Radio Broadcast (May 1928-Apr 1929)

By L. W. Hatry MANY set builders have searched for the ideal form of amplification by plodding their way through combinations of resistance, impedance, and transformer coupling, attempting to effect a compromise between the supposed quality of one system and the wellknown high step-up ability of another. Or, at times, the search for perfection may have been tinctured with the desire for a combination which made use of apparatus already on hand. Whatever the reasons for such combinations, they should be made only with some appreciation of the apparatus and its limitations. Many persons, for instance, wonder where in an a.f. system to place the audio transformer if only one is used. The answer to such questions can always be found. The method is simple. Consider that the voltage step-up: (i.) — in a 20 1 -a is 8. (2.) — in a 199 is 6. (3.) — through a resistor-coupler is I. (4.) — through an impedance-coupler is 1. (5.) — through a transformer is 2, 3, 35, 5, or 6, according to the step-up ratio of the windings. (6.) — through a high-mu tube (240 type) is about 20, or through any tube in a resistance or impedance-coupled stage ought to be about two-thirds of the tube's mu (we have taken the effective step-up to be six in the case of a 20 1 -a type tube). This information is available in magazines, in catalogues, in tube instruction sheets, and in textbooks. It allows one to engineer his a.f. amplifier with the employment of something akin to intelligence and the enjoyment of a feeling near that of competence. As a preliminary to this engineering feat, choose some audio system you know to be functioning in a way that satisfies. As an example, let us use a two-stage amplifier with 3 to 1 and 6 to 1 audio transformers, a 201 -a in the first pmu^m ™ ™»»Read This Box First— 1 THE signal voltage handling capacity of a tube used as an audio amplifier is governed by the amount of bias placed on that tube's grid. Thus, a 20/-A type tube, with 4.5 volts bias (and the corresponding plate voltage of go) will be overloaded and distort if called upon to handle more than 4.5 peak volts. The following table of C and B battery voltages for various tubes used as amplifiers will serve to make the author's article more complete: Tube Type B Volts C Volts Mu 45 15 8.4 20 I -A 67 3 8.4 90 4-5 8.5 '35 9 8.5 90 6 7-9 I 12 '35 9 8.2 '57 10.5 82 90 ,6'. 5 3 ■7' 135 27 2-9 180 40.5 2.9 90 4-5 7 5 210 135 9 7-5 180 10 7 5 222 180 ' ■ 5 60 stage, and a 171 in the second stage, as shown in Fig. 1. The power tube is getting a B voltage of 180 and is properly biased so it is important to remember that its grid-swing limit is about 40 volts peak. To load the power tube, the second a.f. transformer must supply this 40 volts. We shall now determine the characteristics of an amplifier which will fulfill this requirement. For the second or the final audio transformer to give 40 volts, the grid of the tube in the first stage must be getting a voltage equal to 40 divided by the turns ratio of the transformer and the mu of the tube. The overall gain of the amplifier in Fig. 1 will be: T Vt = 201-A T 3 X 8 X 6 = 144 Thus the requisite voltage at the output of the detector must be: -^2. =0.28 Volts '44 For sake of argument we shall accept this value of 0.28 volts as being average in future calculations. It has been the writer's observation, however, that many signals overload the 171 with an amplifier of this type, which would indicate that the output voltage of the detector sometimes exceeds 0.28 volts. This holds true for a set in Hartford, Connecticut, which, of course, is comparatively surrounded with high-powered broadcasters. If the detector will put out 0.56 volts, twice that calculated above, we can use a 3 to 1 transformer instead of the 6 to 1 transformer in Fig. 1 with somewhat better frequency characteristics. Presuming that the a.f. amplifier will be satisfactory if up to the grid of the power tube it has a voltage multiplying ability of 144, the business of figuring equivalent combinations of resistance or othercouplings is easy. For instance, in a threestage resistance-coupled amplifier, such as is shown in Fig. 2, using 201-A tubes, the gain up to the grid of the power tube will be (bearing in mind that a resistor-coupler has a gain of 1 and that the actual amplification through the tube is about two-thirds of its mu): Rc Vt = 201-A Rc Vt = 201-A Rc 1X6 X 1X6 X 1=36 If the second tube has a grid bias of 4.5 volts FIG. I 23 FIG. 2