Radio Broadcast (May 1928-Apr 1929)

OUR Readers Suggest" is a clearing bouse for short radio articles. There are many interesting ideas germane to the science of radio transmission and reception that can he made clear in a concise exposition, and it is to these abbreviated notes that this department is dedicated. While some of these contributions are from the pens of professional writers and engineers, we particularly solicit short manuscripts from the average reader describing the various "kinks," radio short cuts, and economies that he necessarily runs across from time to time. A glance over this "Our Readers Suggest" will indicate the material that is acceptable. Photographs are especially desirable and will be paid for. Material accepted will be paid for on publication at our usual rates with extra consideration for particularly meritorious ideas. — The Editor. A Power Unit Voltage Divider THIS department has published, from time to time, contributions describing various home constructed voltage divider systems and other devices for the improvement of power units designed prior to those incorporating more modern apparatus. Several of these arrangements have provided for C-bias potentials. The circuit of a compact voltage divider unit, recently placed in the market by Electrad, is shown in Fig. i. The various resistors incorporated in the unit are all variable, providing for any desired seven positive and negative potentials within the limits of the power supply output. The "Truvolt" divider may be incorporated in the output circuit of a new power unit or used to provide special potentials from a ready-built supply Noisy reception can often be traced to disintegrating resistors in the voltage distribution system of a power supply device, a difficulty that is readily eliminated by substituting the Electrad divider unit for the original resistor system. The old resistors should be removed and the divider connected between the points of the highest potential, i.e., between the highest B voltage post and B negative or the highest C bias terminal, if such is provided on the power unit, Terminal i on the divider is connected to the high voltage side of the line, and Terminal 7 to the low voltage side. Each potential secured from the divider should be bypassed with a i.o-mfd. condenser to the post used as B negative, as suggested by the dotted lines in Fig. i, which shows the terminal arrangement for the potentials generally required. However, the divider can be connected to the receiver in many ways to secure practically any voltage distribution. Obtaining Screen-Grid Bias OOR the most efficient operation of screen* grid receivers a positive bias on the screen grid is required. Few power units designed prior to the introduction of the 222 tube provide the proper screen voltage. By means of a Duplex Clarostat, connected as a potentiometer between the negative and plus 90 volt posts on my power supply set, 1 am able to secure the best biasing potential. The circuit is simple and is shown in Fig. 2. Both adjustment screws should be given about two and a half turns up (from a tight adjustment) and then the lower screw adjusted until signals are amplified most efficiently. The bias potential should be bypassed + 180V. + 135V -40 V. C -9V.C The circuit arrangement of the Truvolt divider. This unit can be used for the rejuvenation of old power supply sets as well as in the construction of modern designs unit. FIG. 2 A simple way of obtaining the correct positive bias for the screen grid on a screen-grid tube. This bias varies with circuit, tube and available control grid bias with a i.o-mfd. eondenser, indicated in dotted lines in Fig. 2. A. R. Coates, New York City. Testing for Soft Tubes IT IS the general custom to test for a "soft" * tube by operating it under an excessive plate potential. The blue haze, caused by ionization, is an indication that the tube is soft. Unfortunately, a test of this nature often changes the characteristics of the tube, occasionally lessening the efficiency of what otherwise would be a good detector. A perfectly safe test, more compatible with the tenets of laboratory procedure, may be effected with a small spark coil, such as that employed in the Ford, Model T, ignition system. If one of the secondary terminals is not already wired to the primary (as is the case with the Ford coil), make this connection, using a couple of dry cells 364 to energize the primary. Take the tube in hand, covering the glass as much as possible, and touch any of the terminals to the high tension lead from the coil, watching the while for glow. No sign of ionization indicates a hard tube — or one full of air. In this latter case, of course, the filament would burn out when connected across the usual battery circuit. A pale greenish glow, close to the inside surface of the glass, indicates about the right amount of gas for a good detecting tube. If the glow is purple, and is confined to a small area directly around the plate and filament, the probability is that there is too much gas for efficient detecting action. Alfred A. Ghirardi, Stapleton, N. Y. Matching Condensers and Coils ■ in Tandem Tuned Circuits \ A/HILE the general procedure for * ™ matching isolated coils and condensers is fairly well understood, I have never read anything on the process involved in matching coils and condensers already connected in a receiving circuit. The following system will be of value to the experimenter who desires to match an r.f. tuning combination more closely than can be done by the trial and error method on a station. The method employs an oscillator with a meter in the plate circuit to indicate resonance. It is not essential to know the wavelength to which the oscillator is tuned, as long as it is within the broadcast band. Fig. 3 shows a circuit that can be employed as an oscillator. The number of turns of wire wound on a 35" winding form is indicated on the diagram. The size of wire is not important. The additional materials are about three feet of rubber covered wire and a small clip facilitating a temporary connection to the grid side of the circuit being tuned. Cut the wire in half and twist two ends of it together for about one inch, making a small condenser. One of the two remaining ends is soldered to the clip and the other to the grid terminal of the oscillating tube. FIG. 3 An easily constructed oscillator for use in matching circuits in single control receivers, and other purposes. Any one of the several oscillators previously described in Radio Broadcast may be used