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Show kuM 111 i- iTf-BlM I 1 Mllgg..i-...i-.Btj: BngrTV.iilMrt I I )V p l i A a I so o Jo- J o c ( oj jo L. oj y A Push-Pull Amplifier Without the Regular Input and Output Transformers. Transform-ers. A Potentiometer Is Used to Divide the Secondary Voltage of Transformer Between the Two Tubes. Two Choke Coils Are Used In the Output and the Loudspeaker Is Connected Directly Across the Coils Without Any Stopping Condensers. Use of Push-Pull Amplifier Scheme Can Be Put Together With Parts to Be Found in the Workshop. By GEN. T. C. H. BRANNON, In Radio World. When it is desirable to have considerable consid-erable undistorted power for the loudspeaker loud-speaker and neither power tubes nor high voltages are available, a push-pull push-pull ampliUer can be rigged up, usually usual-ly out of parts that are already at hand. The diagram shows such an amplified. Tl is an audio-frequency transformer. It must be one of high grade and preferably of high ratio, say 6 to 1. Rl and R2 are two resistors re-sistors of about .5 megohm each. R3 Is a high resistance potentiometer about .5 megohm. This potentiometer is connected between the two fixed resistors and it is used for the purpose pur-pose of dividing the secondary voltage equally between the two tubes. It is difficult to find two resistors of exactly ex-actly equal ohmage to make the potentiometer po-tentiometer unnecessary. Of course, if a potentiometer of about one megohm is available, Rl and K2 need not be used, R3 alone being sufficient. The grid battery is put in the lead which runs to the slider on the potentiometer, poten-tiometer, with the minus terminal pointing toward the slider. . No Condensers Used. In the output side two choke coils LI and L2 are used in place of an output out-put transformer. These choke coils can be the secondary windings of audio-frequency transformers. Cheap transformers, which should not be used in any high-quality amplifier, can be used here to good advantage. The two choke coils are connected in series between the two plates of the tubes. The mid-point, or Junction of the two windings, is connected to the positive of the plate battery. The loudspeaker Is connected from plate to plate across the two choke coils. Note that no condensers are used in series with the speaker. Similar Sim-ilar circuits have been recommended S . in which a 2 mfd. condenser has been placed on each side of the speaker, with the admonition that each must stand the voltage of the plate battery. When two condensers of 2 mfd. each are used in series the effective capacity capac-ity In series with the speaker is only one microfarad, and this is so low that it seriously cuts down the lower notes in the signal. The loss of the low notes Is too heavy a price to pay for a habit which serves no purpose. It is apparent that there is no DC voltage across the loudspeaker, and no direct current will flow through the loudspeaker when the condensers are omitted. Hence the condensers should be omitted. Tubes to Use. Since the loudspeaker is connected from plate to plate without a step-down step-down transformer, the loudspeaker should have a very high impedance to take full advantage of the power delivered de-livered by the tubes. The Internal resistances re-sistances of the tubes are in series with the speaker, and therefore the speaker should have twice the usual impedance. We cannot change the impedance of the speaker at will to match the series connection, but we can use two speakers in series. But using two speakers is not a practical solution. It is better to use two tubes which have a low output impedance, e. g.. two 371 tubes, the impedance of which is around 2.000 ohms each, requiring a load impedance of 4,000 ohms each. The impedance of the speaker when connected across two such tubes should therefore be 8,000 ohms. Most good speakers are suitable. Such tubes as the 220 and 17 2 may be used. The list of parts Includes the following fol-lowing : Tl One good audio-transformer, ratio 6 to 1. Rl. R2 Two .5 megohm grid leaks (Lynch). R3 One .5 megohm potentiometer. LI, L2 Two audio-frequency choke coils or transformers. One double mounting (Lynch). Two UX sockets. Two amplifier tubes of low output resistance. Ten XL binding posts. One baseboard 4 by 7 inches. |
