Radio age (Jan 1927-Jan 1928)

RADIO AGE for February, 1927 terminals of the "A" battery of the amplifier tube. Th^e device is similar to a rheostat but has terminals at both ends and a sliding contact in the middle which is connected with the lower end of the secondary coil of the first transformer, or to the ground. There is one advantage in using a radio-frequency amplifier with a regenerative detector, namely, it may stifle the howls of the detector tube before they goad the neighbors to the point of committing justifiable homicide. The safest recommendation for the average fan who is just striking out into multitube construction is to make the first tube a regenerative detector, with a variocoupler between it and the antenna, use the second tube as an audio-frequency amplifier, and be careful in tuning to keep the tube from howling. Single-circuit regenerative receivers are almost sure to become a nuisance to listeners for blocks around, but with coupled circuits between the antenna and the tube, careful operation usually will prevent any serious disturbance. As selectivity is important in these days of multiplying broadcasters, it is better to spend a little more money and secure more satisfactory results than are likely to be obtained from the simplest and least expensive outfit. Secure a good variocoupler and two variometers adapted to the wavelengths to be covered. It is almost if not quite impossible to cover the entire broadcasting band efficiently with one receiver, so it is good policy to aim to bring in one station with maximum strength and clarity and then take whatever else comes along for good measure. A station can be selected that has programs you like to hear, and whose wave is near the middle of a waveband that includes other good stations. The receiver that will bring in the one station satisfactorily will probably reach well up and down the scale and miss only those whose wavelengths are at the extremes. There will be enough in any case. To complete the outfit, secure two tubes, good sockets, an audiofrequency transformer, batteries, rheostat," phones, grid leak and condenser of the correct resistance and capacity values, and jacks for the phone plug. A loud speaker is not likely to work satisfactorily with so small a set, but a horn to which the phones can be attached may make it possible to hear programs from the near-by stations without wearing the headgear. A crystal detector can be added, so that the local stations can be brought in when the battery is being charged or when the tubes are out of commission for any other reason. A Simple Circuit CIRCUIT by circuit, the following two-tube hook-up is easy to understand. See figure 2. The aerial connects with the stationary plates of the variable condenser. The rotor of the condenser connects with the primary coil of the variocoupler. The other end of the primary connects with the ground. That completes the antenna circuit. Run a wire from the secondary of the variocoupler to a variometer, and another wire from the Regenerative detector and first stage of radio frequency amplification, with ciystal detector for use when tubes are cut out The Magazine of the Hour other terminal of the variometer to the grid leak and condenser, which usually are mounted with common terminals. The other terminal of the grid leak and condenser connects with the grid terminal on the tube socket. The other end of the secondary of the coupler connects with one filament terminal on each of the two tube sockets, the terminals that are not connected with the rheostats. Whether these are positive or negative terminals will depend on whether the positive or negative end of the "A" battery goes to the negative end of the "B" battery. Some tubes require a* polarity specified in the directions, and some may be connected either way. The "B" battery connection, however, is always with the positive terminal toward the plate. The above connections complete the grid-filament circuit of the detector. The plate is connected with the second variometer and the other terminal of the variometer goes to a positive terminal of the "B" battery. Nearly all "B" batteries have several positive terminals, provided with convenient clips, so the voltage may be adjusted easily after the testing begins. The negative terminal of the "B" battery goes to the first jack, whose other terminal connects with the two filaments and the secondary of the variocoupler. This completes the detector plate circuit, which is closed when the phone plug is placed in the jack. The detector jack is provided with two terminals that should be connected with the two ends of the primary coil of the audio-frequency amplifying transformer. The secondary of the transformer connects with the grid of the amplifier tube on one end and with the filament on the other. The filament terminal used should be the same that connects with the secondary of the variocoupler. The plate of the amplifier tube connects with one terminal of the second jack. The other terminal of the jack goes to the positive terminal of the "B" battery. This completes the wiring for the set, unless a crystal detector is connected between the detector grid lead and phone jack, with a (Please turn to page 49)