Exhibitors Herald and Moving Picture World (Apr-Jun 1930)

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912 EXHIBITORS HERALD-WORLD June 7, 193 1 m SOUND PICTURES BLUEBOOK SCHOOL . . . WIDE FILS' ANSWER TO BLUEBOOK SCHOOL QUESTION NO. 42 By F. H. RICHARDSON BLUEBOOK SCHOOL QUESTION NO. 54. — Describe an electric storage battery. Let us see just what you really know about it. THE QUESTION WAS: Are amplifying tubes connected, in series or in parallel? How is it that when a steady negative voltage is applied to the grid, the grid voltage is nevertheless fluctuating ? In what way would you clean the contact studs of an amplifying tube? In certain types of equipment the photoelectric cell amplifying tubes are held in suspension by springs. Why is this and what precautions are necessary? WELL, gentlemen, out of 642 answers to this question I am able to pronounce only four as worthy of being passed. Those four were prepared by Gayle Grubauch, Frank Dudiak, Karl Friedrich and G. T. Baldy. If the makeup chap over in Chicago will give us sufficient space I will print three of the answers. First, Karl Friedrich says: “In aswer to Question 42, will say that amplifying tubes may' be connected in a number of different ways. If you refer to the tube as a whole there is the cascade connection, the parallel connection and the push-pull connection. If you refer to the filaments only, then there is a choice between the parallel and series connection. “There are two voltages applied to the grid of an amplifying tube. The steady negative voltage supplied by a C battery or substitute, and the fluctuating voltage supplied by the signal we wish to amplify. The result is a fluctuating voltage. The studs of amplifying tubes may be cleaned with an eraser. In some types of equip ment the tubes are mounted in spring suspension cradles in order to prevent mechanical vibrations from being picked up by the tubes and amplified through the horns. In this type of equipment it is necessary to see that the cradle swings free and does not come into contact with any part of the projector or other object that has any tendency to vibrate.” That, I believe you must all agree, is both concise and comprehensive as an answer. Brother Friedrich evidently knows his amplification onions pretty well. My compliments. Next we will listen to Brother Dudiak — no, I think I will conserve space by quoting only one section of it, as follows: “When a C battery is connected to the grid circuit it gives a steady current to the grid. This current must be fluctuated, however, so that it may be multiplied in the vacuum tube. This is accomplished by the input wire being connected to one side of the battery and the current leaves the battery through the negative terminal into the grid. Now, since the current in the input wire is vibratory, it will cause the battery current to flucuate accordingly. That’s the whole thing in a nutshell.” And now, thirdly and lastly, we lend ear to G. T. Baldy, who lives and works up Canada way. He says: “As it would not be practicable to secure all the required amplification from a single tube, tubes are connected in series to secure increased power by steps, the plate output of one tube going to the grid of tl next, through suitable connections. [I “connections” a transformer or condens is meant, I assume. Anyhow, one or tl other (or is it both?) are used. — F. H. E Two tubes in parallel are used in the la | stage of amplification before the pow 1 finally reaches the horns or loud speakei | [This is so that more power may be ha j died without overloading. — F. H. R.] “Although a steady voltage is applied 1 battery C to the grid, the current reach.ii' it from the input circuit [photoelectric cel is vibratory, representing as it does tl voice, music or what-not. This additic causes the entire grid current to vibral I [That is not exactly correct because r “current” reaches the grid from the C ba tery. It is merely a voltage bias set up 1 the connection. — F. H. R.[ “The contacts on amplifying tube stui must never be cleaned with anything but rubber eraser. “In some types of equipment the phot electric cell amplifying tubes and their hou ■ ing are carried or suspended upon ligl rather long coil springs. This is for tl purpose of absorbing the vibration incide: to the operation of the projector mecha ism, such amplifiers being attached to tl motion picture projector. This cradle mu swing entirely free. If it touches the pr jector, no matter how slightly, then tl vibration will be picked up and heard in tl sound.” WIDE FILM CHARACTERISTICS A DENVER exhibitor writes in to say, T* “I have been told that the wide film picture is stereoscopic. Will you be good enough to advise me as to whether or not this is true? Will a theatre which is rather long and narrow be able to use wide film? Will not the having of so much film in or on a projector largely increase the fire hazard ?” Any fire which might occur would have double the amount of film to feed upon. However, even so I regard that of small importance. If projection rooms are properly constructed and the port shutters properly hung and fused, audiences need never know there is a projection room fire in progress. I do think, however, that it may be well to run a vent pipe from the upper magazine of wide film projectors to the open air, so arranged that it will be opened only in case of fire — opened automatically, of course. Although no stereoscopic camera is used, it seems to be a fact that there is some apparent stereoscopic effect in the wide film screen image. Just why this is so I don’t know, nor is the effect very pronounced. As to a long, narrow theatre using wide film, that depends upon just what width “narrow” may mean. It is impossible to say now what may ultimately be considered the minimum wide film screen image permissible. Many have also asked about the speed of projection of wide film. I have delayed answering these inquiries because I was advised there was to be an increase, but just what it was to be I was unable to a certain with any degree of certainty. Tl speed of Grandeur has been 20 pictures p. second. I was authoratively advised th this is to be changed to 24. However, must be remembered that because of tl fact that each picture is higher, more wic film passes a given point at any projectic speed, as counted in pictures per secon than would pass with 35-mm. film. Tl increase is as 90 feet and 112 3/10 — that to say, with a speed of 90 feet of 35-mr film per minute, 90 feet of film passes pi minute, but if the same number of framt were projected per second with wide fib 112 3/10 feet of film would pass per minut Sound men tell me that this acceleration i < speed gives better results in recording tl high frequencies in sound.