November 1929 Radio-Craft
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
Vreeland Corporation was an
early radio manufacturer located in Hoboken, New Jersey, with multiple patents on
file for innovative circuits. The
selector system mentioned here was originally filed in the U.S. Patent and Trademark
Office in August of 1927 and had not been awarded by the time of this November 1929
article in Radio−Craft magazine. In fact, it wasn't until five years later,
in 1932, that the patent was finally assigned. The official description reads in
part, "The general purpose of the invention is to receive the component frequencies
of such a band with such uniformity as to avoid material distortion of the modulated
wave, and to exclude frequencies outside of the band which the system is designed
to receive. Another purpose of the invention is to provide means for shifting the
position of the band in the frequency scale at will, by a simple adjustment, so
that the system may be readily adapted to receive modulated waves of any desired
carrier frequency, including the side bands of such modulated waves." That sounds
to me like a standard heterodyne system with selective filters. It seems the filter
characteristic with a wider inband region and sharp cutoff is what make it unique.
One line in the patent states, "... a radio receiving system embodying a plurality
of my [emphasis added] band selector units, one being
associated with a collector ..." Note the use of the possessive "my," by Frederick
K. Vreeland, which I doubt would ever appear in a contemporary patent filing. Mr.
Vreeland authored books such as
Maxwell's Theory and Wireless Telegraphy as well as
Camp Buildings and Scout Shelters for the Boy Scouts
Building a 1930 Electric Receiver
A super-sensitive set of simple construction
incorporating the latest principles in radio design.
By Clyde J. Fitch
The modern factory-built radio set is so far advanced in design that one hesitates
now to construct his own; yet with a little care and patience a commercial-looking
job can be turned out at home that will excel the latest commercial models. The
receiver shown in the accompanying illustrations was designed for sensitivity, tone
quality, and ease of operation; while it incorporates new features not found in
other receivers. It was built for installation on the Florin Lenox estate in the
Adirondack Mountains, a location which is immune from interference of any kind except
an occasional thunderstorm; consequently unusual selectivity was not required. However
this set compares with the best in selectivity.
Locals with a 2-foot aerial! Absolutely no batteries about which to think twice.
The power of two '10s in push-pull at the flick of a switch and turn of a knob!
All station selection obtained by the turn of another knob. "Wind-your-own" selectivity!
Three stages of screen-grid amplification (guaranteed non-whistling) and a screen-grid
"power detector." And audio quality with a realism which will startle the critical
ear. When we heard that our old friend Clyde J. Fitch had built, at moderate cost,
a receiver which met these qualifications, we asked Mr. Fitch to tell you all about
it. And so - we not take pleasure in presenting
"The 1930 Electric Receiver"
It comprises a Vreeland band-selector system which feeds into a three-stage untuned
R.F. amplifier using A.C. screen-grid tubes and iron-core untuned radio-frequency
transformers. A screen-grid detector is used with one stage of resistance-coupled
audio amplification and one of push-pull using type '10 power tubes. This makes
seven tubes in all, four screen-grid type '24, one type '27, and two '10's; not
counting the '81 rectifier and '74 voltage regulator used in the power pack.
The band selector, the theory of which is explained below, passes a 10-kilocycle
band of wavelengths at any location in the broadcast spectrum, depending upon the
adjustment of the tuning dial. This allows the carrier-wave with its accompanying
"sidebands" (which represent the music), to pass through from the antenna to the
amplifier uniformly, giving distortionless reception in this part of the set. This
is true also of the radio-frequency amplifier, which employs the old Acme iron-core
transformers, used several years ago in that company's reflex sets. These transformers
give very efficient amplification over the entire broadcast band, and associated
circuits do not oscillate ("'whistle," howl, etc.) at any point on the dial when
used with screen-grid tubes. Two type-R3 transformers, (RFT1 and RFT2 in Fig. 1)
with one type-R4 (RFT3), gave the best results; the type-R2 was found unsatisfactory.
