November 1929 Radio-Craft
People old and young enjoy waxing nostalgic about and learning some of the history of early electronics.
Radio-Craft was published from 1929 through 1953. All copyrights are hereby acknowledged. See all articles
Vreeland Corporation was an early radio manufacturer located
in Hoboken, New Jersey, with multiple patents on file for innovative
Vreeland band 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
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.
The use of a screen-grid detector with a stage of resistance-coupled
audio amplification is another feature that gives high, distortionless
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).
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.
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.
An optional but experimental design of the
"band-selector;" Selectivity is governed by C; R closes the
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
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
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
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 supetheterodyne 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
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."
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.
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
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 AB.
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
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
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.
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.
AA 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.
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
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 shielded.
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 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. 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
Details of "band-selector" coils. (Two are
required.) The use of different tuning condensers will necessitate
a different number of turns.
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
With capacitative coupling, the coupling decreases with increase
of frequency. Therefore the selectivity is greater at the lower
wavelengths 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 broadcast band.)
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 sheet copper.
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
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.
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 this unit.
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.
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
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.
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.
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.
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.
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
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
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
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
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 condensers,
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 maximum sensitivity.
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."
Parts Required for "1930 Electric" Receiver
Posted December 18, 2015