Your RF Cafe
March 1930 Radio News[Table of Contents]
These articles are scanned and OCRed from old editions of the Radio & Television News magazine. Here is a list of the Radio & Television News articles I have already posted. All copyrights are hereby acknowledged.
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.
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 point.
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 accompanying hook-up.
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.
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 audio-frequency amplifier.
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 frequency extreme.
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 channel.
A pair of -45's arranged in push-pull fashion complete the audio amplifier.
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 unit.
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 audio channel.
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