No doubt there are other iron-core radio-frequency transformers that may be used
instead, in case the constructor has difficulty in finding the Acmes.
The "band-selector" detail. Varying the number of turns in L
determines the selectivity of the receiver.
Fig. A - Front of set, showing panel layout: note convenient
adaptability to any panel size. The "volume control" is R5; while C2-C3 is the "station
selector" or tuning knob. A light, rigid, monel-metal sub-panel is used.
An optional but experimental design of the "band-selector;" Selectivity
is governed by C; R closes the circuit.
The use of a screen-grid detector with a stage of resistance-coupled audio amplification
is another feature that gives high, distortionless amplification.
Here (in New York City) the set picks up locals with an indoor antenna two feet
long; with no antenna it picks up nothing, since it is well shielded. Quality is
unusually good, for the reasons mentioned above, and operation is simple, as the
illustration (Fig. A) shows; one knob (C2-C3) being used for tuning, and the
other (R5) for a volume control (which is easy to regulate because it does not detune
the set or cause it to oscillate and squeal or howl).
In the illustrations, Fig. A shows the front view; Fig. B the top with
the covers of the shield cans removed; Fig. C the rear, and Fig. D the
power pack. The panel was cut to the size shown, merely to fit a special cabinet.
Construction of the Set
The metal base should be made first; that illustrated here was a sheet of monel
metal 13 1/2 x 29 1/2, inches and 1/32-inch thick with the corners cut out to a
depth of an inch each way and the sides bent down and soldered at the corners, making
a "pan"11 x 27 x 1 1/4·inch deep. This work should be done by a tinsmith. Aluminum
may be used instead, but it should be at least 1/16-inch thick, as it is mechanically
weaker than monel metal; since aluminum cannot be soldered easily, the corners should
be strengthened with brass angles bolted in place. Even a good wooden baseboard
may be substituted.
Dimensions for drilling the base are not given, as these will depend upon the
particular parts used. The general layout shown in Fig. B can be followed without
difficulty. The sockets are mounted with their terminals underneath; so that practically
all the wiring is below, out of sight and well protected. With the exception of
the socket holes, the drilling of the base may proceed as the set is being assembled,
by the use of a hand drill.
The Band Selector
The band-selector coils L1 and L2 consist each of 85 turns of No. 28 wire wound
2 inches in diameter, with a center tap, as detailed in Fig. 2. (Hammarlund
space-wound coils were used.) They are clamped by two strips of Bakelite, which
carry the three terminals for the two end connections and center tap. Brass angles
are used to mount them in the proper relation to the metal base, inside the shield
cans as shown.
The two 0.00035-mf. tuning condensers C2, C3 are mounted on the shield cans as
illustrated; both on the same shaft and with a single-dial control.
The 23-plate "midget" antenna coupling condenser C1 is mounted on an insulating
strip of Bakelite which is fastened to the upper left-hand corner of the right-hand
can, (Fig. B). The 9-plate "midget" trimming condenser C4 is mounted directly
on the other shield can, as shown in the same figure.
The coupling coil L3 consists of 4 1/2 turns of wire wound on a vacuum-tube base.
This is plugged into a standard socket, mounted on the set base as shown. The two
ends of the coil are connected to the filament prongs of the tube base. This completes
the band-selector parts; wiring and adjustment will be described later.
Many explanations have been published about this new method of radio tuning,
but the practical man wants more practical data and a clearer understanding of the
theory without wading deeply into mathematics; therefore, let us begin at the beginning.
and explain why bandpass tuning is necessary for undistorted reception.
Suppose we start at the transmitter, but forget the old idea of modulated carrier
waves and look at the situation from a different viewpoint; a viewpoint where even
the layman can get a clear insight into radio transmission. From our method of heterodyne
reception (as used in the superheterodyne receiver) we know that when two alternating
currents of different frequencies, F and F1, are combined, two other frequencies
are produced, equal to (F + F1) and (F -- F1); making four distinct frequencies
in all. (Harmonics of these will also appear, but they are useless for our purpose.)
