Movietone Bulletin (August 1928)

The How and Why EVERAL projectionists, movieS tone and otherwise, have asked the same question, though of course in somewhat different words. Boiled down it is this: Just how and why does a vacuum tube amplify sound? A fair question, though not an easy one to answer in such manner that the non-technical man _ will have a clear understanding of the matter. However, provided the engineers will permit what I shall say to get past them, I will attempt to explain in non-technical language. Please remember that it may be necessary to make some sacrifice of strict technical correctness, if I am to really make you understand. In the main, though, the things set forth will be correct. I hope they will also be intelligible. In a vacuum tube such as we use for sound amplification, there are three elements, or “electrodes” as they are usually termed by technical men. They are called respectively the “filament” the “grid” and the “plate.” The grid is located between the filament and the plate. It is in effect an electrical valve which controls amplification of sound, as will hereinafter be set forth. The filament is heated by the “A” battery and. when so heated has what is termed “electrons” boiled out of it. Put in another way, it throws off electrons, which consist of and are tiny particles of electric energy, or current, negatively charged. Electrons are only boiled out or thrown off by a hot conductor in a vacuum. They travel in straight lines and at high velocity. The plate is positively charged by the “B” battery. It is connected to the output circuit, or in other words to the sound reproducing apparatus. Positive attracts negative, as you all know, hence, (disregarding the grid for the moment,) since the plate carries a positive and the filament a negative charge, the plate would attract to it most of the electrons thrown off by the filament, or indeed practically all of them if the plate voltage be high enough. But between the filament and the plate is the grid, carrying a negative charge supplied by the “C” battery, and that is the real secret of the whole business. Not only is negative attracted by positive as before set forth, but it is repelled by negative. That is to say, two negative charges will not attract each other, but on the contrary will repel each other. Remembering, then, that between the negative electron-emitting filament and the positively charged plate is the negatively charged grid, it follows that the flow of electrons will be opposed, and opposed in exact proportion to the negative voltage applied to the grid, hence any variation in or of the grid voltage will cause a corresponding change in the plate current, and it is by changing the grid voltage that we are able to alter the plate current at will. The amount of power required to operate the grid and cause the desired changes is negligible, but we can take a relatively large amount of power from the plate. In other words in the vacuum tube we have the means for using a very small amount of power to control a relatively large amount of power. It requires a very slight exertion to push down the accelerator of your car, but the use of that small effort operates to release and make available the full power of the engine. Vacuum tube amplification action is much the same. It is a valve action. In fact, in Europe vacuum tubes are called “valves.” The impulses from the movietone photo-electric cell are very, very weak—much too weak to operate even the most delicately adjusted apparatus. It would, therefore, be impossible to transform them into audible sound until and unless they be made much stronger. We therefore conduct these weak impulses to the grid of a vacuum tube, where they act to set up exact copies of themselves in the plate circuit, only those “copies” are vastly stronger than the original impulses. The more intense the fluctuations, the louder will be the sound produced for any given degree of amplification. And thus, through the flow of electrons from filament to plate we have amplification, and by the action of what amounts to an electric valve (the grid) we are able to control the amplification and secure the desired results in sound amplification. Recapitulation: The heated filament throws off negatively charged electrons, the quantity so emitted being in proportion to the temperature of the filament, up to its operating capacity. The flow of these electrons constitutes the current which will operate the sound reproducing apparatus when governed and controlled by the voltage impressed upon the grid, which latter comes from and is controlled by the photo-electric cell at the projector. I might add that since the capacity for amplification by one vacuum tube would be entirely inadequate to operate a loud speaker, two or more of the tubes are connected in series, or in parallel, like batteries, by which means the power is built up to any desired point. Note: The Wise Ones need not waste time starting to pick the foregoing to pieces. All I have attempted to do is to make clear the basic principles upon which vacuum tubes operate in producing sound amplification. When that is understood, the study of the technique becomes far more easy; also I am firmly convinced that men who have a clear understanding of the basic principles upon which any apparatus depends for its action will, in the end, be able to handle the apparatus far better and more efficiently. T shall follow this article presently with another, explaining just in what manner a vacuum tube is made to act as a current rectifier. Later, other phases of the apparatus will be explained in a similar manner. If there are any points you would like to have dealt with, please advise the editor and it will be attended to as soon as possible. Movietone Mike knows his onion tops when it comes to film technique. Everybody says his new girl is negative. Mike says that’s all right—he’ll develop her. Part of the 5th commandment was aimed at projectionists—honor thy fader.