Radio broadcast .. (1922-30)

RADIO BROADCAST. 3.5 NEXT TO CORE 50 LAMINATIONS e-p-r.-^y ^ KILOC5CCLES Fig. 6 15,000 turns of No. 40 of 7100-ohms re- sistance wound over the primary. The over- all voltage amplification (Fig. 4) of the 201 A tube and the transformer was measured for frequencies from 200 up to 8000 cycles, first with a core of 50 laminations and then with 30, 15, 5, 3, 1, 0.2, and 0 laminations. It will be observed that the removal of most of the iron has a relatively small effect upon the amplification over the greater portion of the frequency range, however, the effect of the core is rather pronounced at the low frequencies. There- fore, in order to retain the low notes a considerable amount of iron is required in this particular transformer. By 0.2 of a lamination is meant that portion of a single lamination which passes through the opening in the coil and which has a length equal to that of the coil or approxi- mately one inch. The increased a_mplifica- tion due to this small amount of iron over that with no iron at all is pronounced below 200 cycles. Above 4500 cycles, re- gardless of the number of laminations, the core has practically no effect on the am- plification other than to prevent an over exaggeration of the higher frequencies. Regardless of the amount of iron the maximum amplification occurs at 6500 cycles which is rather surprising and indi- cates that the introduction of iron does more than merely increase the induction. The maximum amplification is approximately twice that of the lower frequencies. The form of the curve suggests a reso- nance effect which is due to the distributed capacity of the secondary and the grid- filament capacity of the tube to which it is connected. It is evident that the in- troduction of resistance in the primary circuit would tend to flatten out this curve and make the amplification much more uniform throughout the frequency range, and this is confirmed by Fig. 5. A change of plate voltage would have so_mething of the same effect as shown in Fig. 6 as this will change the internal resis- FREQUENCY IN KILOCYCLES tance of the tube. A decrease in plate potential from 90 to 67.5 volts for a 201A with normal grid bias causes an increase in plate resistance from approximately 10,000 to 14,000 ohms and at the same time de- creases slightly the amplification factor. A decrease of plate potential from 67.5 to 45 volts would increase the resistance from 14,000 to 18,000 ohms. There are certain other factors that will have a secondary effect. Series II: The primary of this trans- former has 5000 turns of No. 40 wire with fREOUENCV IN KILOCYCLES Fig. 7 FREQUENCY IN KILOCYCLES Fig.* Fig. U a resistance of 2500 ohms. The secondary has 15,000 turns of No. 40 wire with a resistance of 6100 ohms. The amount of iron was varied as in Series I but the gen- eral appearance of the curves is quite different. There is a more pronounced spreading of the curves (see Fig. 7) and the maximum points occur at different fre- quencies, the maximum amplification for FREQUENCY IN KILOCYCLES Fig. 72 the air core occurring at 6000 cycles. In- troducing iron has the general effect of lowering the frequency at which the peak occurs. As the amount of iron was increased the maximum came at 5200, 4500, 4000 r 3500, and then increased to 4800. The difference between this series and the last was due largely to the increased capacity of the secondary winding which was placed next to the core. Another possible differ- ence was due to a change in leakage re- actance. There is also a greater spreading of the curves for the different values of plate voltage (see Fig. 8) than for the corresponding case where the primary was. wound over the secondary. Series III: This is a commercial trans former for which there is no information regarding the number of turns or the turns ratio, but the ratio is probably between 2.5 and 3 as indicated by the amplifica- tion at the lower frequencies. The primary resistance is approximately 2000 ohms and the secondary 7500. It is considered a high-quality transformer. Figs. 9 to 13. show the effect of changing the plate voltage, adding resistance in the plate circuit, and varying the capacity across, the secondary of the transformer for three different values of plate voltage with normal bias in each case. It should be noted that the effect of adding capacity is. to decrease materially the amplification at the higher frequencies. The fact that the amplification varies considerably with frequency does not necessarily indicate that the quality will be poor. The maxi- mum amplification for normal plate volt- age and grid bias occurs at 4200 cycles, and is approximately twice that at low frequency. Series IV: This is a commercial trans- former haying a 6:1 ratio and giving an amplification of approximately twice that of the last transformer at low frequencies. The increase of amplification with fre- quency is less pronounced than for the last transformer, the maximum being about 30 per cent, greater in this case, although, due to the larger distributed capacity, it occurs at 3200 cycles and de- creases quite rapidly for frequencies Fig. 44 • NOVEMBER 1929