"Modulation" Simply Explained
At left is illustrated the sharp "cut-off" by which "volume distortion"
is caused in ordinary tuned-radio-frequency sets; at the right the desirable "flat-top"
effect which may be obtained by proper "band-selector" design.
Since radiation of electric energy from the transmitting antenna must take place
at very high frequencies to be efficient, a high-frequency generator is used at
the transmitter. For broadcasting, frequencies from 500,000 to 1,500,000 cycles
per second are used, each station having its own assigned operating frequency. This
frequency is called the carrier-frequency, or "carrier wave." We will call this
frequency f. Now, suppose we combine with this carrier frequency, f,
the sound-frequencies (or music and speech-frequencies produced in the studio) and
see what happens; keeping in mind that the music- and speech-frequencies range from
about 50 to 5,000 cycles per second. We will call these the audio-frequency band,
or just "AB."
From the above heterodyning action, we learn that four distinct frequencies will
result; namely, the carrier-frequency f, the audio-frequency band AB, and
the bands (f + AB) and (f-AB).
The audio-frequency band AB will not be radiated from the antenna, because its
frequencies (50 to 5000 cycles) are too low for efficient radiation. The carrier
frequency, f, will be radiated as will also the frequencies (f + AB)
and f - AB). These latter two are called the sidebands, and they contain
all the music and speech, because they contain the audio-frequency-band component
In explaining radio transmission to a layman, we can simplify this still further
and merely state that radiation takes place at very high frequencies; therefore
at the transmitter we add a high frequency, say a million cycles, to the sound frequencies,
so that they can be radiated, and later remove the high frequency at the receiver.
The ordinary person will grasp this heterodyne idea much quicker than he can grasp
the modulated-carrier-wave idea.
We see from the above that a group of frequencies (namely, f, f
+ AB, and f - AB) are radiated from the broadcast transmitter having a
maximum difference of I plus and minus 5000, or a total separation of 10,000 cycles,
or 10 kilocycles. For example, using a carrier of 1,000,000 cycles, a band of frequencies
from 995,000 to 1,005,000 cycles will be radiated. Our receiving set must therefore
tune in all these frequencies at once in order to receive all the music; or the
sopranos will be lost in the ether, and this might not always improve the programs.
Hence, the use of a band-pass or selector.
Fig. B - An airplane view of "The 1930 Electric Receiver."
Simplicity is a key-note. A convenience is socket L3 into which the coupling coil
of the "band-selector" is plugged. Condensers C1 and C4 are mounted on the ends
of the aluminum shield cans.
A view of the interior of S3. looking from the front of the set.
The fixed condensers are soldered to each other. To obtain maximum efficiency from
the R.F. transformers, they are mounted away from all metal work. There is no interaction
between these units.
The power unit. Figures 709, 854, and PF281 are parts numbers.
The power-transformer primary is provided with taps for line-voltage compensation.
One voltage regulator, V9, is used.
Fig. C - Rear view. The arrangement of the binding posts
is clearly shown in this illustration. Tube shields which thread on were used to
prevent noisy contacts. Note the neatness of this powerful screen-grid-type radio
receiver. The parts used are standard.
Details of "band-selector" coils. (Two are required.) The use
of different tuning condensers will necessitate a different number of turns.
The ordinary radio set must be selective in order to tune in a single station
without interference from others. Tuning must be sharp. But sharp tuning cuts the
sidebands; it weakens or eliminates the higher audio frequencies, and causes the
music to sound deep and muffled. Instead of tuning in the complete ten-kilocycle
band of waves, it covers a band of only about four or five. This is shown in the
response curve of Fig. 5, which only has good response over a 4-kc. band. Each
tuned circuit in the receiver has one definite "peak" where the response is maximum,
and this is adjusted to be in resonance with the carrier-frequency.
