watts per tube)
(Same characteristics as RCA805)
(l25 watts per tube)
In the case of all the above tubes, two tubes of the same type
may be used for Class "B" modulators.__________________ _________________
(l)* See chart, page 37.
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The use of one type tube in both the audio and the radio frequency
stages reduces the number of types necessary and the number of
spares to be kept on hand.
It would be possible to use slightly larger
tubes, such as the RCA806, with a rated output of 250 watts per tube,
which would necessitate other types for the modulators, or the use
of two of the larger tubes, increasing the tube cost and accomplish­
ing nothing.
Since all the tubes listed fall in the price class between ten
and fifteen dollars, there is lettle to choose among them, except
personal preferences.
Any of them will serve the purpose admirably
and should give long and trouble-free life.
Four of the RCA805 tubes
are being used in the KDFAi transmitter to modulate a five hundred
watt carrier, and have been in service for nearly five thousand hours
with no trouble of any kind.
It is interesting to note in passing that as little as three
years ago, the cost of tubes for a 250 watt final radio frequency
stage was not $27.00 as in the above case, but $60.00 or more.
In addition, four tubes were necessary, adding to the complexity of
the circuit.
Of course, a larger tube could have been used, but then
the cost would have amounted to $75.00 or more per tube.
This is
mentioned merely to show the rapid strides made in lowering tube
costs, accomplished through increased tube efficiency and increased
When we consider the larger transmitter, ten kilowatts or more,
the picture is entirely changed.
Tubes for such high powers must be
liquid cooled, since no manufacturer has so far successfully made a
tube which would withstand the tremendous heat generated, or the high
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voltages necessary, a requirement for reasonably high efficiency.
Thus it is necessary to use some form of cooling.
Since water is
inexpensive, plentiful and, when free of impurities, non-conducting,
it is most commonly used for indirectly cooled tubes.
Since there is
considerable conductivity in most tap water, distilled water is
generally more satisfactory.
The water can be cooled by an outdoor
spray or fountain arrangement, making use of the cooling properties
of the air, after which it is pumped back into the station to be
used over again.
Tubes must be specially constructed for water-cooling.
cooling chamber takes the form of a jacket around the plate of the
tube, which is not glass inclosed as is the air-cooled tube, but is,
itself, the outside of the tube.
The distilled water is pumped through
the jacket and past the cylindrical plate of the tube, then the water
is cooled as described in the previous paragraph, and returned to the
tube jacket by means of centrifugal pumps.
The cost of the water-
cooling equipment is high, and requires extra precautions to
minimize leakage.
There is a choice in water-cooled high power transmitters
between class "B" linear amplification or class "C" amplification.
Either method is perfectly satisfactory, but, as their operation is
quite different, many engineers have marked preference for one or the
Tube cost in any event will be found the most important single
We will consider as an example, a five kilowatt set.
In this
case, there is a choice between high and low level modulation.
Tubes readily available includes
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Amperex 892, rated at 5. fcw. modulated class "C".
2.5 kw. class "B" linear.
Amperex 220C (ratings same as 892).
KCA892 (same as Amperex 892).
BCA892R, rated at 2.9 kw. modulated class nCn.
The RCAS92E is a special tube of the water-cooled type, but
using air-cooling by means of large hollow aluminum fins fitted on
the socket, through and around which air is forced. Since the 892R
is rated at only 1 kw. for linear service, it would not be suitable
except as a class "C" amplifier, for a five kilowatt transmitter.
The cost of each of the tubes listed, less sockets, runs between
$250. and $325. There is little choice among them since they should
all give long life operation and have been proven in operation.
For five kilowatts, then, class "C", the approximate cost for
the final stages would be summed up thus:
Final radio frequency stage, one 892 t u b e .... $285.
Final class "B" modulator, two 892 t u b e s ..... 570.
t o t a l .... $855.
For five kilowatts obtained by means of a class "B" linear
Fihal linear amplifier stage, two 892 tubes . . . .
To the cost of the linear stage would be added complications
in the driver stage, so the final cost for the two transmitters
would be very nearly the same.
Similar figures could be given for 25 kw.
sizes, except
fifty kilowatts, in which case it is customary to usetwo type
tubes in class HBn linear, since thera’is no tube manufactured
which carries a fifty kilowatt rating in class "C". Two 862 tubes
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operating at 25,000 volts will give 50 kilowatts output.
The 862
costs $1650.
It is beyond the scope of this paper to discuss completely the
desigji of such large transmitters.
However, more information on
water-cooled tubes and their operation will be given later, as well
as methods used in considering the different items of design applicable
to all sizes of transmitters.
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Chapter Three.
Discussion of Two and a Half Kilowatt Transmitter.
As was mentioned previously, it is the outputs between one and
five kilowatts in which there is a question as to the choice of the
best type of circuit and tubes.
First, a dedision must be made as to whether air-cooled or watercooled tubes will be used. Then we can choose the circuit.
Water-cooled tubes, for years commonly used in the larger trans­
mitters, have been found to have good life expectancy, are fairly
reasonable in cost, and are generally well known and understood by the
average engineer. Their disadvantages include:
1. They require complicated cooling systems.
2. Extremely high voltages are necessary for reasonable efficiency.
3. They are difficult to handle.
4. The filament power required is much higher than for air-cooled
5. A specially built, well insulated socket and water jacket are
Following is a list of typical tube3 with their service records,
taken from the files of WLW and KDFN. The water-cooled tubes are from
WLW's records, the air-cooled are from the records of KDFN.
Type no.
Serial no.
Life in hours.
0 (Defective
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From this chart it may be observed that life expectancy for
both air-cooled and water-cooled tubes is high.
Most manufacturers
guarantee their tubes for a period of not more than six months or
1000 hours service, depending on which occurs first.
However, the
tubes are usually designed to last for a much longer period, and
the guarantee is merely to guard against defective tubes.
In taking up the matter of tube costs, we can most easily get a
complete picture by making a chart that shows tube costs under differ­
ent conditions of operation.
Since, under certain conditions the cost
of the driver stages is quite a factor, we should include the cost of
both the drivers and final stages.
