Radio broadcast .. (1922-30)

NOVEMBER, 1926 MODERNIZING THE AT WATER KENT MODEL 20 51 an external high quality power am- plifier in place of the audio amplifier in the set. Third, we may replace the amplifier unit in the set with an equally compact, but infinitely better resistance-coupled amplifier. Fourth, we may replace the ampli- fier in the set with an impedance- coupled one. All four types of amplifiers will give most excellent results. The first three will be described here. A TRANSFORMER-COUPLED AMPLIFIER AS THE amplifier supplied with the set is of the transformer- coupled type, it is very simple to replace the old type transformers with a pair of the new high quality audio transformers and to make pro- vision for the use of a C battery. The Atwater Kent receiver is not equipped with a C battery. As the excellence of an improved amplifier of this type will depend almost en- tirely upon the transformers used, it is important that good instruments, such as Rauland Lyric, Amertran De Luxe, Jefferson, or the new General Radio 200 A be employed. There are also several other suitable trans- formers now on the market. The first step is to remove the set from the cabinet and then take out the six screws that hold the two transformers to the shelf. Turn the set over and cut the four wires, under the shelf, that go to each of the transformers. Then mount the two new audio transformers in the places formerly occupied by the old transformers. PLATE VOLTAGE Speaker or Output Device FIG 2 RADIO BROADCAST Photograph Having substituted new transformers for those supplied in the re- ceiver, we have a few battery changes to make. This combination picture should make clear the wiring to the A, B, and C batteries. The voltages specified are naturally only approximate grid bias supply, while that on the second trans- former connects to the negative 9-volt tap. We are assuming that a 2OI-A tube is employed in the first audio stage and a Ceco type F type tube in the second (output) stage. The regular minus A terminal serves the double duty of minus A and +C binding post, as is evident in Fig. 2. There are two good reasons for using a C battery. One is economy (a C battery greatly prolongs the life of the B battery), and the other is audio quality. Without proper C voltages it is impossible to obtain good audio quality, as discussed by George Crom in his article starting on page 745 of RADIO BROADCAST for October, 1925. We now have an amplifier which, when used with a 2oi-A tube in the first stage (middle socket) and a power tube, such as the Ceco F in the last stage (end socket), and with batteries connected as shown in Fig. 2, will give exceedingly fine results in connection with a good loud speaker. 11 is well to remember that, within certain limits of practicability, the higher the B voltage used on the last tube, the better, as high B voltages permit the use of high C voltages, which decrease the possibility of the amplifier overloadingon loud signals. An accompanying table, on this page, gives approximate B voltages for any value of C voltage for sev- eral different power tubes. The ux-iyi tube requires very much less B voltage for a given value of C volt- agethananyoftheothertubes. Thus, when considerable volume is wanted, and high B voltages (180 or so) are not available, the 171 is a good tube to use. The amplification con- stant of the 171 is very much less, however, than that of the Ceco type F, for example. Also, while it is desirable to use an output device with any power or semi-power tube in order to keep Speaker GRID BIAS VOLTAGES This table gives approximate grid bias voltage for three last-stage tubes. Values slightly dif- ferent than those given may be tried until best results are obtained Three of the four wires cut from each of the transformers should now be connected to the corresponding terminals of the new instruments by running them through small holes which have been drilled through the brown bakelite shelf near the different new transformer terminals. It will be found necessary to solder extension leads to the different wires in order to make them reach the terminals of the new transformers. The connections of the cut wires are as follows: Green wire goes to +B terminal on trans- former. Yellow wire goes to Plate terminal on trans- former. Black wire goes to Grid terminal on trans- former. Red wire is not used. Cover end with tape to prevent short-circuit. It will be noticed that no connection has been made to the terminal on each transformer marked minus Fil. These terminals are to be used as minus C binding posts, as the Atwater Kent re- ceiver is not provided with C battery binding posts. That on the first transformer should connect to the negative 45-volt terminal of the FIG. 3 The battery connections to the Model 20, the resistance-coupled amplifier having been added, are as shown here. The out- put device connections are also given. If this latter is not used, the loud speaker will be connected directly to the P and B posts. The underside of the resistance amplifier panel is shown in Fig. 4 on page 52. The Amperite and fixed condens- ers are mounted beneath this panel fll I! L HOP Device RADIO BROADCAST Photograph