International projectionist (Jan-Dec 1936)

16 INTERNATIONAL PROJECTIONIST October 1936 would otherwise result and permitting greater amplification. The 6N7 is simply a double triode — two 3-element amplifying tubes built into a single glass envelope. A tube of this type is desirable for use as a phase inverter, since the internal balance of both triodes will always be the same, and any change in emission due to deterioration of the cathode or other causes will react equally on both plates. The construction and action of the 6L6 tubes have been fully described in International Projectionist.! The Speech Circuits Speech input from the p.e. cells is wired to the two "input receptacles" shown at the extreme left of the diagram. The negative input line is grounded directly under the lower input receptacle. The positive lead from both photocells is traced from the receptacles up, right, down through 3 megohms, right through % mfd. and down to ground. Regarding the cathode and anode of the photocell as the poles of a generator delivering speech a.c, the resistance just mentioned and the %-mfd. condenser in series with it constitute the load to which the power is delivered. A fluctuating voltage-drop exists across that load when the photocell is in operation. One side connects to ground through the righthand plate of the ^-mfd. condenser and hence to the cathode of the first 6J7, which also is grounded. The other side, from the top of the 3-megohm resistor, connects to the control grid of the first 6J7 as follows: up, left, through the microphone jack contact as drawn, right through the .01-mfd. coupling condenser and right to grid. Thus, the fluctuating voltage-drop existing across the load placed upon the photocell is extended to the grid and filament of the first amplifying tube, creating a fluctuating change in the voltage difference between that grid and cathode. A corresponding but greater fluctuating change then appears in the plate current across the tube. When the mike jack is in use, the plug lifts the top contact out of touch with the central prong and thereby opens the p.e. cell speech circuit. The photocells in consequence will remain inoperative while the microphone is connected. The Electronic Equalizer The cathode and plate of the first 6J7 may be considered the poles of a generator of amplified speech a.c. From the plate trace right and down through 1 megohm, through 25,000 ohms, through 1-mfd. and to ground. From ground trace back to cathode. The 25.000-ohm resistor just mentioned is paralleled by two shunt circuits. One runs from the top of that resistor right through 2/10 megohm (or 200,000 ohms) and then t I. P. for June, 1936, p. 22. down through 25,000 ohms and left to the lower end of the 25,000-ohm resistor first mentioned, hereafter called the source resistor for the equalizer circuit. The second parallel path around that source resistor may be traced from its upper end upward, right, up through a potentiometer, right, down through .0005 microfarads, down through 25,000 ohms and left as before to the lower end of the source. Disregarding that second path for a moment, the 200,000-ohm resistor and the 25,000-ohm resistor shunted around the source may be considered a tapped voltage divider. The tap at the junction of the two connects to the grid of the next 6J7 tube through the .01-mfd. coupling condenser. The ratio of 25.000 to 200,000 is 1 to 8, and l/8th of the source voltage is coupled to that grid. The second shunt path, however, through the potentiometer, varies that ratio in accordance with the setting of the control and with sound frequency. Assuming the potentiometer to be set for zero resistance, the condenser alone will be paralleled across 200.000 ohms. The condenser will have almost no effect with reference to extremely low frequencies, since it will present a very high reaction to them; but at higher frequencies it will serve as a better conductor and decidedly modify the ratio of 8 to 1. Setting the potentiometer at zero thus results in increased highfrequency volume with very little change in low-frequency volume. The effectiveness of the condenser, however, is reduced by bringing potentiometer resistance in series with it, and at maximum potentiometer setting its influence is comparatively small at any frequency. The chart printed in International Projectionist for June, 1936 (p. 16), shows the range of tonal response that can be obtained by means of this equalizer. The circuit itself, as just traced, is electrically identical with the equalizer circuit diagrammed on page 15 of the same issue of LP., but the resemblance is concealed by the fact that the drawings are laid out differently. There is also one difference in values, in that the right-hand 25.000-ohm resistor of the present circuit is represented, in the earlier drawing, by a resistance of 10,000 ohms. It may be instructive to compare Final Mirrophonic Article To Appear Next Month PUBLICATION of the second and final article relative to the W. E. Mirrophonic Sound Reproducing Sys-tern, scheduled for this issue, has been deferred until next month. Photographs and drawings of certain Mirrophonic system components could not be prepared in time for this issue, and the presentation of this data in part was deemed inadvisable. The concluding article of the Mirrophonic series will appear in the November issue. the two diagrams in detail, insofar as the electronic equalizers are concerned, and to note how identical electrical arrangements underlie a wide difference in superficial appearance. Phase Inverter and Output Circuits The charge on the control grid of the right-hand 6J7 tube being varied by the input circuits just traced, the plate and cathode of that tube may be taken to represent the terminals of a source of amplified speech a.c. Cathode goes directly to ground. Plate is connected to the upper end of a %-meg. resistor, which returns to ground (trace downward and left) through 1 microfarad. A shunt path exists through the 1/10-mfd. coupling condenser and down through the ^-megohm volume control. Any desired portion of the voltagedrop across that volume control is applied, by means of the slider, to the left-hand grid of the 6N7. The cathode of the same tube returns to ground through the 3,000-ohm grid bias resistor. Now, one of the functions of the 6N7 is to provide push-pull input to the power tubes of the last stage, for which purpose the two plates of the 6N7 must be 180 degrees out of phase. But the plate of any tube is always 180 degrees out of phase with its grid, plate current decreasing as grid charge increases, and vice versa. Therefore, in the case of the 6N7, the right-hand grid is supplied with speech voltage by the left-hand plate, and the right-hand plate, being 180 degrees out of phase with its own grid, will be out of phase with the left-hand plate also. From the left-hand plate trace up, right through the 1/10-mfd. coupling condenser, down through ^ megohm and left to the right-hand grid. The two plates of the 6N7, being suitably displaced as to phase, can then be used to drive the two control grids of the output tubes. From the left-hand 6NT plate trace up, right through 1/10 mfd. and down to the control grid of the top 6L6. From the right-hand 6N7 plate trace right, down, and right through 1/10 mfd. to the control grid of thelower 6L6. However, it. will be noted that the sound voltage supplied to the upper 6L6 has been amplified only once in the 6N7 ; while the sound voltage to the lower 6L6 has been fed back to the right-hand of the 6N7 and amplified twice. A proper correction must be applied, in the interests of good quality, to insure that the speech voltage supplied to both 6L6 tubes shall be equal in value. Return to the left-hand plate of the 6N7 and trace up, right through 1/10mfd., and down through ^ megohm, through 8,000 ohms, and right and down to ground. The two series resistors, 250,000 ohms and 8,000 ohms, constitute a voltage divider with a ratio of about 30 to 1. The right-hand 6N7 grid is connected to the tap between those two resistors, and does not receive the full plate output of left half of that tube, but only about l/30th of it. In any properly designed phase-inver