Design of the Selector
The band-pass selector has two tuned circuits, loosely coupled together. The
two circuits give two peaks, adjusted so as to be dose together, making a somewhat
flat-topped graph 10 kilocycles wide, as shown in Fig. 6. The curve has two
small humps at the top c used by the two peaks of the two circuits. The sides slope
down about as in the curve of Fig. 5, but the width at the base, using carefully-designed
circuits, is no greater than in the former curve. Therefore, selectivity is as good
when using the band selector as it is with the ordinary method of tuning, and it
responds to the entire band of transmitted frequencies more evenly.
Fig. 7 shows the circuit diagram of a band selector; this is the system
used in our complete schematic, Fig. 1. The two tuned circuits are indicated
at A and B, and are coupled together by the common inductance L. The circuits A
and B have each the same sizes of coils and condensers, and are designed to cover
the broadcast range; therefore any good broadcast coils and condensers may be used.
Magnetic coupling must not exist between the coils; therefore they must be thoroughly
The degree of coupling is determined by the value of the inductance L; this,
also, determines the spacing between the peaks of the two resonance curves, and
consequently the width of the band of frequencies that it will pass. If L is increased,
coupling is increased and the width of the curve, Fig. 6, is increased. If
L is decreased, the width of the curve is decreased. The correct value of the coupling
inductance L will give us the desired 10-kc. width. 4 1/2 turns, wound on a tube
base, is about the correct number. Several such coils may be made, ranging from
2 1/2 to 6 turns, and are easily interchanged by mounting them in a standard socket.
The smaller the coil, the greater the selectivity. You can therefore adjust the
selectivity to suit your particular requirements.
The width of the curve (Fig. 6) does not remain constant over the entire
broadcast band, but varies with the frequency. Using the circuit of Fig. 7
with inductive coupling, the width increases with increase of frequency; because
the reactance of the inductance L is greater at the higher frequencies, and the
degree of coupling is greater. Hence it may pass only the correct10-kc. band at
the upper wavelength settings, but a 10-kc. band at the lower. As this will be objectionable
in some localities, the system shown in Fig. 8 may be employed instead.
The only difference is that the two circuits are coupled by a common capacity
C, instead of by an inductance. (When using the coupling condenser C, the grid circuit
of the first tube in the R.F. amplifier will be open. The resistor R, of about 2
megohms, should be connected across the condenser C.) This gives just the opposite
effect to that obtained with the inductance; the larger the capacity, the less the
degree of coupling, and vice versa. A coupling condenser of 0.025-mf. is about the
correct size. This also may be mounted into a tube base, together with the resistor
R, and plugged in the socket in place of the coupling coil.
With capacitative coupling, the coupling decreases with increase of frequency.
There-fore the selectivity is greater at the lower wave-lengths than at the higher,
which is just the opposite effect to that obtained with the inductance. Different
sizes of condensers may be tried to get the correct balance. (Perhaps a combination
of capacity and inductance may be found to give a 10-kc. curve through the entire
Summing up, the receiving aerial picks up all the stations on the air, but the
band selector allows only the band of frequencies transmitted by anyone selected
station to pass through it and into the R.F. amplifier.
R.F. Amplifier and Detector
The radio-frequency amplifier and detector is a complete unit in itself and wired
independently. The mounting of the apparatus is indicated in cross-section in Fig. 3,
and the reproduced photographs clearly show the unit and its location on the base.
The copper shield can (S3) of a Remler "Infradyne" intermediate amplifier was used
by the writer, and was just the correct size for inclosing the parts; a metal top
was required, however, measuring 4 by 16 inches. All the parts are mounted on this
top, so that it can be removed for inspection or repair without removing the can
from the base. The can is 2 3/4 inches deep inside, and can easily be made up of
The sockets are equally spaced on the metal top, as shown, while the radio-frequency
transformers are mounted below them and away from the can. Below the detector socket
is fastened a strip of bakelite supporting one of the by-pass condensers; the other
condensers are soldered together and mounted between the sockets. Eleven one-microfarad
condensers in all are used in this unit. Condensers of this capacity were selected
to by-pass thoroughly all the connections in each stage, so there would be no tendency
to oscillate due to coupling of the common connections. In ordinary sets this effect
is negligible; in "high gain" receivers using screen-grid tubes "common coupling"
becomes a serious problem.
