Radio Broadcast (May 1928-Apr 1929)

24 RADIO BROADCAST MAY, 1928 and the correspondingly correct B voltage (in which case it should not be called upon to handle more than 4.5 volts signal voltage), it would be overloaded when required to handle the six volts which would deliver 36 a.f. volts to the grid of the final tube. This latter figure is not even enough to load up a 171 fully. The bias on the second tube could be increased, with correspondingly greater handling capacity, but this in turn would necessitate a higher B-battery voltage. It will be seen, therefore, that the second 201 -a audio tube would be nicely loaded with a 4-volt grid-swing, in which case it would be able to supply the final tube with 24 grid volts. A 171 type tube with only 135 B volts and a C bias of 27 will be adequate to handle these 24 volts. With this amplifier the detector must output 0.67 volt (24 -s 36) to load the 171, which is quite a lot, if for no other reason than that local stations are likely to be necessary for so high a voltage. It will be agreed, then, that the usual resistance coupled amplifier with 201-A type tubes is not very satisfactory. But suppose we use 240 type high-mu tubes with a working mu of 20. Here are the figures: Rc Vt = 240 Re Vt = 240 Rc 1 X 20 X 1 X 20 X 1 =400 The 240 will take a grid-swing of 3 (with 180 volts B, 3 volts C). It can overload a 171 on 180 volts if it gets more than 2 grid volts because 2 multiplied by 20 (the gain in the tube) is equal to 40, the maximum handling capacity of the 171 Hence, if three resistance-coupled stages are used, and since we have already decided that an amplification of 144 will satisfy for average conditions of detector output, the use of 240 type tubes is probably foolish. By dropping the plate resistor of the first tube to 100,000 ohms, the latter's step-up can be reduced to about 12. That would give an overall step-up of 240 to the grid of the last tube. A 201-A type tube in the first stage, with a step-up of only 6, will reduce the overall gain to 120, which begins to fit better with the desirable figure. Now let us consider a single resistance-coupled stage. There has been a lot said about it in some of the more active sections of the press: Rc Vt = 240 X 20 X Rc 1 = 20 That is obviously no good. Why? The detector would have to output 2 volts in order to put 40 volts on the grid of the power tube. What about one resistance-coupled and one transformer coupled stage, such as that shown in Fig. 3? Vt X Rc 60 It will be seen that the gain is insufficient in such a combination, too great a detector output being necessary. If, however, the transformer were a high-grade 6 to 1 unit instead of a 3 to 1 unit, the overall gain would be 120. which is much better. Now let us consider the following three-stage a.f. amplifiers, shown in Figs. 4 and 5, in which FIG. 3 the second audio tubes will handle about 4.5 volts grid voltage (since they are biased accordingly): Rc Vt = 201-A Rc 1 X 6 X T 3 Rc Vt X Vt = 201 X 6 -A X A > Rc Vt = 201-A X 8 X 1 = 108 T 3 = U4 the gain of the amplifier, the smaller the detector output voltage necessary to load up the power tube. Various three-stage combinations, with their overall amplification, are listed below. The necessary C voltage for the tubes may be determined by reference to the table on page 23. In the first case the next-to-the-last tube will be badly overloaded if the signal is strong enough to load a 171 to its maximum handling capacity of 40 grid volts. In practice, of course, the volume control on the receiver would be turned down as soon as overloading of the 201-A, manifest as distortion, became apparent, and thus the 171 would be working uneconomically. To deliver 40.5 volts to the 171 the next-to-the-last tube Rc Vt = 240 Rc Vt Rc 1 X 20 X X 20 X 1 = 400 Rc Vt = 201-A T Vt = 201-A T 1 X 8 X 3 X 8 X 3 = 576 Rc Vt = 201-A T Vt = 201-A T 1 X 8 X 2 X 3 X 2 = 256 Rc Vt = 201-A T Vt= 201-A T 1 X 8 X 2 X 8 X 3 = 384 Rc Vt = 240 X 20 X Rc Vt X 201-A 8 X T 3 =480 h|i|iH ^N|i|iH 4|iMi|ip 4|l|H|l|ij— 1 FIG. 4 has to handle 6.75 volts on the grid. If this tube were given a raise in bias to 9 volts with 135 plate volts, it would be capable of handling this voltage without overloading. Merely moving the a.f. transformer to the third coupling position, as in Fig. 5, requires that the second audio tube have a grid-voltage of only 1.68 to produce 40.5 volts on the grid of the power tube. Judging from the fact that many set owners find two high-mu tubes highly desirable in a resistance — or impedance-coupled amplifier, the opinion that an amplifier step-up of 400 is sometimes useful, seems logical. Of course the higher FIG. 5 From all of the foregoing come a few useful' rules, which may be outlined as follows: (1.) In a combination of transformer-coupling with any other type of audio-frequency amplification the transformers should be in the last stages, and the transformer with the greatest step-up should be in the last one. (2.) Always make certain that the tube before the power tube will not be overloaded before the power tube is fully loaded. To find this out is merely a matter of simple division or multiplication. (3.) When judging the performance of two amplification systems, calculate what the overall step-up for each is before deciding what has happened. There are other simple rules that it is good practice to heed. They may be stated as follows: (a.) It is always best to require least of the detector. This is possible when the a.f. step-up is high. (b.) High a.f. step-up is no help to the detector if the volume control follows the detector, and the less the step-up in the audio end the more this is true. (c.) A detector gives greater undistorted output with increased B voltage, within limits. Operate it according to the needs of your set.