Note On The Singleended Pushpull Amplifier

Much interest has been shown in the article on the push-pull amplifier circuit in the October, 1951, issue of the Experimenter. Some of our correspondents have cited earlier work in which series-con-nected output tubes have been used. Since others may also be interested, we reproduce here their combined references, all of which are U. S. Patents.

Hoklen 1,999,327

Smith 2,247,316

Artzt 2,310,342

White 2,358,428

Etter 2,423,931

Rockwell 2,446,025

Stodola 2,488,567

Stachura 2,561,425

A study of these patents appears to show that none of them have anticipated the basic circuit of Figure I of the October, 1951, Experimenter, which is reproduced here. The essential element of this circuit is the symmetrical drive of the series output tubes without the vise of a transformer. Each output tube is driven with a voltage from cathode to grid so that the two tubes operate in the same fashion. The usual attempts at driving these series tubes without a transformer result in one being driven as a cathode follower, while the other is driven as a straight amplifier or in the drive of one tube by some portion of the output voltage. With either of these systems the true push-pull symmetry of Figure 1 is lost.

Another point raised by several of our correspondents concerns the heater-to-cathode voltage of the upper output tube. When the series circuit is used, this voltage is usually so high that a separate heater winding must be used for the upper output tube. This winding should then be connected to a d-c potential corresponding to the average potential of the upper cathode. Sometimes, lie-cause of the high a-c voltages developed, this separate winding must be connected directly to the cathode rather than to a derived d-c potential. Then it is desirable to use a shielded, twisted pair for the connection from the transformer to the heater. When tubes like the Type 6AS7 Twin Triode are used in the output, however, this separate heater winding is not usually necessary.

The connection of one side of a separate heater winding to the cathode adds a capacitance across the output, load. The effect of this capacitance is usually insignificant. For example, it can usually be kept to less than 200 ti/ud; and then for two Type 6L6 Pentodes as output tubes the frequency at which the reactive current is equal to the load current is 500 kc. This figure applies for a 1650-ohm load and, if lower-impedance tubes

Figure 1. The basic single-ended push-pull amplifier circuit, showing the series-connected output tubes supplying a common load and driven by a cathode-follower phase inverter.

Figure 1. The basic single-ended push-pull amplifier circuit, showing the series-connected output tubes supplying a common load and driven by a cathode-follower phase inverter.

are used or if more care is exercised in reducing the stray capacitance, the effect is even less important.

Those who want additional information on the amplifier should find the paper on "A Single-Ended Push-Pull

Audio Amplifier" in the January, 1952, issue of the Proceedings of the Institute of Radio Engineers helpful. Reprints of this paper are available on request from the Editor of the General Radio Experimenter.

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