The wiring of this amplifier is clearly shown in the complete schematic diagram,
Fig. 1. The space enclosed by dotted lines S3 indicates the connections inside
The 100-ohm resistor R1 , between the R.F. cathodes and ground, gives a 1 1/2-volt
"C" bias on the R.F. control grids. The 5000-ohm resistor R2, connected between
the detector cathode and ground, gives 5 volts bias on the detector control grid.
These resistors are mounted in the amplifier unit but are not shown in Fig.
Schematic diagram of the "1930 Electric Receiver." The shield
cans for the various units are indicated by dotted lines. Resistor R6, once adjusted,
need not be changed unless the characteristics of V4 change appreciably. A dynamic
reproducer may be substituted for the electrostatic reproducer indicated. Resistors
RB, R9 prevent undesirable oscillation in the push-pull stages. Every by-pass condenser
shown should be used.
Construction of the R.F. transformers. The secondary is arranged
in "pies" to reduce self-capacity to a minimum. "Flat" response is obtained by variation
of the number of primary and secondary turns.
Fig. D - Perspective view of the power pack, which should
be mounted at a distance from the set chassis. Power transformer PT should be at
the point most distant from audio transformer AFT. Filter chokes with generously-proportioned
cores greatly help to obtain humless operation.
Connections to the amplifier unit are brought out through holes in the bottom.
There will be two heavy leads for the 20 1/2-volt A.C. supply to the heaters. Large
flexible wire should be used for these, as the current for the four tubes will be
There are four other connections to the amplifier, colored preferably as indicated
in the diagram; blue for volume control (R5) regulating the screen-voltage of the
R.F. tubes; brown, for detector screen-voltage control (R6); green for the detector
plate; and red for the 180 volt "B" supply. The shielded control-grid leads are
brought out through the top, as shown in the photographic illustrations. The first
one comes from the band-selector shield can, as will be seen from Fig. B.
Suitable untuned transformers for the R.F. amplifier can be constructed according
to the illustration, Fig. 9. The core consists of strips of very thin silicon
steel transformer laminations 1/2" wide by 2 3/8" long, stacked up to a thickness
of 3/16", as shown. On each side of this is placed a wooden form 5/32" by 1/2" by
2 3/8" long, so that the finished core assembly is 1/2" square. It is bound together
with a few wrappings of waxed paper.
The primary winding is "random wound" over one half of the core length. No. 38
S.C.C. wire is used. (This is indicated in the illustration for clearness as a single
layer.) The secondary is also of No. 38 S.C.C. wire wound in ten sections, or "pies,"
equally spaced over the entire length of the core as shown. A layer of waxed paper
is placed between the two windings.
The wavelength band which the transformer efficiently covers depends upon the
number of turns in the primary and secondary. For the first and second stage units,
RFT-1 and RFT-2, the primary should have 35 and the secondary 83 turns.
For the third stage, RFT-3, the primary should have 26 and the secondary 43 turns.
As there will be some variation in these transformers due to different qualities
of iron, it may be necessary to add or subtract wire from both primary and secondary
to obtain the best results. This can easily be done experimentally after once operating
the set to determine if it amplifies uniformly over the entire broadcast range.
The Audio Channel
The audio amplifier comprises one stage using resistance-capacity coupling (V5)
and one with push-pull (V6-V7). Resistance-capacity coupling was selected for the
first stage, because it is the most efficient method when used with a screen-grid
detector. The plate-circuit resistor R3 used at a plate voltage of 180 is 250,000
ohms, which allows about one milliampere to pass through the detector with a grid
bias of 5 volts and a "screen" voltage of 75. The screen voltage is adjusted by
the 500,000-ohm potentiometer, R6, mounted on the base. When once set, this requires
no further adjustment.