Class "C" Final, Class "B" Modulator.
Final R F stage, one 228A tube .......
Final class "B” audio, two 228A’s
Plate Voltage.
$ 225.
6000 volts.
RF driver, two 810 tubes
Audio driver, four 845*s
"B" Linear Final.
one892 .........$
Modulate R F driver, two 810*s
m AB"
......$ 742.
modulator, four 845’s
10000 volts.
......$ 352.
Class MBW Linear Final.
Wot practical with present tubes, since eight 750TH's would
be required for two and one-half kilowatt output.
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Class "C" Final, Class "B” Modulator.
Final R F stage, two 750TH*s
Plate voltage
$ 350.
Final class nB w modulator, two 750TH*s
RF driver, two 810' s
$ 7b7 .
........ ......
Audio driver, four 845's
4000 volts.
When, air-cooled tubes are compared with water-cooled tubes, it
is seen that the former have a great advantage.
In most of the m o d e m
transmitters, it is common practice to add a certain amount of forced
draft to keep the ambient temperature as low as possible.
By drawing
air from the top of the transmitter enclosure, and with an intake at
the bottom, it is possible to reduce the temperature considerably
and thus provide a greater margin of safety for condensers and other
parts which are affected by high temperatures.
Water-cooling, however, is not such a simple matter.
As mentioned
previously, it is best to use distilled water or some non-corrosive,
non-conducting liquid, since a conductive cooling medium such as tap
water will allow high leakage losses from anode to ground and
encourage formation of scale on the plate, and so reduce the cooling
The lower cooling efficiency brought about by scale may
cause hot spots on the plate, which in turn will allow gas to be
released internally, and destroy the filament emission.
For these
reasons, the extra cost of distilled water seems to be very much
It might be noted in passing that the majority ofwater-cooled
tubes are unsuitable for the higher frequencies, because of their
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high, interelectrode capacities.
As it is possible to get more effective cooling with water,
the hulk of the tube is much less than for air-cooled tubes for
the same plate dissipation rating. However, the added space required
for pumps, cooling coils and other cooling equipment will offset the
advantage of size in the tube itself.
Since it is impossible to remove all conducting materials from
water, even distilled water, it is best to reduce the leakage current
through the water by the use of a porcelain cooling unit. This
porcelain coil is built in a helical form to give a length of fifteen
feet or more. The unit should be installed in close proximity to the
tube socket, and the tube socket is often mounted inside the coil. (2)
The cost of the auxiliary equipment for the cooling system is
normally a one cost item, that is, there should be little upkeep
required. However, the first cost is considerable. Following are ap­
proximate prices on representative items required:
Belays for shutting off high voltage in case of failure of water
circulating system (two required)..................... $40.00
Porcelain cooling and isolating coil ...............
later pumps (two required) .........................
In addition, the following will be needed, one for each
water-cooled tube in use:
Water jacket ......................................
Mounting insulator and clamp for water jacket
Filament transformer, one for each tube except in the
case of two-phase filaments, for which two are required.50.00
(2). long, "Porcelain Cooling Unit". Electronics, October, ’38. P. 24.
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Cooling equipment, then, costs from $400. to $500. for the average
To be completely impartial, we must consider the cost of sockets
for the air-cooled tubes.
In most cases, ten dollars per socket will
cover the cost of a socket for any of the air-cooled tubes in common
The only exception to this rule is in the case of the new
forced-air-cooled RCA tubes, RCA891R and RCA892R, the sockets for
which are $60.00.
However, these two types are essentially water-cooled
tubes RCA891 and RCA892, in which the water jacket has been replaced
by a large finned socket and cooling shell through which air is forced.
Characteristics are similar to the comparable water-cooled tubes,
except that the plate dissipation is reduced by about fifty percent.
Plate voltage requirements are of extreme importance in any
transmitter, since the cost of transformers, rectifier tubes, filter
condensers, and other parts increase rapidly with increased voltage.
For voltages up to about six thousand, regulation pole transformers
are satisfactory, used in three-phase circuits with outputs up to
twenty kilowatts.
For higher voltages or for greater power, specially
will be required at much higher cost per unit of
Since pole transformers are made in quite large quantities,
their cost is not excessive, as no special designing is necessary.
•*ti choosing rectifier tubes, the commonly used 872A is suitable
up to 3500 volts in single-phase full-wave circuits.
A three-phase
connection will allow an output of 8700 volts*
RCA bulletin, "Air-cooled RCA 5 kw Transmitter, type 5D. RCA
Manufacturing Co., Camden, N.Y.
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For voltages in excess of 8700, it will be necessary to use the
669 or similar tubes. The 869 tube is suitable for voltages up to 7000
in a single-phase full-wave circuit, or 20,000 volts in three-phase
full wave circuits. The cost of the 872A is $10.00, while the 869 is
priced at $125.00.
Filter condensers are also an important item especially at the
higher voltages. For example, a 5000 volt, 2 mfd. condenser sells for «
about $35*00 while a 10,000 volt, 2 mfd. condenser costs more than
Insofar as the voltage rating of the filter chokes is concerned,
the cost is only fractionally more for 10,000 volts than for 5>000
volts. In addition, if higher voltages are used, the current rating
could be lower for the same power output from the filter. In the final
analysis, the cost would be very nearly the same.
From the above, it is obvious that the lower voltages have many
marked advantages.
As regards ease of handling, air-cooled tubes seem to have the
advantage. Unless water-cooled tubes are cooled with distilled water,
it is necessary to dismount the tubes and clean the plates at least
once a month to keep down scale formation. Even if distilled water is
used, it is necessary to check and replace gaskets periodically; pumps
must be repacked, and the water lines must be watched closely for
leaks. It is no small matter to replace a water-cooled tube because
it is a part of a necessarily leak-proof water line, and must be
carefully installed and checked for water leaks before operation can
be resumed. This may often mean a loss of valuable air time.
Air-cooled tubes may be replaced with comparatively little
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trouble or lost time, since ail that is necessary is to remove the
tube from its socket, replace with another, make the necessary
connections to plate and grid, after which operation can be resumed.
