International projectionist (Oct 1931-Sept 1933)

INTERNATIONAL PROJECTIONIST September 1932 FIG.l point, but this time because the filament is not heated sufficiently. Improper plate and grid voltages have similar effects. Consequently it is very important not only to have good tubes in the amplifiers, but also that the applied voltages be of the correct values. Let us consider a tube with low emission. If the signal is of a small value, so the plate current can follow the signal without getting too close to the saturation point, then the amplification will be without distortion, even with a poor tube. In Figure 2 this smaller signal is shown by the dashed line. The curve of Fig. ure 1 is repeated here to show how the smaller signal compares with the one described previously. Notice that the normal peak of the smaller signal does not go as high as the distorted peak of the large signal shown by the dotted line. This gives rise to another thought: if there are no spare tubes on hand, or if the spares are all poor ones, they can often be used in the earlier stages of an amplifier even though they are not capable of perfectly amplifying in the final stages. It also brings to mind the fact that the best tube available should be used in the third stage of the 41-A amplifier. The same is true of the voltage amplifier in an RCA set. Use the two best tubes in the third stage. Other amplifiers using the same type tubes in more than one stage should be treated accordingly. Proper Voltage Values When the amplifiers are equipped with meters it is easy enough to see that the voltages are kept at the proper values. Quite often it is not possible to read the plate voltage, but this does not matter, as the proper flow of plate current is an indication that the voltage must be correct. If the plate voltage and the grid bias voltage should both be wrong at the same time, there would exist a condition where the plate current could be of the correct value but the tube would be distorting. This fact would be quickly shown, however, by the plate current meter, because, even though the plate current would be normal when no signal was being put through the amplifier, as soon as a signal would be applied to the grid, the meter would \^iggle back and forth more or less violently, unless the signal is very small. This is a good thing to watch anyway, to see if the stage is amplifying as it should. It is generally considered that the plate current can vary as much as 10% without producing any noticeable distortion in the sound. However, it is desirable that the meter be steady. You will notice on making this test with a poor tube that the meter will become steady when the volume is cut down, unless the tube is entirely gone. As the volume is again increased the needle starts to wiggle more and more, each loud note or word causing the needle to jump. With a good tube in the socket, but with some of the voltages having wrong values, the meter acts the same way. It must be borne in mind that even a good tube, having all voltages normal, will give a similar indication when the tube is overloaded. Any tube is capable of handling a signal of only a certain magnitude; when this is overstepped the tube overloads and the meter starts to wiggle. This is a perfectly normal condition, so some judgment must be used when making this test. On ordinarily loud reproduction none of the tubes should overload, and the plate current meter should remain steady. On extremely loud shrieks, crashes, etc., it is allowable that the needle make quite large excursions from the normal. However, when the needle moves at all while the volume is kept at the average level, something is wrong, and the most likely point of trouble is the tube. RCA System Amplijiers When no plate current meters are supplied with the set, it is not so easy to see what or where the trouble is. When there are two voltage amplifiers in the installation, (as have many rca equipments ) , the first thing to do is switch to the other amplifier. If the trouble clears up, this quickly shows the trouble to be in the first amplifier. Should the trouble remain, it is probably in the power amplifier. If there be two or more power amplifiers, the monitor is plugged into one of the others. This will give good reproduction in the monitor when the trouble is in the amplifier that had previously been connected to the monitor. If the trouble has been narrowed down to one of the power amplifiers, see how the tubes look in that amplifier. If any of them look abnormal, they should be replaced. If such is not the case, they must be replaced one by one, until the trouble is corrected or all the tubes have been exchanged for good ones — showing something else to be wrong. Usually the only visual indications of a bad amplifier tube in the RCA power amplifiers are too much blue glow or the plates being red hot. When the plate of one rectifier tube is cool and the other rectifier plate is red, the one with the cool plate usually is no good. The same is true of the amplifier tubes. When the plates of both the amplifier tubes and both the rectifier tubes are red hot at the same time, some other part of the amplifier has developed trouble. NEW CURTAIN TRACK FROM AUTOMATIC DEVICES AFTER a long period of development and installation under actual operating conditions. Automatic Devices Company of Allentown, Pa., has recently announced the new Steelite light-duty curtain track. A large number of theatres, schools, churches and similar auditoriums of modest size are limited in their appropriations for stage draperies and heretofore have been compelled to use home-made wooden curtain tracks or other unsatisfactory travelers to keep within their budget. Use of Steelite removes these restrictions by offering a steel track with all the advantages of rigid, fireproof construction at a cost as low as that of the troublesome wooden traveler. The Steelite track is distributed by drapery houses and equipment dealers throughout the country and samples will be gladly sent upon request to Automatic Devices Company, Allentown, Pa., manufacturers of Silent-Steel heavy-duty curtain track, Allentown automatic curtain machines and Stabilarc motor-generators, well known in the film industry. ■!■