Radio Broadcast (May 1929-Apr 1930)

RADIO BROADCAST A B c r~\ D E ^ V/ A B C D E F TYPE OF TUBE CX-112A CX-112A CX-112A CX-371A CX-371A CX-371A NUMBER OF TUBES 2 2 2 2 2 2 CIRCUIT PUSH-PULL PUSH-PULL PUSH-PULL PUSH-PULL PUSH-PULL PUSH-PULL Ig (mA) .002 .46 4.L .002 0.6 6.7 THIRD HARMONIC 5.0 % 10.2% 24.2% 5.6% 10.5% 21.0% Po (WATTS) .45 1.16 2.62 1.37 2.89 6.38 INPUT (VOLTS R M S) L9.7 36.5 82.4 59.5 86.5 175.0 Fig. 5 out a small fraction of one milliampere d.c. which flowed through it. In some earlier listening tests we used resistors bridging the input and output to overcome a slight tendency to sing at some high frequency. While no difficulty was experienced with singing in the set-up shown in Fig. 6, we continued the use of bridging input and output resistors. The percentage of harmonics registered by the oscillograph may be somewhat high because of loss of fundamental in the output inductances, but it is not believed that this discrepancy is very great. In Fig. 5a is shown the wave form of the output current when 19.7 volts (r.m.s.) is indicated on the meter, Eg, in Fig. 6. The load resistance RP was 18,100 ohms. The peak value of the 19.7-voIt (r.m.s.) signal is 27.9 peak volts. This is slightly over twice the bias voltage which was 13.5 volts. Each tube was drawing about 2 microamperes of grid current. On increasing the signal voltage the grid current increased rapidly and both the upper and lower peaks of the output current were seen to flatten somewhat. When the signal reached 36.5 volts (r.m.s.) each tube had an average d.c. grid current flow of 460 microamperes. (Fig. 5b.) The r.m.s. plate current in Fig. 5a was 5.0 milliamperes and the power output 450 milliwatts, or 225 milliwatts per tube. In Fig. 5b the r.m.s. plate current was 8.0 milliamperes and the power output 1160 milliwatts or 5.15 times the output per tube obtained in Fig. 5a. Fig. 5c shows the result of increasing the signal to an extreme value of 82.4 volts (r.m.s.). The power output is 2620. milliwatts or 11.6 times the output per tube obtained in Fig. 5a. The data and the results of an analysis of Fig. 5 (a, b, and cl are shown in Table 1 The second harmonics in Fig. 5 (a, b, and c) in order of increasing signal, are, respectively, 2.6 per cent.. 6.8 per cent., and 4.8 per cent. The percentage was calculated from the ratio of harmonic to fundamental. Considering second harmonics alone as a measure of the distortion, the quality is good for any signal amplitude. The corresponding amounts of third harmonic were, respectively, 5.0 per cent., 10.2 per cent., and 24.2 per cent. The percentage of higher harmonics was relatively small. Evidently the third harmonic is the principal distortion component from a push-pull amplifier, the percentage increasing (roughly) in direct proportion to the signal voltage. Similarly it will be found that with a single-tube amplifier the percentage second harmonic (principal distortion component) increases in direct proportion with the signal voltage. As the operation of the single tube extends into the grid-current region, or into the region of plate-current saturation, and the second derivative of the plate current with respect to plate voltage is negative, third harmonic distortion is produced and the second harmonic distortion will not increase in direct proportion to the signal voltage. These tests were repeated with two cx-371a tubes. Fig. 5 (d, e, and f) shows the wave form of the output current. The corresponding data are given in Table I. The results indicate, as in the tests with the cx-112a tubes, that for highest quality output the signal voltage amplitude should not exceed twice the normal bias voltage for the tube. The load resistance required for maximum undistorted power output from a single tube, considering 5 per cent, of second harmonic as a criterion of distortion, is equal to approximately twice the plate resistance of the tube. In the push-pull stage the load resistance has less effect upon the percentage of harmonics. Since the maximum power output from a tube (or any source of power) is obtained when the resistance of the load equals the resistance of the tube (source), the maximum power output will be obtained from the push-pull stage when the load resistance equals the sum of the plate resistances of the tubes. With the load resistance equal to twice the plate resistance of one tube and normal bias voltage, the distortion is usually negligible until the signal is large enough to start a flow of grid current. The flow of grid current lowers the grid-filament resistance of the tubes putting a load on the input transformer. In Fig. 6 the input impedance was shunted with two 10,000-ohm resistors. The effect of loading the input transformer was then eliminated. The data, Fig. 5 (a to f) in Table 1, show an increase in the bias voltage due to the flow of grid current through the d.c. resistance of the input. Figs. 2, 3, and 4 and the data of the table show an interesting result due to the flow of the grid current through the impedance of the input circuit. Fig. 2 shows the wave-form of the output current when the resistances R in Fig. 6 are 100 ohms. The signal was 39.0 volts (r.m.s.), the d.c. grid current 1.05 milliamperes, and the output alternating current 10 milliamperes. The resistances R were then changed to 10,000 ohms. For the same signal voltage, 39 volts, the grid current was reduced to 0.45 milliampere and the output alternating current was reduced to 9.15 milliamperes (see Fig. 3). Increasing the signal until the d.c. grid current rises to 1.05 milliamperes, as it was initially, the required signal voltage is found to be 52 volts (r.m.s.) The output is 10.4 mA. a.c. The output is 8.3 per cent, higher than was obtained with the lowimpedance input, though the signal voltage was increased 33 per cent. Fig. 4 shows the wave-form of the output current . It is apparent that the distortion also is greater. In other words, a low-impedance input circuit is to be preferred. Table I — Data and Results of Analysis Figure D.C. Plate Cur. (I/>) Milliamperes D.C. Grid D.C. Grid Bias, (Ec) Cur. Volts Milliamp. Grid Signal (Eg) R.M.S. volts A.C. Plate Cur. (//) R.M.S. Mil. Power Output cv Milliwatts Per cent. 2nd 3rd 5A* 5B* 5C* 26.4 34.0 46.0 13.5 14.7 24.4 .002 .460 4 1 19.7 36.5 82.4 5.0 8.0 12.0 450. 1160. 2620. 2.6 6.8 4.8 5.0 10.2 24.2 5D** 5E** 5F** 60. 81. 40.5 40.8 56.7 .002 .60 6.7 59.5 86.5 175.0 12.9 18.7 27.9 1370. 2890 . 6380 . 3.1 .4 1.2 5.6 10.5 21.0 2t 3t 4t 33.5 30 . 5 33.7 14.5 16 8 19.4 1.05 .45 1.05 39.0 39.0 52.0 10.0 9.15 10.4 1612. 1350. 1745 . Input resistance 100 ohms 10000 ohms 10000 ohms Harmonic m 5lh 1.7 1.8 1.3 .3 1.1 13 2.3 2.0 0.9 2.0 *2 CX-112A tubes in push pull. **2 cx-371a tubes in push pull f 2 -cx-I12\ tubes in push pull. Ep = +180 volts. Ec = + 180 volts. Ec = + 180 volts. Ec : 13.5 volts. Rp = 18.100 ohms. = 40.5 volts. Rp = 8260 ohms. 13.8 volls. Rp = 16.120 ohms. 220 • • FEBRUARY 1930 •