Since both air-cooled and water-cooled tubes become very hot
during operation, most stations keep a pair of heavy leather gloves
handy for grappling with the tubes.
The gloves together with a heavy
cloth or chamois pad, will allow removal of even the hottest of tubes
in a few seconds without danger of dropping the tube or burning the
If this is not done, it may be necessary to w a i t many
precious minutes before handling the tube.
A check of the filament voltages and wattage ratings of the
various air and water-cooled tubes shows that, on the average, watercooled tubes with equivalent maximum electronic space current.
example, the Eimas 750TH with a maximum space current
( d. c.
plate current plus d. c. grid current) rating of 1.25 amperes
consumes lbO watts in heating the filament.
For the Amperex or Western
Electric 228A water-cooled tube, the maximum space current is 1.25
amperes, the same as the 750TH, but the filament power is 880 watts.
The RCA892 with a filament power of 1320 watts, has a rated space
current of only 2.0 amperes.
A factor whioh offsets partially the seemingly great difference
in filament power is that air-cooled tubes operating at lower plate
voltages require higher space currents for the same output.
tubes such as the 750TH with an output rating of 2500 watts for two
tubes, uses 320 watts of filament power while the 228A tube with the
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same output, 2500 watts, requires 8S0 watts filament power. The reason
is that while the air-cooled tubes use an efficient thoriated filament
operating at fairly low temperatures, the water-cooled tubes use
tungsten filaments which must be operated at much higher temperatures.
The use of tungsten filaments in water-cooled tubes is made necessary
by the difficulty of obtaining a high vacuum. Even a reasonably small
amount of gas will greatly reduce the efficiency of a thoriated type
of filament, and the amounts left in water-cooled tubes will tend to
permanently harm the filament. High gas content in water-cooled tubes
is due in part to the large metal surface exposed to the internal
vacuum, and to the fact that it is difficult to get a good seal between
the copper plat& and the glass envelope.
Air-cooled tubes, on the
other hand, are much more easily pumped to a high vacuum because there
is much less metal in the tubes, the metals used are more easily
de-gassed than is copper, and the tube elements are completely
enclosed in a glass envelope, removing the problem of sealing metal
to glass except for the relatively small element leads.
As stated above, since thoriated filaments operate at lower
temperatures, it is plain to see that the power required to raise the
filament to operation temperature is much less than in the case of
a tungsten filament.
A filament starter is adviseable with all the larger tubes
includihg the air-cooled, although in the case of air-cooled tubes,
it is permissable to start the filament heating at only a slightly
reduced voltage to reduce the initial rush of current. As the
temperature of tungsten increases, the resistance increases quite
rapidly and the current is reduced accordingly. At room temperature,
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the current may be several times the operating current. A reduction
of 25 or 30# for about 30 seconds will serve to protect the tube
When considering water-cooled tubes it must be kept in mind that
the starting voltage has to be low since the cold resistance is very
low compared to the filament resistance at the high operating
temperature necessary with water-cooled tubes. It is best with watercooled tubes to apply not more than 25$ voltage to start with and
increase the voltage slowly by means of rheostats or a series of time
delay relays, so that two or three minutes are consumed to bring the
filaments to operating temperature. Also, when the filament of a
water-cooled tube, which may be as much as an eighth of an inch in
diameter, is turned off, it should be reduced slowly to prevent
straining and buckling the filament. Since the filaments in air-cooled
tubes are much smaller in diameter and operate at lower temperatures,
there is less expansion and contraction, and a smaller change in
resistance. It is usually not necessary to cool air-cooled filaments
slowly; they may be turned off at once after the conclusion of the
days operation. A very complete discussion of the care and operation
of water-cooled tubes may be obtained from the different manufacturers,
notably RCA, Western Electric, Amperex, and Federal Telegraph companies.
The foregoing is not to prove the superiority of one type of cooling
over another, but to give the engineer an idea as to what to expect
from different tubes.
How that we can make a decision as to the choice between aircooling and water-cooling, the next important step is to discuss the
type of circuit to use. First, a short discussion of the more common
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methods of modulation is in order. These methods are as follows:
1. Class "£n linear amplifier in connection with a modulated B. F.
driver. The linear amplifier operates at approximately cutoff "bias
so that the plate current is zero with no excitation.
2. Class "C" amplifier, plate modulated by a class "B" modulator.
The class ”C" amplifier operates with high grid bias so that a
considerable amount of grid drive is required before the plate
current commences to flow. The class "£n modulator operates so
that only a small plate current flows without audio input.
3. Grid bias modulation. The audio signal is mixed with the
incoming radio frequency signal on the grid of the final stage.
Grid modulated amplifiers operate in class "£" or class "A" and
are relatively low in efficiency.
4. Heising modulation, a class "C" amplifier is plate modulated
by a class nA n modulator. Since class HA" modulators are very low
in efficiency, Heising modulation is fast becoming obsolete.
Class "B" linear simplifiers are used for amplifying a modulated
radio frequency signal as stated above. Since we want distortionless
output, it is important that the output voltage be proportional to the
input voltage. Linear amplifiers are also known as distortionless
amplifiers, because of their property of magnifying the signal fed to
the grid so that the output signal is increased in voltage and
identical in shape to the grid input signal. One of the chief dis­
advantages of the linear amplifier is its low efficiency, which is
between 23 and 35 percent. This means that the plate must dissipate
at least twice the power output obtainable. The big advantage of the
linear class n£" amplifier, aside from its distortionless character,
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is the reduction in size of the final audio stage. Since the radio
frequency signal is audio modulated in a stage previous to the final
stage, much less audio power is required. The usual proceedure in
medium power transmitters is to modulate the driver stage (the radio
frequency stage just preceding the final stage). The driving power
required for a linear stage is much lower than that required for a
class "C" stage, and will vary with different tubes, hut is usually
from two to five percent of the input to the filial stage. Since the
output must he free from distortion and phase shift, it is necessary
that the driver tuhe he capable of delivering considerably more power
than the peak requirements, with nearly perfect regulation. That is,
with an increase in the audio signal, the radio frequency power
delivered to the grid must remain practically constant. It is often
adviseable to load the driver stage with a constant non-inductive
load so that the regulation will he improved from no load to full
load, since the more heavily loaded amplifier will give more nearly
constant voltage output than one that is almost completely unloaded
during certain parts of the cycle. A satisfactory load will usually
he one that loads the driver stage about as much as the peak power
requirements for the class "B" stage. Then a modulated driver with
a rating of three times the maximum power delivery to the final stage
would he suitable.
