Radio age (Jan-Dec 1925)

RADIO AGE for August, 1925 The Magazine of the Hour 73 Economy of Arrangement Is Vital \ S WILL be seen, the part of the •^* assembly taken up by the receiver proper is very small, the panel measuring 10% long and 11 yi deep, but for some purposes the total height with the batteries included may be too great to be practicable. In such a case, the battery compartment can be made separate, terminating the receiver portion at the board shown running over the tops of the batteries. This, however, makes it necessary to reconnect the batterieseverytimethat the receiver is set up and as a result it is a decided nuisance. As laid out in the drawings, there is ample room for a fifth "B" battery if it is desired to operate with a plate voltage of 112j-£ volts, or a sixth "B" battery if we wish to operate at 135 volts. Further, there is room for one spare "A" battery if it is desired to carry this replacement along on a trip. The voltmeter (VM) is. a very desirable instrument, particularly when dry cell "A" batteries are used, for it at once indicates the drop in voltage due to weakening batteries. Unless we have some means of testing the voltage occasionally, we are likely to believe that the set is out of order when the voltage drops, and waste much time chasing for trouble in the wiring when the difficulty actually exists in the cells. Just because automatic filament controls are installed for the regulation of the filament current is no reason why the voltmeter should be omitted. Voltmeters must be connected properly according to polarity, and you must be guided by the markings on the instrument. If the polarity is wrong, then the needle indicator will be thrown against the wrong end of the scale. The size of the voltmeter should be so chosen that the full voltage will bring the needle near or slightly beyond the center of the graduated scale where the divisions are the largest, and the battery voltage should not throw the needle to the far end of the scale. Suppressing Oscillations Free oscillations in the radio frequency stages are the greatest difficulty in the construction of a radio frequency or reflex type of receiver, and we must devise some system for stopping these oscillations if we expect to get the full output of the set. In fact, most of the trouble reported with sets having radio frequency stages can be traced to improper or imperfect methods of damping down the oscillations. There are a number of methods of stopping oscillations, among which are the potentiometer, neutralizing condensers, bias batteries, etc., but in thisreceiver we have simplified the problem by the use of a plate reactacne coil marked (L3) on the diagrams. Coil (L3) consists of four or five turns of wire wound on the end of the tuning coil. One end of this coil is connected to the plate of the first radio tube as shown, while the other end is left opened or is unconnected. As one end is opened, only capacitative current will flow from the plate into the coil, and the magnetic coupling is therefore very feeble, as it should be. Some little experimenting will be required in adjusting this coil before it just stops the oscillations. It may be that the plate connection must be connected to the other end of (L3), or that the whole coil must be wrapped on the other end of the tube. Varying the number of turns, or the distance of (L3) from (L2) may be required. Just because you have not hit the proper combination on the first trial is no proof that it will not work. The losses are at a minimum with this arrangement, and there are no separate controls as when a potentiometer is used. In addition to the compensating coil (L3), the "C" battery will be an aid in keeping down oscillations and increasing the sensitivity of the first two tubes. In the center, and at the bottom of Fig. 1, will be found a detail of a small radio frequency choke coil which can be used when an audio frequency transformer secondary is not available. This consists of about 450 turns of No. 36 D. S. C. wire wound on a cardboard or bakelite tube as shown, and is mounted on the upper shelf (F) of the set. Some little experimenting may be required to get the most effective number of turns, but the coil is not very critical to the turns, and for most apparatus the number of turns shown will come very close to the best effect. We must have enough turns so that the tubes will not paralyze on the higher wavelengths through leakage of the R. F. current to the ~"B" battery. In the lower right hand corner of Fig. 1 is a detail of the resistor assembly used in the audio frequency stages. We can assemble the resistors and grid leaks on the shelves by means of clips as shown, or better yet, we can buy these completely assembled units from ovir advertisers, at a reasonable figure. Summary TN tuning this set, the adjustment of -* the detector rheostat (R) is of great importance, for there is one position of the rheostat where the detector tube is the most sensitive and gives the greatest volume. This generally occurs when the rheostat is about one-half on, and it is seldom necessary to turn on this tube to full brilliance as with the amplifier tubes. LET OUR HOOKUPS BE YOUR GUIDE IN The Magazine of the Hour Let America's best known radio experts keep you informed of the latest in technical radio. Here are a few of the radio men who write for RADIO AGE monthly: John B. Rathbun, Frank D. Pearne, Edmund H. Eitel, Paul Green, Brainard Foote, H. Frank Hopkins, Armstrong Perry, McMurdo Silver, Roscoe Bundy, Arthur McCullah, and many others. 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