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JAN -6 193^
INTERNATIONAL PROJECTIONIST
VOLUME VI
NUMBER 3
DECEMBER 1933
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THE ELEMENTS OF|AUDIO AMPLIFICATION
Charles Felstead
THE functioning of the thermionic vacuum tubes and their operation in audio-frequency amplifiers is more or less of a mystery to most persons. Yet it need not be, for if the fundamental principles underlying the action of vacuum tubes are once mastered, the subject is one that is not difficult to comprehend.
Fortunately for the purposes of this treatise, all of the tubes used in theatre sound projection systems are of the simple three-electrode type; although those that are employed as rectifiers function simply as two-electrode tubes. Some of the new tubes used for radio reception have as many as seven electrodes, such as the 6A7 pentagrid converter with its filament, cathode, four grids, and plate.
Even though he might never have occasion to make repairs on the audiofrequency amplifiers in the sound projection equipment, it is well for the projectionist to know the principles on which that portion of the apparatus functions. Since a stage of amplification is composed of little more than a vacuum
tube and its associated input and output coupling devices, a thorough understanding of the theory of the vacuum tube makes it easy to comprehend the operation of an amplifier.
To make the presentation of the data as clear as possible, the work has been divided into three sections. This first part deals with the construction, and the theory of the functioning, of elementary two-electrode vacuum tubes and their use as rectifiers; while the second and third parts discuss the functioning of three-electrode tubes and their operation in audio-frequency amplifiers. The Diode Tube
As early as 1884, Thomas A. Edison made the discovery that hot bodies in vacuo had the property of permitting the passage of electrons to a cold electrode in the same vessel. This property was called the "Edison effect", and was first detected in incandescent lamps. Later experiments resulted in the development of the two-electrode vacuum tube, or diode, by Fleming in 1896.
The original diode was known as the "Fleming Valve", and consisted of a
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hot filament surrounded by a cold metal plate within an evacuated glass bulb. The filament and glass bulb used resembled the filament and bulb of a large automobile headlight lamp. Before discussing the functioning of this device, it will be necessary to examine the action of a heated filament in vacuo.
The metal that forms the filament of the tube is composed of atoms which are constantly moving in all directions within the metal. There are some electrons that have escaped from atoms that are likewise moving within the metal from one atom to another, and their average velocity is greater than that of the atoms. If the metal of the filament is heated, the speed at which the atoms move increases with the increase of temperature.
The heating of the filament is accomplished by passing an electric current through it, in the same manner that the filament of an electric light is heated. If this heating of the metal were done in air, the emitted electrons would fall back to the filament, and oxidation of the surface would occur; so in the fol