The plate resistor R3, grid leak R4, and 0.05-mf. coupling condenser C17 are
mounted on the base, as shown. The locations of the '27-type A.C. tube, the two
'10s, and the input push-pull transformer AFT are clearly illustrated.
Underneath the base are mounted the detector R.F. choke L4; the 0.0005-mf. by-pass
condenser C16; the 1500-ohm "C" bias resistor R7, for the first audio tube; the
two 50,000-ohm stabilizing resistors R8 and R9, in the push-pull grid returns; and
the by-pass condensers. A 1-mf. condenser C18 shunts the 180-volt and ground terminals;
2-mf. (C19) is the by-pass capacity for the 90-volt output; and 1-mf. (C20) by-passes
the first-audio "C" bias resistor.
The two plate terminals 1 and 2 of the output tubes are mounted on a Bakelite
strip at the right end of the base, while the other terminals are along the back,
arranged as indicated in Fig. B.
The center tapped output choke used with this set is mounted in the cabinet with
the reproducer. It has a value of 400 henries and a carrying capacity of 60 ma.
(To prevent parasitic oscillation the halves are balanced at the factory for absolute
electrical symmetry on both sides of the center tap.)
It is fitting to observe at this moment that a primary fault in average push-pull
circuits is that malady variously known as "feedback," "interference," "harmonics,"
etc.; in plain words, parasitic oscillation. It manifests itself in an above-normal
plate current; and it may, or it may not, be accompanied by a high-pitch whistle,
and distortion. It does not make any difference whether it causes distorted reception,
the idea is that a fault exists which is going to cost the owner some money unless
it is remedied. As exceptional precautions have been taken in the design of this
receiver to prevent parasitic oscillation in the push. pull stage, there is almost
no likelihood of trouble from this source. In addition to measures mentioned above,
the author points out the use of two stabilizers or oscillation suppressors, R8
and R9, in the "grid return" lead of each power tube. This connection is possible
only when the input transformer secondary has a balanced, two-section winding. (To
by-pass these resistors would be to defeat their purpose. The "grid bias resistor"
R14 serves an entirely different purpose and it is necessary that this unit be properly
by-passed, - the purpose of C25.) A single resistor in a single return lead would
be "common" to both tubes and the "isolation" desired would not be obtained.
By carefully following the diagram, Fig. 1, you should have no trouble in
wiring the set. Be sure to use large cable connections for the 7 1/2-volt filament
and the 2 1/2-volt heater supply. The practice of transposition is followed with
all filament leads; that is, they are twisted to eliminate any possibility of an
induction hum. The metal base is grounded, also the "B-" lead from the power pack.
The band-selector connections should be as direct as possible. The leads should
be well insulated and securely soldered. After the panel is put in place, the condenser
shaft lined up and dial mounted, the set is ready for operation.
Type '45 or '10 Power Tubes?
During the early stages of the design, the question arose as whether type '45,
type '10 or type '50 tubes should be used. Each has its advantages and disadvantages.
(The constructor is recommended to experiment with the necessary power equipment
for type '45 tubes, where the power-handling ability of the type '50 tube is not
needed for driving dynamic reproducers.) Type '10 tubes, however, were selected
because it was intended to use an electrostatic reproducer, the polarizing potential
for which would be available with the operating voltages of type '10 tubes; whereas
the use of type '45 tubes, which do nor operate at such high plate potentials, would
necessitate a special "polarizing" unit for the electrostatic reproducer. (The "undistorted
power output" of two '10s is about the same, at the voltages shown, as that of two
'45s at 250 volts, plate.) The question of single or push-pull operation was settled
almost instantly; as the push-pull arrangement, on many counts, is far superior
to a single tube. It will be noticed that coupling condensers have been dispensed
with; this arrangement is possible only when the output is push-pull, for the direct
current (which would ordinarily circulate through the reproducer winding or matching
transformer primary, when contact is made directly to the plate - in the case of
"dynamic" and "electro-magnetic" reproducers), is balanced out by this connection.