At first thought, it would seem that, since a class "Bn amplifier
is biased to cutoff, there would he no current flowing in the plate
circuit without audio modulation, hut it should he remembered that the
radio frequency carrier is being applied to the grid at all times; thus
the plate current is nearly constant and the instantaneous current will
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
vary at an audio rate above and below the average plate current. Since
the plate must dissipate a constant amount of energy, with or without
modulation, the efficiency is low and large tubes must be used to
obtain appreciable output.
Since a class "Cn amplifier is biased beyond cutoff, so that with
low grid driving power there is no plate current flowing, the class "C"
amplifier is sometimes called an efficiency amplifier, because the
efficiency of the final is governed to a certain extent by the value
of bias and by the grid driving power. Most class "C" amplifiers are
biased so that plate current flows about 120 to 150 degrees of the
cycle, a value which gives optimum efficiency commensurate with a
reasonable amount of driving power. While it is possible to bias the
stage to four or five times its cutoff voltage, so much extra grid
driving power is required for only slightly higher efficiency that
it is not considered good engineering practice.
A class "C" modulated amplifier, since it is itself being
modulated, will require an audio signal sufficiently large to give
100 percent modulation, or a value equal to half the input power to
the radio frequency final stage. Thus, the input to the audio modulator
will be required to be about 80 percent as much as the input to the
final amplifier. Two tubes are required in class "B" audio circuits
for good quality since one tube will operate only during the half cycle
that its grid is positive. Thus, to get a complete cycle of audio
output, two tubes operating 180 degrees out of phase with each other
are required.
Driving power for the E. F. stage will run much higher than for
the linear amplifier, since the grids are biased highly negative. The
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
"bias voltage most be overcome and a large positive grid swing obtained
in addition to obtain large output. About ten percent of the output
power must be supplied by the driver.
For the class "B" audio stage, an audio signal of very high
quality must be available, necessitating a class "A" or class "AB"
audio driver. Audio driving power equal to about seven percent of the
audio output of the modulator will suffice.
For grid modulation, the efficiency is very low, only 20 to 25
percent. However, the audio signal required is very small, on the
order of two to four percent of the output from the modulated stage.
Grid modulated amplifiers are difficult to tune and, because they
require such large tubes, are little used in broadcast amplifiers,
except at times as a modulator for a linear amplifier circuit.
Up to thiB point, it will be seen that the linear amplifier seems
to have the advantage insofar as complicated circuits and driving power
are concerned. However, there are two points on which the linear
amplifier is inferior to the class "C" modulated amplifier. The first
is comparative stability and ease of tuning; the other is that under
most conditions, larger tubes and much higher plate voltage must be
used in linear circuits to obtain an output equal to that of the class
nC" circuit which operates with comparatively small tubes and at more
reasonable plate voltages.
In the adjustment of a class "C" stage, plate circuit tuning, bias
adjustment, driving power adjustment, antenna loading and audio loading
are not especially difficult, and when once adjusted, will usually
give little or no trouble. Stability in class HC" modulated amplifiers
is limited only by the care used in building and in design.
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la linear amplifiers, the plate impedance must be carefullyadjusted so that the tube impedance and that of the tank circuit
are very nearly the same.
The grid bias must be adjusted so that
the tube is operating exactly at cutoff -without excitation, since
carrier shift will result with incorrect bias, as well as the
possibility of having excessive or too little available modulation
depending on whether bias is too low or too high.
In other words,
if the linear amplifier is operated beyond cutoff, it is possible
to have complete modulation of the positive peaks even though the
class "C" amplifier were operating at only 60 or 70 percent
modulation, the class "C” amplifier in this case being the driver.
Driving power is also a critical point in linear amplifiers.
.Excessive radio frequency excitation will often result in a negative
carrier shift, while too little R. F. excitation will reduce the
output and cause overheating of the linear stag©.
The loading to the antenna must be adjusted carefully to match
the impedance of the plate and tank.
Excessive distortion and reduced
output are two common results of incorrect antenna loading.
The other class "B" linear adjustments are routine, as in the
case of class "C" amplifiers.
However, linear amplifiers are more
subject to parasitics and any small change in loading due to any one
of several causes, may give trouble.
Hence, linear amplifiers are
usually in need of closer supervision and must be more thoroughly
and carefully tuned for high quality operation.
Grid bias modulated amplifiers require careful adjustment of the
bias, the plate load and antenna load, and the R. F. excitation must
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be closely adjusted for satisfactory operation. Grid bias modulated
amplifiers are probably slightly more stable than linear amplifiers
but not nearly so trouble-free as a class "C" modulated amplifier.
In summing up the comparison in tuning ease of the three systems,
we find that the class "C" stage has no especially critical adjust­
ments, the linear and grid modulated amplifiers have four each.
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Chapter Pour
A Practical Design Example,
laving taken up the advantages and disadvantages of the various
tubes and circuits used in m o d e m broadcast work, let us put our
conclusions to practical use by designing a transmitter rated at 2%
kilowatts output.
While a one or a five kilowatt transmitter could have been chosen,
it was felt that the 2g kilowatt size offers the most for the money
invested and is probably not so well known as it should be. That size
operates at little more expense than the one kilowatt size, the
original cost is only slightly more and gives a power gain of four
decibels. On the other hand, 5 kilowatts is a power gain of only three
decibels over 2% kilowatts, and 5 kilowatts output requires higher
voltage with all its attendant difficulties, as well as much higher
As we have seen, while linear amplification is less expensive as
far as tube cost is concerned, it is more complicated than and not as
stable as a class "C" amplifier. Air-cooled tubes seem to offer less
complications and probably would cost less over a period of time than
water-cooled tubes,
for these reasons, a class HC" final, class nB" modulated
transmitter was chosen in this particular case. Under some conditions
it might be better to use one of the other combinations.
