International projectionist (Jan-Dec 1935)

J unitary 1935 INTERNATIONAL PROJECTIONIST 15 circuit runs down and left to the tapped primary of the power transformer. The right-hand secondary of this transformer provides low voltage power to terminals 4 and 5 of the connection block just mentioned, for use outside this amplifier. That power lights the signal lamps of the remote volume control used with this amplifier. The volume control itself is the 500,000-ohm resistor R-23, shown just left of the Type 56 tube in the center of the drawing. The sliding contact is moved by a small a.c. motor that can be operated from a remote location. A bank of signal lamps at the operating point indicates its setting. The right-hand secondary of the power transformer lights those lamps. There are no rheostats and no metering arrangements in the filament circuits of Figure 2. The heaters of the two left-hand tubes, types 24-A and 56, are powered by the extreme left-hand secondary of the line transformer, which is also capable of supplying additional current at the same voltage to a singlestage pre-amplifier through terminals 6 and 7 of the connection block in the extreme lower left-hand corner. The filaments of the push-pull type 45 tubes are heated by the second secondary from the left. There is no occasion for filament rheostats or meter readings, because the filament voltage will always remain the same while the power line voltage stays constant. The tap switch in the primary of the power transformer can be set to compensate for high or low voltage lines. The filaments of the tubes and the operation of the amplifier are not troubled by minor variations in the heating current. Wide fluctuation of power line voltage is, of course, another matter, to be cared for by installation of voltage control equipment. The plate power of Figure 2 is derived from the full-wave rectifier tube, type 80. The positive output is, of course, at the filament. The line may be traced right, and then upward through L-8 and L-9, past the filter condensers C-20, C-21 and C-22. From the top of coil L-8 one branch can be traced right and then upward to the center tap of T-4 primary; thence to the grids of the push-pull stage. From the filaments of that stage to the center tap of R-27, through the bias resistor R-28. From the ground connection at the negative side of that resistor to the ground connection just right of Resistor R-24, about two inches left of the lower push-pull tube. Thence left and down, past the filter condensers, to the mid-tap or negative of the plate secondary of the power transformer. Returning to the mid-tap of the pushpull output transformer T-4, another branch may be traced upward and left to the top of R-26, down through R-26 to the plate of the type 56 tube. From the cathode of that tube down through L-31, and right and down through the bias resistor R-24, thence left and down to the center-tap of the plate secondary. Still another branch runs left from the top of R-26, down through R-19, and left through R-20 to the screen grid of the type 24A tube. Still another branch, following the positive line across the extreme top of the drawing, runs downward through R-16 and R-17, right, up, right, down through R-81, and R-18 to the plate of the type 24A. One more branch runs down through R-12, left, down and left to the " + 90" terminal of the left-edge connection block. This provides bias voltage for two photoelectric cells. The grid bias of the push-pull tubes is furnished by the voltage drop through R-28 (just left of T-4), which is in series with the plate current through those tubes. The grids return to the negative side of that resistor through the common ground connection. The bias of the '56 tube is supplied by the voltage drop through R-24, through which the plate current of that tube returns from cathode to ground. The grid of the same tube is connected to the ground or negative side of R-24 through the volume control potentiometer. The bias of the control grid of the '24 tube is derived from the drop through R-21, to which the control grid connects through the secondary of the input transformer, T-2. Speech Circuits of Figure 2 The speech input to Figure 2 is through the four upper terminals of the left-hand connection blocks. The first and fourth are the photo-cell input line, the other two being used for special inputs, as from announcing microphone. C-10 and R-15 are part of the elaborate frequency-controls of this amplifier. The plate of the 24-A type tube is coupled through C-14 to the volume control potentiometer R-23, the drop across which is applied to the grid of the second stage, type 56 tube. C-ll and C-23 are frequency control condensers. The speech component in the plate circuit of the '56 tube may be traced upward through L-7 and down through C-19, thence through the primary of coupling transformer T-3, and back to cathode. L-6 and L-7, C-17 and C-18 are frequency control devices. The input to the push-pull stage is from transformer T-3, and the output through T-4, the secondary of which is designed for impedance match to the power amplifier following. It is also provided with a tap connection through which it can be coupled directly to the speakers in emergency. The manner in which the frequency control devices seen in the speech circuits of this amplifier can be adjusted in the theatre to adapt its characteristics to acoustical conditions, recording technique, and other variables not controllable in the factory, is explained in detail in the manufacturer's instruction book, which accompanies each installation. Such elaborate controls were not provided in 1928. The amplifier shown in Figure 2 requires of the projectionist only the following: 1. Throw main line switch on in the morning and off at night. 2. Use of more elaborate test equipment, requiring real electrical knowledge, to find trouble. 3. Understanding of tube types and circuit arrangements not used in earlier equipment. From these illustrations and the discussion of them the meaning that improved amplifiers have for the projectionist should be obvious. They mean less trouble but more responsibility. Or, to attempt to sum up the difference in a phrase, they substitute the requirement of better and more thorough electrical knowledge for those of cleaning storage battery tops and adjusting rheostats. NOTES ON LOUDNESS, PITCH Dr. Harvey Fletcher BELL TELEPHONE LABORATORIES MUSICIANS employ three terms to describe different aspects of the sensation they experience when listening to musical tones. These are pitch, loudness, and timbre, although the term quality, or tone color, is sometimes substituted for timbre. Most textbooks on physics have taught that these psychological characteristics are related in a simple way to three corresponding physical quantities: frequency, intensity, and overtone structure. The relationship between pitch and frequency, and between loudness and intensity, has been thought to be one of direct correspondence: the pitch of each note corresponding to a definite frequency, and the loudness of each note to a definite intensity. The relationship between harmonic structure and timbre has had no such simple formulation, but at least the timbre has been thought to depend on overtone structure alone. Bell Laboratories Studies Studies in these laboratories, however, have shown that no such simple relationships exist, that each of the psychological quantities — -although depending chiefly on the corresponding physical