What the constructor is most interested to know is that the frequency-discrimination
of coupling condensers has been eliminated and better reproduction results.
Where "electrostatic" reproducers are used, it will be an advantage to connect
directly to the plate, as will be observed from consideration of the output connections
of Fig. 1. Of course, the use of a dynamic reproducer is optional; its two
leads being connected to output posts 1 and 2.
The Power Pack
Parts Required for "1930 Electric" Receiver
The power pack (Fig. D) is also mounted on a metal base. This base (when
shaped) is 10" x 18"x 1 1/2" deep. Only one illustration, besides the diagram (Fig. 4)
is given as this unit is comparatively simple to build. The list of parts gives
the details of the apparatus used in the power pack, all of which is mounted on
top of the base.
Each of the filter condensers (C21-22, C23-24) consists of two 4-mf. units. Two
of these are connected in series, giving a total capacity of 2 mf. across the highest-voltage
side. One of the other 4-mf. sections is connected across the 425-volt lead and
the fourth across the resistance bank.
The latter consists of one 3000-ohm, 100 watt, resistor R10; one 2000-ohm, 100-watt,
resistor, R11; one 3000-ohm, 20-watt, R12; and one 4500-0hm, 20-watt resistor, R13;
all connected in series as shown and mounted on a small Bakelite panel. A 1000-ohm,
100-watt, "C" bias resistor R14 is also mounted on this panel, and by-passed by
the 1-mf. condenser C25. Be sure to use well-insulated wire for making all connections.
Resistor R10 functions as an over-all voltage control, "absorbing" the potential-difference
between 180 and approximately 400 volts.
Choke L5 has a greater current carrying capacity than L6. Its rating is 120 ma.,
20 Henries, and D.C. resistance of 210 ohms; L6 being rated at 60 ma., 50 Henries
and 600 ohms. It will be seen that the power tubes derive their plate supply through
L5, but not through L6. This connection results in absolutely humless operation
(so far as the filter is concerned) when the output tubes are in push-pull; a little
more filtration being desirable for a single-tube-output design.
The 110-volt A.C. connection to the power transformer passes through the switch
SW on the panel of the set. The filament connections between the power pack and
the set should be made with large wire; so that there will be no appreciable voltage
drop. The pack should not be placed too close to the set; because A.C. hum may be
picked up by the set if it is placed in the magnetic field of the power transformer.
This completes the assembly of the parts, with all the apparatus mounted, the
radio-frequency amplifier unit all wired and its leads coming out the bottom, through
holes in the base.
Adjustment and Operation
With everything hooked up properly, simply turn on the switch and the set is
ready for operation. To adjust the band selector, tune in a station and turn the
volume down. Then set the antenna-coupling condenser C1 at maximum and retune the
station while changing the trimming condenser. For maximum sensitivity, the two
tuned circuits must be in resonance; and this condition is obtained by carefully
adjusting the trimming condenser C4. A screwdriver may be used to adjust the two
"midget" condensers by cutting holes in the shields and slotting the shafts of the
If the set is broad in tuning, decrease the capacity of C1 and readjust C4. In
this way, the correct capacity for your particular antenna can be found. A trimming
condenser for the first tuned circuit has been found unnecessary; but one may be
used to facilitate the adjustment. The adjustment should be tried at both upper
and lower ends of the scale.
The tubes also may be shifted around, for they vary slightly, and some work better
as a detector than others. The detector screen voltage should also be adjusted for
Using a good electrostatic or dynamic reproducer, this set will give unusually
good tone quality with very little A.C. hum; and you will be well pleased with the
ease in operation and sensitivity. With the '10 push-pull amplifier you can obtain
sufficient volume for any occasion. If preferred, '45 tubes may be substituted by
making changes to suit in the power pack.
Acknowledgement is made here of the courtesy of Mr. R. H. Siemens, chief engineer
of the Radio Construction Laboratories, who kindly provided laboratory facilities
and technical aid during the design and construction of the "1930 Electric Receiver."
Posted August 30, 2022
(updated from original post