Radio broadcast .. (1922-30)

748 Radio Broadcast P RADIO FREQUENCY AMPLIFIER NEUTROFORMERS ' S UV 201 A PS UV 201 A P S CRYSTAL DETECTOR P S UV201A A + B FIG. 3 as explained in the April, 1924, RADIO BROADCAST, The radio-frequency transformer T2 is of the untuned type and covers a wavelength range of 200 to 5,0 meters. Much of the instruction as outlined for the three-tube circuit of Fig. 3 may be followed in the construction of this receiver. Especial care should be exercised in the selection of a suitable audio-frequency transformer and crystal. JACKS, AND HOW THEY ARE USED THERE are as many types of closed circuit jacks as there are manufacturers of them. It should be remembered that jacks are used for the sole purpose of providing a mechanical cutin on any part of a radio circuit. In the natural progress of things, jacks came to be used in more elaborate ways. One manufacturer produces 14 different kinds of jacks. Not all of them can be explained here but several of the more well-known type are illustrated in Fig. 5, A and B. In A, the several styles are outlined and their use is depicted in B. For the output jack in the stage of an amplifier or a plain detector circuit we have that as shown in i . This is called the single open-circuit type. I n 2 is shown a single-closed circuit type permitting the throwing in or out, automatically, of the primary of a transformer. Another method of performing the same operation is shown in 3. That shown in 4 is fundamentally the same as that shown in i except that a filament control is included which breaks the filament circuit when the plug is withdrawn from the jack. In 5 we have practically the same as that shown in 2 with the addition of the filament control as explained. The method of controlling one or more filament circuits by means of jacks is shown in 6. A PUSH-PULL POWER AMPLIFIER IN THE construction of a push-pull amplifier, the selection of satisfactory parts plays an important role. In fact, the ultimate successful operation of the unit depends upon this consideration more than anything else. The input stage audio transformer should have a low ratio, say 2 to i, so that the voice and music is not unnecessarily distorted before it reaches the push-pull transformers. The action and theory of operation have already been explained in past issues of RADIO BROADCAST and will not be repeated here. The suggested circuit is shown in Fig. 6. Any standard type of tubes may be used in a unit of this kind but of course uv-2Oi-A's or better still, power tubes such as the 202 or the w. E. 2i6-A will be more suitable for this type of work. The value of C battery will vary in proportion to the amount of B battery used. A table of C battery voltages was included in THE GRID for January, 1925. For the tubes suggested 120 to 150 volts B battery will be sufficient. CALIBRATION CURVES ALMOST any type of receiver may be charted and calibrated so that a graphical curve is produced that may be used for reference purposes. Instead of rumaging through numerous papers having dial settings numerically listed, it is much easier and more efficient to refer to the s.hipshape wavelength curve that also helps you to locate new station positions on your tuning dial. Neutrodyne and tuned radio-frequency receivers, superhetrodynes, and crystal reflex receivers furnish the most accurate curves. In the regenerative type of receiver, several combinations of inductance and capacity will tune to the same station and the curve system of tuning is not so dependable or reliable. A specimen of a wavelength graph curve is shown in Fig. 8. The several points marked on the diagonal line indicate station positions. For instance 492 meters, WEAF, is located on the vertical border to the left; running rcross horizontally on the 492 line we note where it is intersected by the diagonal line. Then, coming down vertically, it is found that this position is equal to 81 on a 100 segment semi-circle of the dial. This base line is comparative to and indicates the markings on the dial. To prepare a wavelength curve it is well to have commercial graph paper that is obtainable in most stationery shops. The dial markings are laid off at regular, equal intervals along the base line and the wavelength range, usually from 200 to 600 meters is hid off in a V TI UNTUNED RFT T2