International projectionist (Jan-Dec 1936)

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December 1936 INTERNATIONAL PROJECTIONIST 23 plate of VT-3 right to the primary of T-2, down through that winding, left, up through C-9, and right to cathode. From the two outside terminals of the secondary of T-2 trace to the right to the grids of the two output tubes. From the filaments of those tubes follow the filament leads down, left and down to the second secondary of T-4. From the center-tap of that secondary trace up, left, up through C-ll as far as possible; then right, up through R-12, right, and up and left to the center-tap of the secondary of T-2. From the plates of VT-4 and VT-5 trace the load upon those tubes to the right to the output transformer primary; from Terminals 4 and 5 of that winding left through the two small meter resistors, then down as far as possible; left to the third junction, down and right through C-10-C-13; down through the 60-ohm section of R-5 to the inid-point of the second secondary of T-4; then back along the filament leads to the filaments of VT-4 and VT-5. The output transformer secondary, like the input transformer primary, is center-tapped to provide a choice of coupling impedances. The 'Harmonic Equalizer' The harmonic content, or harmonic distortion of an amplifier, which impairs tone quality and causes sound to seem unnatural, is created by the inherent nature of amplifying tubes. The pattern of the output sound wave never matches exactly the pattern of the input sound wave. The extent, of the distortion created by tube action depends upon the conditions under which the tube is operated. The grid bias is among the operating conditions that have an impor introduced into the sound by the action ot the tube. These frequences bear a definite arithmetical relation to the original or fundamental wave, and are called "harmonics." If the fundamental frequency be, say, 500 cycles, there vvill be a second harmonic of 1,000 cycles, a third harmonic of 1500 cycles, and so on. Harmonic distortion is that action, on the part of the amplifying circuits, that alters the pattern of the sound frequency in a way which corresponds exactly to the addition of harmonic frequencies. Conversely, if suitable electrical means are used to draw off or filter off harmonic frequencies, the distortion is removed and sound resumes its original clarity. A further discussion of harmonic distortion, and of one method for removing it (reverse feedback) was discussed in the last issue of I. P. (Nov., p. 19). The amplifier of Fig. 1 of the present article also incorporates means for removing harmonic distortion but the method in no way resembles reverse feedback. A different arrangement is used to achieve the same result. The W. E. 91A amplifier traced herein last month used a single-end output stage instead of push-pull output. Reverse feedback, which removes all harmonic distortion, made this possible without sacrifice of quality. The 86type amplifier now under consideration gets rid of one-half the harmonic distortion [the second and other even harmonics] by the common device of a pushpull output stage. The even harmonics balance out in the primary of a pushpull output transformer, where they meet 180 degrees out of phase. The third and other odd-order harmonics, however, remain in ordinary push-pull circuits, the output tubes. Trace again from Terminals 4 and 5 of the primary of T-3 through the meter resistors (which are too small in value to be considered) ; thence down, left to the third junction^ down, right through C-10-C-13, down through 60 ohms to the center-tap of the second secondary of T-4, and then back to the filaments of VT-4 and VT-5. Sixty ohms and 16 microfarads constitute the only impedance in this line. The second and other even-order harmonics cannot manifest themselves in. the primary of a push-pull output transformer, where they meet out of phase and cancel, but commonly appear asvoltage in the return between the centertap of that primary and the output fila-. ments. They also appear in the spaceof the output tubes themselves. The very low impedance of 60 ohms in series with 16 microfarads, in the present circuit, prevents the even-order harmonics from developing any appreciably large voltage drop in the filament-toprimary-center-tap lead, and thus concentrates them almost entirely in the space of the tubes. But in the tubes they react with the fundamental frequency to produce third and other oddorder harmonics which are 180 degrees out of phase with the odd-order harmonics already present there. The result is, that while even-order harmonics are kept from the speakers by meeting, out of phase in the primary winding of the push-pull output transformer, oddorder harmonics are cancelled in the tubes themselves by the generation, within those tubes, of the identical harmonic frequencies in reverse phase. The action in the tubes is very similar to that which takes place in the mixer, FIGURE 1 tant influence upon the accuracy of amplification. But with the most favorable of operating conditions there is always some degree of distortion which is manifested as unnatural sound. The input and output wave patterns of an ampliying tube can be studied in a number of ways, and analysis reveals that the effect of the distortion now being considered is exactly the same as if certain extraneous frequencies were and a certain amount of distortion appears in the sound. The. "harmonic equalizer" in Fig. 1 consists of the 60ohm portion of the grid bias resistor, R-5, and the 16 microfarads of C-10-C-13. These two units are in series with the return from the center-tap of the primary of the output transformer to the filaments of Cfo ,,Cl3 S§ (§> £XT. H.V. SUPPLY