It is best to start with the final stage and work back through the
exciter stages to the oscillator in the case of the radio frequency
portion, or to the audio input in the audio section. Since the Eimae
tube answers all the requirements for the final radio frequency
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stage, and is approved by the Commission (see F. C. C. approved tubes
in chart section), tvo 750TH's will be used, operating under the
following conditions:
Plate voltage . ..................... 4000. volts.
Plate current....................... 900- milliamperes.
Efficiency set by F. C. C
Power input ....................... 3^00. watts.
Plate dissipation .................
1100, watts.
Plate power output
2500. watts.
Incidentally, commencing July First, 19^0> the Federal Communi­
cations Commission is requiring that all broadcast stations use the
direct method of measuring transmitter output, that is, the antenna
current and the antenna resistance at the point of coupling from the
transmission line are measured and the power into the antenna is
computed from the formula:
power equals current squared times antenna resistance.
However, usually it will be found that the efficiency factor set by
the commission is not far wrong and the current, voltage and power
figures given above will be quite accurate.
Since the driving stage must be capable of an output of eight
to ten percent of the input to the final stage, a power output of
about 3^0 watts should be available at good regulation. The RCA810
triode is a new tube just recently released
by the Radio Corporation
of America with characteristics similar to the widely used RCA805 but
with a heavy duty filament and capable of considerably more output than
the 805 tube. The 810 tube is capable of an output of more than 200
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■watts operating at low plate voltages.
Low voltage is desirable at
this point since the audio drivers as well as several other tubes can
then operate at t h e same voltage, keeping to a minimum the number of
power supplies.
Then the choice for this stage will be two RCA810
tubes tinder the following conditions;
■Plate voltage
Plate current
Estimated plate efficiency
Plate input power
55• percent.
Plate dissipation power
Plate power output
In order to reduce the amount of grid driving power requirements
on this stage, and since plate voltage is of less importance than in
the final stage, the driver stage is not driven as hard as in the final
Bias amounting to about 1-gr times grid cutoff voltage is
sufficient for the driver stage.
In the voltage amplifier stage preceding the RCA810 tubes, output
equal to about six percent of the input to the driver stage is re­
An output of 625 x .06 or 37.5 watts is sufficient.
To obtain 40 watts of radio frequency power an 810 tube could be
used, although a beam type tube such as the RCA814 is probably more
The advantage of a tetrode or pentode type tube in the lower
stages is the additional isolation afforded the crystal stage from the
final stage. Because of its high amplification factor, a large change
in load on the plate would have little effect on the grid impedance*
Another advantage of the type 814 is its low driving power requirement
and a maximum plate voltage rating of 1250 volts, which allows the same
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plate voltage to be used on both the 810 and 814 tubes.
Since the type
814 does not require neutralization, it is possible to simplify the
circuit somewhat.
The type 814 will operate under the following conditionsj
Plate voltage
1250. volts.
Plate current
100. milliamperes.
Plate input power
Estimated plate efficiency
Plate output power
125. watts.
60. percent.
75. watts.
The output is much more than required, but it will do no harm
and may allow the efficiency of the 810 driver stage to be increased.
Any increase in driver efficiency will allow delivery of more power
to the final grids, resulting in increased efficiency from the power
amplifier stage and greater power output.
Since the 814 is a beam power tube, and as such, has very high
power sensitivity, the grid drive required for 75 watts output is only
about a watt.
The 814 stage could be operated directly from the crystal
stage, but, to keep the heating of the crystal as low as possible, it
is usually advisable to run it at very low plate voltage and reduced
power input. For this reason, and since another lower power stage adds
little to the oost, it is best to include an additional stage between
the oscillator and the 814 stage.
Output from this additional stage is
not important, so an 814 driver stage consisting of an RCA802 pentode
tube is seleoted.
This type, like the 814 tube, has high power
sensitivity and requires less than a quarter watt driving power.
type 802 can operate at a plate voltage of 400 volts, which same
voltage will also be used on various other tubes.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
The 802 stage will operate under the following conditions*
Plate voltage
400• volts.
Screen voltage
200. volts.
Plate current
30. mil H a m p e r es .
Screen current
7. milliamperes.
Plate power input
12. w a tts.
Estimate;- efficiency
50. percent.
Plate power output
6. watts.
For the oscillator an
voltage and with its power
RCA802 tube isused, operating at low
output limited to about two watts, which
is more than enough to drive the 802 first buffer stage.
Since the
plate current is very low and nearly constant, it is better to use
a series dropping resistor
or voltage divider to lower the plate
voltage, rather than use a
separate power supply.
The plate power may
be obtained from the same source as the first buffer stage.
The 802 oscillator will operate under the following conditions*
Plate voltage
250. volts.
Screen voltage
150. volts.
Plate current
Plate power input
20. milliamperes.
Oscillator plate efficiency
Plate power output
5. watts.
40. percent.
2. watts.
The above lineup of tubes should give very satisfactory operation,
and should be extremely stable, since a surplus of grid power is always
Pentode and beam tubes are used wherever possible, so that
only two stages need neutralizing.
This makes for simpler construction,
since single ended coils may be utilized, the voltage across the
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coils is less, and condensers with lower voltage ratings may he used.
The audio portion of the transmitter will he considered in the
same way as the radio frequency, that is, from the high power stages
and progressing hack to the input.
For the class "B" audio modulator stage, the amount of high
quality audio power required is equal to one half the input power
to the final power amplifier stage, for 100 percent modulation.
In other words, an audio power equal to fifty percent of the radio
frequency input power will completely modulate the radio frequency
carrier so that the carrier envelope will vary from zero to twice
the unmodulated carrier intensity. Then, since the input to the
power amplifier stage is 36OO watts, an audio power output of 1800
watts will he necessary for 100 percent modulation.
