International projectionist (Jan-Dec 1945)

may be either microphone or phonograph output. In the case of the latter, one side of the line is grounded. The secondary of the input transformer is connected directly across the 100.000 ohms of the volume control, which thus constitutes the load upon the secondary winding. That portion of the potentiometer which is included between the slider and ground is the source of speech voltage for the 77 tube, the connection to grid being direct and to cathode running through 2 microfarads. The speech output of that tube may be traced from plate right, down through Vs megohm, right, and down through 8 mfd. to ground, from ground up, right, and through 2 mfd. to cathode. The y2 megohm plate resistor may be regarded as a source of voltage, one side of which connects through .02 mfd to the control grid of the 89; the other side running through ground and 2 mfd. to the cathode of that tube. The 89 is triode-connected; its screen and suppressor grids being electrically extensions of the plate. The primary of the coupling transformer, in series with 8 mfds., constitutes the load upon the output of the 89 tube. The secondary winding of the coupling transformer supplies the source voltage driving the 79 output tubes; the voltage developed between the top of the winding and the center-tap driving the upper 79; that developed between the center-tap and the lower end of the winding being applied to the grid and cathode of the lower 79. In each of these tubes the two sections are jumped together, so that each acts not as a two-section tube, but as an enlarged simple triode. They are loaded by their respective sections of the primary of the output transformer. The secondaries of that transformer provide speech current at a choice of impedances to match any desired loudspeaker combination. The reader will have noted that this amplifier is far simpler than a common theatre amplifier — just as the dynamotor is simpler than an arc supply motorgenerator. There is nothing in Figure 1 that ought to give the slightest trouble to any projectionist. Auxiliary Sound Services A common use of auxiliary sound devices is the reinforcement of live entertainment by means of a microphone. The essentials of the system are very simple: a microphone, an amplifier, and a loudspeaker. The details may be more complex in that several speakers may be needed to cover the whole audience; other sources of sound such as a record player or radio tuner may be incorporated; there may be more than one microphone. Such additions do not compromise the basic simplicity of sound fakp^k Blank of Zoloft FIGURE 2 reinforcement; they merely add wires and switches and so on, and may alter the calculations for impedance match — on the other hand they help to simplify the work of trouble-shooting. Where there are several sound sources and several speakers it is very easy to find out by switching, or by unplugging connections, whether a given trouble is in the sound source, the amplifier or the speaker. Where there is more than one highimpedance sound source, such as several microphones of high impedance types, the amplifier will be equipped with a "mixer", either built-in or added as an auxiliary. The basic mixer circuit is shown in Figure 2, which represents an external or added type. It is too simple to need tracing in detail. There are three identical tubes (in this case pentodes wired as triodes). There are three sets of input terminals, accomodating three high impedance sound sources. Each tube acts as one-stage amplifier for the sound source with which it is connected. Each of these one-stage amplifiers has its own volume control. The various inputs are "mixed" by the simple fact that the plates and cathods of all the tubes are wired in parallel. The output runs through the overall volume control shown at top right. In Figure 2 power supplies are derived from the amplifier through the socket shown at lower right. An indicator lamp bulb is connected in parallel with the three filaments. The identical arrangement may be built into the amplifier, as an integral part of its circuits. Suppose Figure 2 were thus built into Figure 1. Look at Figure 1 — find the 25,000 ohm resistor drawn just under the 77 tube — trace left to the four-way, grounded junction point. Now cut the wires leading upward and leftward from that point. Again, cut the wire leading leftward from the control grid of the 77 tube. Now discard everything above and to the left of the connections you have just broken in Figure 1, and substitute for them Figure 2. Wire the upper output terminal of Figure 2 (top right) to the grid of the 77 tube in Figure 1 ; and wire the lower output terminal of Figure 2 to the four-way grounded junction just mentioned. Now Figure 1 has a high impedance mixer input instead of a low-impedance input transformer. Of course, the foregoing relates only to speech circuits, and it still would be necessary to draw power from Figure 1 for the power input socket shown at the lower right of Figure 2. For use with Figure 1 the tubes of Figure 2 should be of the 6C6 type, since the 57 does not have a 6-volt heater. Practical Problems In any entertainment reinforcing system the primary problems relate to power output, number and placement and matching of speakers, and acoustic relations between speakers and microphones. If the power output is the same as that of the regular theatre amplifier, it will of course be adequate for the auditorium. It is not practical to use the regular speakers, because they would throw sound directly into any microphone placed on the stage. A microphone can 10 INTERN ATIONAL PROJECTIONIST