Radio age (Jan 1927-Jan 1928)

RADIO AGE for January, 1927 design takes half the fun out of the job. Be original, and you will discover that some wise guy imitated your design years ago. The radio frequency amplifier belongs between the detector and the antenna. Start at the antenna and work toward the detector. An expert will, of course, figure out the inductance and capacity of his antenna and design it to fit his set. If he does not forget the tin roofs, metal framed buildings, trees and other nearby objects that may absorb energy and cause capacity effects, he may secure better results than the common variety of radio bug. Roughly, the fundamental wavelength of the aerial is about three times its length. It is quite safe, for ordinary purposes, to run a wire from the garage to the house and trust the variable inductances and condensers to tune to the stations you want to hear. Variable Condenser YOU may place a variable condenser in series between the antenna and the inductance coil that comes next. It will help with the tuning. The inductance coil may be tapped or not. Some prefer a flexible unit, adaptable to wide bands of wavelengths, and others believe in covering a narrow band more efficiently and getting rid of effects caused by dead ends. A 50-turn honeycomb coil, or some other type of coil containing about the same value of inductance, and a .00025 or .0005 condenser work well together for broadcasting wavelengths. Either the coil or the condenser, or both should be variable. The antenna, or the series condenser, if there is one in the antenna circuit, connects with the grid of the amplifier tube. If the rotor of the condenser, its movable part, is toward the ground, connect the grid to the stator. Changing the connections, so that the stator connects with the ground and grid, may make a difference. Try both ways and compare results on the same signal. The filament circuit for the tube is the same as in most tube circuits. The directions that come with the tube may state that the positive terminal of the "A" battery should be connected with the rheostat that regulates the current. If the connections are not specified, try both ways. Any type of amplifier tube may be used except the new kind designed for the last stage of audio frequency amplification only. The correct voltages, as stated in the directions, should be applied to filament and plate. The manufacturers know more about these things than the local standing committee. As few constructors care to spend any large sum for add One stage of radio frequency amplification, including regeneration, transformercoupled to a crystal detector. Either a variocoupler or a radio-frequency amplifying transformer may be used in coupling the plate circuit of the tube to the crystal detector citcuit ing an amplifier to a crystal set, dry-cell tubes are likely to be chosen. A potentiometer is a useful device in radio frequency amplification. It is known also as a stabilizer, for it helps to prevent the tube from oscillating. It resembles a rheostat in having a coil of resistance wire as its principal element, but instead of being connected in series it is shunted across the terminals of the "A" battery. There is a variable contact in the middle of the coil as well as the terminal contacts at the two ends. From this middle contact a wire leads to the grid, sometimes by way of the inductance coil. The result is that the grid is made positive and the grid circuit absorbs an appreciable amount of power. The effect is similar to that of a "C" battery used to place a biasing potential on the grid. The Magazine of the Hour It may be difficult to find a potentiometer, unless you shop by mail. I asked three local radio dealers in my town for once and every one of them asked me what that thing was. A wire rheostat can be used by eliminating the connection between the tongue and the end of the coil and connecting the tongue with the grid. The ends of the coil are connected to the filament or "A" battery terminals. The plate of the tube is connected to the primary coil of the coupler. If a single-circuit crystal set is used, the plate is connected, through a condenser, to the end of the single coil to which the detector is attached. The condenser prevents the "B" battery current from entering the detector, where it might fuse the cat whisker to the crystal. The positive terminal of the "B" battery is connected with the other end of the coil and the negative terminal is connected with the "A" battery. Either minus-to-plus or minusto-minus connections may be tried. So long as the positive terminal of the "B" battery is kept away from the filament connections there is no danger of burning out the tube. A resistance is connected in series between the plate and the "B" battery. The output resistance of a tube often is specified by the manufacturer and 11,000 to 15,000 ohms may be required for efficient operation. If a suitable resistance element is not at hand, an old-time amateur device may be tried. Draw a line on paper with a lead pencil or India ink. Place the paper on a little base of dry wood or bakelite where it will form a connection between two screws or binding posts that touch the ends of the line. Test the resistance and change it by erasing the line, or part of it, and making a new line that is thinner or fatter. A short, fat line probably will come nearer providing the proper resistance than a long, thin one. Where coupled circuits are used, the variable tuning con