To obtain 1800 watts of audio power, and always remembering that
two tubes must he used in class "B" for distortionless output, two
750TH tubes can he used. A glance at the tube chart shows that two
750TH's operating with JQOO volts on the plates, will give 2000 watts
output. However, operation at 3000 volts would necessitate another
power supply, since the radio frequency power amplifier operates at
1+000 volts. 4000 volts may he used on the class "B" stage also by
a reduction in the peak plate current. An additional advantage is
that with reduced current, higher quality is more easily obtained than
if the tubes were operating near their peak output. At UOOO volts, the
two tubes could supply up to 3000 watts without exceeding any of their
ratings. The higher voltage also tends to simplify the matching of
the tubes to their modulation transformer, since higher impedance
circuits give better
audio regulation.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
The 75 OTH tubes operate tuider the following conditions:
Plate voltage .
4000. volts
Plate current- maximum signal-
. . . 750. milliamperes.
Plate to plate load impedance . . . .
8000. ohms.
Plate power input
3000. watts.
Plate efficiency- maximum signalPlate power output
60. percent.
1800. watts.
Maximum audio grid driving power for the modulator should he
from five to seven percent of the plate input power to the modulator
under conditions of maximum modulator output, hut under the present
conditions, where the output is only ahout sixty percent of maximum
of which the modulator is capable, four percent audio grid driving
power is more than enough. This reasoning is h o m e out hy the tuhe
manuals of the different manufacturers.
To obtain 120 watts of distortionless audio, it is best to use
class "A" or class nABn amplification, and there should he an excess
of audio for best quality, or rather, the power available should he
considerably more than the operating power. The most logical choice
seems to he push-pull-parallel RCA845 tubes which are almost uni­
versally used for the audio driving stage in transmitters from 500
watts to five kilowatts.
The four 8^5 tubes will operate in class HAB" under the following
Plate voltage .....................
Zero signal plate current
Maximum signal plate current
Maximum signal power input
1250. volts.
SO. milliamperes.
HOO. milliamperes.
500. watts.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Maximum signal efficiency
42. percent.
Maximum signal power output.......... 210. watts.
Since only 120 watts are required, and since distortion increases
with an increase in plate input power and audio grid driving power,
the four 845 tubes should supply very high quality audio, with little
or no measurable distortion. Little driving power is requited for the
845 stage since the grids are driven into positive or grid current
region only on maxinnxn peaks. A voltage amplifier stage will suit the
purpose admirably. For this purpose, two SOJ tubes have been chosen,
operating at a plate voltage of 400 volts. The 807 tubes are sturdy,
have long life, are easily obtainable, and will put out a good amount
of power if required. In addition, they have an available amplification
of around a hundred, and require little audio grid driving power.
They may be operated in class "A" with an output of 16 watts. In class
"ABH they are capable of 80 watts output per pair.
The push-pull SOJ stage will operate as follows:
Plate voltage.....................
Plate current
400. volts.
.......... 120. milliamperes.
Plate power input .................
48. watts.
Efficiency (class "A")
30. percent.
Power o u t p u t .....................
15. watts.
The voltage amplification factor under the above conditions will
approach fifty.
At this point it is best to stop and see how much voltage amplifi­
cation will be required to bring the audio level up enough to drive the
grids of the final audio stage to the required output. The usual
transmitter is designed to allow 100 percent modulation with input to
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
the audio channel at zero level, or a power of .006 watts across a
500 ohm line.
The power required to drive the final audio stage is
120 watts, a level of plus 44.5 db., where
dbs equal 10 x logqo
(output power/input power).
Then a total gain from the input to the grids of the 750TH tubes
of 44.5 db. should be sufficient.
Additional gain amounting to ten
db. should be allowed if a regenerative feedback cirouit. is used,
an addition much to be desired.
Feedback is used to reduce distortion,
hum and tube noises, as well as increasing the audio stability b y
feeding a portion of the output signal back into the input stage or
one of the intermediate amplifier stages in an out of phase relation
with the original signal.
If the feedback voltage is correctly applied
it will be in opposition to the tube noise and distortion voltages
generated within the tubes themselves, and will act to attenuate them.
This may be more fully studied in several very fine articles appearing
in periodicals. (4).
Then allowing 10 db. for inverse feedback, and another 10 db. for
losses in the circuits, it is found that there is a total gain of 44.5
plus 10 plus 10 db., or 64.5 db.
Since the power gain is zero, the
voltage gain through a transformer will be nearly equal to the ratio
of the transformer.
Then the power gain for each step is as followsi
Class MB" driver transformer
........... minus 5 db.
Bell, W. A., ^Inverse Feedback.
pp. 47-49.
(step down)
Radio Magazine, Oct., 1938.
Nalley, "Negative Feedback Applied to Class ’B ’ Audio. Radio
Magazine, July, 1937. pp. 54-57.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
8^5 driver stage . ,
8^5 input network
minus 2 db. (resistance loss),
7 db.
807 voltage amplifier
plus 34 db.
807 input network
minus 2 db. (resistance loss).
total —
plus 32 db
Since we require a total of 64.5 db., 32»5 db. are still necessary
for full output.
By adding a push-pull stage using a pair of 1603 pentodes,
it is possible to realize a gain of 100 or more or at least 4o db.
voltage gain. In addition there is a voltage gain in the input
transformer of around 10 db., so that there will be an excess of
voltage gain.
The 1603 tubes may be operated as follows:
Plate voltage . . . . . . . . . . . .
Plate current per t u b e ...........
25O. volts.
2. milliamperes.
It is also possible, when necessary, to operate the two 1603 tubes
to obtain a quarter watt or more output, at a sacrifice in voltage
While no claims are made for the superiority of this design
over others, it is felt that it foims a nice balance between original
cost, maintenance and operation cost, and reliability.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
75 watts
2 watt;
Audio innut
zero db.
25 watt
210 watts
Figures above line show approximate output from preceding stage.
A zero level of .006 watts is used.
1800 watts
r 1603 >
f 1603 >
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
(condensed, from RCA Technical Manual TT3).
Heater voltage .....................
6.3 volts.
Heater current.....................
