March 1930 Radio News
Wax nostalgic about and learn from the history of early
electronics. See articles from
Radio & Television News, published 1919-1959. All copyrights hereby
Radio News magazine used to publish Information Sheets each
month that readers could easily cut out and insert into a notebook as
a handy reference. For example, one Information Sheet presents basic
information on the current-producing color sensitivities of common elements
like cesium, rubidium, and potassium, along with that of the human eye
for comparison. Another Information Sheet has a table of resistor values
useful for constructing a range-selectable voltmeter from a simple milliammeter.
Also, just as today you can buy a nearly complete AM or FM radio in
the form of an IC (plus a handful of external components), it was possible
even in 1930 to buy a complete radio receiver for integration into a
chassis either as just a radio or as part of a combination unit that
might also contain a record player. Some spec sheets for a few of those
are included here as well just so you can see what they looked like
- including all the vacuum tubes and interstage coupling transformers.
Radio News Information Sheets
Sensitivity of Photoelectric Cells
By Elmore B. Lyford Index No. R-535.31
The activity and sensitivity of a photoelectric cell depends upon
the material with which the inner surface of the bulb is coated. All
metals are photoelectrically active to a greater or less degree, but
the most active ones have been found to be those which chemist call
"rare earths," such as caesium, rubidium, uranium, etc.
No known photoelectrically active substances are sensitive indiscriminately
to all wavelengths of light - each such substance is affected by a definite
band of light waves only, and has its point of maximum sensitivity at
some wavelength within the band. In general. the more active substances
have their point of maximum sensitivity within the visible portion of
the spectrum, while the less active substances respond best to light
in the extreme violet or ultraviolet portion of the spectrum.
The curves given in the accompanying chart show the color-sensitivity
characteristics of three common types of photoelectric cells. These
curves remain practically unchanged regardless of the shape or size
of the cell in which the metal may be employed. The shape and size of
the cell have much to do with its total output, but the relative sensitivity
of the cell to various wavelengths of light is determined almost entirely
by the active material employed.
For purposes of comparison, the color-sensitivity curve of the average
human eye is also shown, and all the curves have been drawn with the
point of maximum response at 100%. This should not be taken to mean
that under the same conditions all of these metals are equally active,
for such is not the case. These curves imply show where the point of
maximum response comes, and how rapidly this response falls off as the
wavelength of the incident light is increased or decreased from this
Multipliers for Small Meters
By Elmore B. Lyford
Index No. R-264.1
Any milliammeter may be used as a voltmeter by using a resistance
of the proper size in series with it. It may be made into a multi-range
voltmeter by using several resistances, and selecting the appropriate
one by the use of a selector switch, as shown in the accompanying diagram.
When deciding upon the proper resistance value to employ as a multiplier,
Ohm's law must be taken into consideration:
The accompanying table give the correct resistance values to use,
with each of the milliammeters shown across the top, to obtain the voltage
range shown in the first column.
If a multi-range voltmeter is desired, several different resistances
may be used in series with the same milliammeter, and the appropriate
one for each reading selected by means of a switch, as shown in the
With a good milliammeter, the accuracy of the voltage readings will
depend almost entirely upon the accuracy of the resistance used, and
these should be the best obtainable. They must also be capable of carrying
a current equal to the milliampere range of the meter used.
Manufactured Receiver Circuits
In the seven-tube Majestic No. 180 receiver two -50 tubes arranged
in push-pull provide exceptionally fine tone quality with little or
no possibility of distortion due to overloading.
Five -27 tubes make up the remainder of the circuit, three being
used as radio-frequency amplifiers, one as a detector and one as a first-stage
Provision is made for either a short or a long antenna and peaking
of the antenna stage is obtained by the use of an antenna trimmer.
A drum switch allows changeover from a radio program to the playing
of phonograph records, by connecting a phonograph pickup in the secondary
circuit of the first-stage audio-frequency amplifier.
The drawing above shows only the circuit diagram of the radio-frequency
amplifier, detector and audio-frequency amplifier portions of the receiver.
The power pack employs two -81 tubes in a standard circuit for full-wave
rectification. The necessary power for the excitation of the field coil
of the dynamic speaker is obtained from the same power pack. A voltage
regulator, connected in series with the primary of the power transformer
provides even volume and distortionless quality.
In the Silver Screen-Grid 30 receiver four screen-grid tubes are
used, three of them functioning as high gain r.f. amplifiers, the fourth
as a screen-grid power detector. The first audio stage, employing a
-27 tube, is coupled to the detector output through a resistance-coupling
unit, so that the impedance relations between the detector plate and
the plate load may be satisfied. Incorporated in this part of the circuit
is a switch which throws in or out a shunt condenser across the plate
resistor so that regulation of the bass or treble notes may be obtained.
In other words, the operation of the switch accentuates one or the other
In the grid circuit of the first-stage audio amplifier tube is a
closed-circuit jack which is intended to take a phonograph pickup for
the electrical reproduction of records through the receiver's audio
A pair of -45's arranged in push-pull fashion complete the audio
A standard power-supply unit provides the plate and filament voltages
for all of the tubes. One feature of the power unit is the line-voltage
regulator which maintains a steady non-fluctuating voltage supply to
the line transformer despite line voltage variations.
Three -24 or a.c. screen grid tubes, two -27 or a.c. heater type
tubes, a pair of -45 output power amplifier tubes and a single -80 or
full-wave rectifier are the tubes which are employed in the Stewart-Warner
Series 950 a.c, operated receiver.
The three screen-grid tubes are used in the three-stage r.f. amplifier,
which is stabilized by the employment of neutralizing or stabilizing
condensers. The tuning condensers are shunted with equalizing capacities
so that perfect alignment of a resistance-coupling unit.
Coupling between the detector and first audio amplifier tubes, both
-27 type tubes is accomplished through the use of a resistance coupling
Push-pull transformers between the first a.f. stage and the pair
of -45 tubes and between the latter and the loud speaker, complete the
Full details of the power supply unit, which follows standard lines,
are given in the circuit above.
The a.c. filament type of tube, otherwise known as the -26, is used
in the antenna coupling stage and the three tuned radio-frequency amplifier
stages of the Kolster, Series K-21 a.c. operated receiver.
The heater type a.c, tube (-27) is used in the detector stage.
These five tubes complete the tuner section which is constructed
as a separate unit, haying its own chassis.
The audio amplifier, consisting of two transformer stages, and the
power supply unit comprise the second unit of the receiver. Connection
between the two units is made through a terminal board or plug arrangement
a shown in the circuit below.
The first audio raze employs a -26 tube while the final audio stage
employ a single -71A tube in the output.
Provision is made for the plugging in of a phonograph pickup so that
the audio channel may be used for the reproduction of phonograph music.
Details of the condenser and resistance hank of the power supply
are shown in detail below.
Posted February 26, 2014