0.9 amperes.
Grid-plate c a p a c i t y ...............
Input capacity..................... 12.0«(mfd.
Output c a p a c i t y ...................
Maximum ratings and typical operating conditions:
Plate v o l t a g e ..................... 500. volts.
250. volts.
Screen voltage
Suppresor voltage
40. volts.
Plate c u r r e n t ....................... ^5. milliamperes.
Driving power (approximate)
0.25 watts.
Power output (approximate maximum) . . 16.0 watts.
RCA802 is a pentode transmitting tube of the heater-cathode type
for use as an R. F. amplifier. The plate connection is Brought out
through the top to maintain low grid-plate capacitance. Neutralization
is usually unnecesary. The 802 tube is especially good as ah R. F.
oscillator or amplifier.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Filament voltage
10.0 volts.
Filament current ...........
Grid-plate capacity
3.25 amperes.
Input capacity
Output capacity
Maximum ratings:
Plate voltage
1500. volts.
Plate c u r r e n t .............
210. milliamperes.
Plate dissipation
125. watts.
Typical operating conditions:
Power output,
class "B*1audio (2 tubes)370. watts.
Power output,
class "CM B. F.
Power output,
class "B" E. F.
215. watts.
57.5 watts.
BCA805 is a high-mu, three electrode transmitting tube of the
thoriated-tungsten filament type for use as a radio-frequency
amplifier, oscillator and class "Bn audio-frequency amplifier.
The plate connection is brought out through a separate seal atthe
top of the tube. The grid is designed so that the amplification of
the tube varies with the amplitude of the input signal.
(condensed from BCA. Technical manual TT3).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Heater voltage.....................
6.3 volte.
Heater current .....................
0.9 amperes.
Grid-plate capacity
0.2 mmf.
Input capacity ......................
Output c a p a c i t y ...................
7.0 mmf.
Maximum ratings:
Plate v o l t a g e
600. volts.
Plate c u r r e n t
25. vatts.
Plate d i s s i p a t i o n
Typical operating conditions:
Plate voltage
600. volts.
Power output,class "AB" audio (2 tubes)60.
80. watts.
Power output, class
12.5 watts.
Power output, class "CH E. F ........... 15-0
25.0 watts.
Power output, class nCHE. F ......... 25.0
37.5 watts.
"B" R. F .........
BCAS07 is a heater type transmitting tube incorporating design
principles involving the use of directed electron beams. This tube
may be classed as a tetrode, although its operation is more nearly
like the pentodes. The exceptionally high power sensitivity makes
this tube excellent for use as an R. F. or A. F. amplifier, frequencymultiplier, oscillator and plate modulated amplifier. In R. F.
applications, the 807 may usually be operated without neutralization.
(condensed from RCA Technical Manual TT3)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
filament voltage ....................
filament current...................
4.5 amperes.
.Amplificationfactor.................. 35.
Grid-plate capacity
Input capacity
8.7 mmf.
Output capacity
Maximum ratings:
Plate voltage... ..................
2000. volts.
Plate current
250. milliamperes.
Plate dissipation
125. watts.
Typical operating conditions:
Plate voltage......................
2000. volts.
Power output, class nB n audio(2 tubes)
590. watts.
Power output, class "B" E. f ........
60. watts.
Power output, classHC" fi. f ........
250. watts.
Power output, class "C" E. f ........
375. watts.
The BCAS10 is a high mu triode transmitting tube with high
perveance, capable of high efficiency at low plate voltages and with
comparatively small amounts of grid driving power. This tube has a
heavy duty filament shielded at both ends, which tends to conserve
filament input power, and eliminates bulb bombardment and stray
(condensed from EGA. Technical manual TT3)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Filament voltage ....................
Filament current...................
3*25 amperes.
Grid-plate c a p a c i t y
0.1 mmf.
Input capacity....................... 13.5 vsnf.
Output c a p a c i t y ..................... 13.5
Maximum ratings:
Plate v o l t a g e
1250. volts.
Plate c u r r e n t
150. milliamperes.
Plate dissipation
50. watts.
Typical operating conditions:
Plate voltage......................
1000. volts 1250. volts.
Power output, class "B" R. F ............. 20.
25. watts.
Power output, class "C" R. F............. 70.
jS .
Power output, class "C" R. F........... 100.
(telegraphy or buffer service)
130. watts.
RCA£>l4 is a filament type transmitting tetrode incorporating
design involving the use of directed electron beams. Type Sl4 requires
no neutralization when used in R. F. applications. The high power
sensitivity makes this tube especially suitable for use as and R. F.
amplifier, frequency multiplier and plate-modulated amplifier.
(condensed from RCA. Technical Manual TT3.)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Filament voltage ...................
10.0 volts.
Filament current...................
3.25 amperes.
Amplification factor ................
Grid-plate c a p a c i t y ..................13.5
Input capacity......................
6,0 mnf.
Output c a p a c i t y ....................
6.5 nmf.
Uaxinrum ratings:
Plate voltage
1250. volts.
Plate c u r r e n t
Plate dissipation
120. milliamperes.
75. watts.
Typical operating conditions:
Plate voltage
Power output,
class" A "
Power output,
class nAB"(two tubes).
1250. volts.
105. watts.
ECA245 is a three electrode power amplifier tube of the thoriatedttingsten filament type. Especially suitable in class "A" audio service,
(condensed from RCA Technical manual TT3)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
e c a i 6o 3
Heater voltage .
6.3 volts.
Heater current.....................
0.3 amperes.
Maximum ratings:
^ Plate voltage
250. volts.
Plate c u r r e n t ........................ 6.5 milliamperes.
Screen voltage
100. volts.
As class "A"amplifier, pentode connection:
Plate v o l t a g e
250. volts.
Screen voltage
100. volts.
Plate current
....................... 2.0 milliamperes.
^Amplification factor
1500. or more.
ECA1603 is a triple-grid tube of the heater-cathode type,
especially suited as a voltage amplifier for audio frequencies.
Voltage gains of several hundred times are obtainable,
(condensed from ECA. Technical Manual TT3)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Amperex 220C
Filament voltage .............
Filament current .............
Amplification factor . . . . . .
Grid-plate capacity
Input capacity ...............
Output capacity
Maximum ratings:
15,000. volts.
Plate voltage ................
Plate current
Plate dissipation
Typical operating conditions:
Plate voltage
Power output,
class"A" audio
. .10,000.
. . .
7,500. volts.
250. watts.
Power output,
classHB" audio. . .15,000.
(2 tubes)
9,000. watts.
Power output,
class"B" R. F.
2,500. watts.
Power output,
class"C" R . F.
5>000. watts,
Power output, class "C" R. F.
. . .10,000.
7,500. watts,
Amperex 220C is a water-cooled tube widely used for audio and
radio frequency applications in transmitters from 2 .5 to 5 kilowatts
output under conditions of modulation.
(condensed from Amperex tube chart #220C)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Amperex 22SA
Filament voltage
21, 5 volts.
Filament c u r r e n t .................
Ul, 0 amperes.
Amplification factor
Grid-plate capacity ...............
23,4 mmf.
Input c a p a c i t y ...................
15,0 mmf.
Output capacity...................
3. 0 mmf.
Maximum ratings:
Plate voltage
6,000. volts.
Plate current
1,500. milliamperes.
Plate dissipation.............. 5,000
Typical operating conditions:
Plate voltage.................. 5,000
6.000, volts.
Power output, class"B"audio
(2 tubes)
Power output,
. . .
9.000. watts.
class“B" R. F. . . .
Power output, class "C" R.
F. . . .
Power output, class "Cn E. F ............
5,100. watts.
Amperex 228A is a water-cooled tube with a power output of 1 to
2.5 kilowatts for broadcast service.
(condensed from Amperex tube chart #228A. )
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Amperex 892
Filament- two unit type for single-phase or two-phase operation.
Filament voltage
11.0 volts.
Filament current..................... 60.0 amperes.
Amplification factor
Grid-plate capacity
32. mmf.
Input c a p a c i t y ....................
.17. mmf.
Output capacity....................... 1.8 mmf.
Maximum ratings:
Plate voltage
15,000. volts.
Plate current
2,000. milliamperes.
Plate dissipation
10,000. watts.
Typical operating conditions:
Plate voltage
10,000. volts.
Power output, class "B" audio . . .
(two tubes)
22,000. watts.
Power output, class "B” £. F. . . .
2 ,500. watts.
Power output, class "C" E. F. . . .
5,000. watts.
Power output, class "CM E. F. . . .
10,500. watts.
Amperex and EGA. 892 tubes are similar in characteristics and
ratings, water-cooled, and with a power output of 2 .5 to 5 kilowatts
in broadcast service.
(condensed from Amperex tube chart #892.)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Eimac 750TH
Filament voltage ...................
7-5 volts.
Filament current ...................
21.0 amperes.
Amplification factor................. 30.
G-rid-plate c a p a c i t y ...............
4.5 mmf.
Input capacity .....................
6.0 mmf.
Output c a p a c i t y ........................ 8 mmf.
Maximum ratings:
Plate voltage
Plate current
Plate dissipation
Typical operating conditions:
Plate voltage
Power output,
class"B" E. F ....
Power output,
class"C" £. F.
2,100. watts,
Power output,
class"C" E. F.
2,250. watts,
Power output,
class"B" audio
(2 tubes)
. . .
3>000. watts.
Eimac 750TH is an air-cooled vacuum triode with an output of
2 .5
kilowatts for two tubes in radio frequency broadcast service.
The grid and plate are made of tantalum, allowing high temperatures
to be attained by the elements with negligible release of gas. The
type 75OTL is the same as the 750TH except for the amplification
factor, which is 13 times for the 750TL.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Approved power ratings of vacuum tubes for operation in the last
radio frequency stage of broadcast transmitters, abridged from the
Federal Communications Commission charts.
-High Level or Plate Modulation*
Power rating
McCullogh Telegraph
2o 4a
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Power ratings for linear amplifiers.
Power rating
2^2 C
Power rating for grid-bias modulation.
Power rating
2o 4a
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Amperex Tube Manual, Amperex'Manufacturing Co., hew York City.
Bell, W. A., "Inverse Feedback," Radio Magazine, Oct., 1938, pp. 47-49.
Broadcasting Yearbook, Broadcasting Publishing Co., Mew York City,
Doherty, W. H., "A Mew Linear Amplifier of High Efficiency, " Radio
June, 1936, pp. 18-21.
(condensed from Proceedings
of Institute of Radio Engineers)
Eitel McCullogh Tube Charts, Eitel McCullogh, Inc., San Bruno, Calif.
Electronics Engineering Manual, Editorial staff, Electronics Magazine.
McGraw Hill Publishing Co., 1938.
Hawkins, J. M. A., "Care of Transmitting Tubes," Radio Magazine, Feb.,
1936, pp. 66-84.
Hawkins, J. M. A., "The Class C Amplifier," Radio Magazine, Feb.,
1936, pp. 58-82.
Long, J. J., "Porcelain Cooling System at WHAM," Electronics Magazine,
Oct., 1938, pp. 24-25.
Marsden, C. P., "Thermionic Snission," Electronics Magazine, Dec.,
1938, pp. 22-32.
national Association of Broadcasters Engineering Manual, .National
Association of Broadcasters, Vfashington, D. C., 1938.
Radio Amateur*s Handbook, American Radio Relay League, West
Hartford, Conn., 1938, 1939.
RCA Tube Manuals, RCA Manufacturing Co., Camden, N.J. (released at
irregular intervals)
Smith, W. W., The Radio Handbook, Radio, Ltd., Los Angeles, Calif., 1938.
Sterling, George E., The Radio Manual, D. Van Nostrand Publishing Co.,
New York City, 1939"! (third edition)
Terman, F. E. Radio Engineering, McGraw Hill Publishing Co., New York
"Tubes, Inc.," Editorial staff, Electronics Magazine, Nov., 1938,
pp. 13-15.
Tubes, Federal Telegraph Co., Newark, N. J., 1938.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
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