International projectionist (Jan-Dec 1936)

THE W. E. 86-TYPE AMPLIFIER By V. M. COUSINS MEMBER OF THE TECHNICAL STAFF, BELL TELEPHONE LABORATORIES This new amplifier is one unit in the modernization program now being furthered by Erpi in its theatre installations, including elimination of the 200A panel, installation of the new multicellular speakers, and the use of the 300A and 262A tubes — all of which have been described in detail in these columns. AMPLIFY: To enlarge or to increase in scope. As defined thus in the dictionary, the word amplify has ordinarily been used to describe the rhetorical process of enlarging an idea, or perhaps of expanding a description or discussion. Whatever the ordinary usage has been and to whatever extent the enlargement has been carried out, it is safe to say that the amplification involved has not even approached the amplification obtained in many types of vacuum tube amplifiers used in various forms of electrical communication and transmission. "To enlarge or increase in scope" is almost ludicrously inadequate to describe a process that increases the electrical energy in a circuit tens or hundreds of billions of times, values which are required of amplifiers in numerous circuits at the present time. Sound picture and public address sys tems require amplifiers capable of providing energy increases of the order of ten billion times, or gains of 100 db, to increase the very small amount of energy received from the photoelectric cell, microphone, or phonograph reproducer to values capable of operating the loud speakers in a theatre or hall at the level required for proper sound reproduction. Besides providing this large amount of amplification for the signal currents, the amplifier must provide — if operated from alternating current — energy losses between the power and voice circuits of a similar order of magnitude, so that crosstalk from power circuit to voice circuit will be maintained at an inaudible level. Gain and Output Level To make available an economical, high-quality amplifier for sound picture and public address systems, Bell Telephone Laboratories has recently developed the 86A, B, and C amplifiers. Incorporating the latest developments in vacuum tubes, transformers, and other elements, these amplifiers provide a gain of 98.5 db and an output level of approximately 15 watts. The circuits and mechanical arrangement of the three amplifiers are essentially the same; they -W/ JmJ ImJ Ism. -fWVi — °s ! a ! -fVW| — ° -rr^ EXT. HIGH > VOLTAGE SUPPLY O 6 10 VOLT. I AMPERE FILAMENT po) 105-125 VOLTS 50-65 CYCLES NOTE A = MILLIAMMETER CONNECTION FIGURE 2 Simplified schematic of the 86-type amplifier circuit [20] FIGURE 1 An installation of a 1086 A amplifier in a motion picture theatre. An 87 A amplifier is mounted on the lower section of the rack differ chiefly in being arranged for different power supply frequencies and output impedances, in order that they may be adaptable to a wide range of uses. As shown in the schematic of Fig. 2, these amplifiers have four stages: the three preliminary stages employing 262A vacuum tubes* resistance-condenser coupled, and the power stage employing two 300A vacuum tubest in a push-pull arrangement. Transformer-coupling is used for the input to the first stage, and as input and output for the power stage. The amplifiers are entirely a. c.-operated: the plates being supplied from a 274A vacuum tube, which rectifies the high voltage supplied by the power transformer, and the filaments being operated on low voltage obtained from the same transformer. The power transformers of all three amplifiers are designed to operate from a primary supply voltage between 105 and 125 volts. The A and B amplifiers are designed for 60-cycle circuits, and the C amplifier for either 50 or 60 cycles. All three may be operated con * I. P., February, 1933, p. 21. tl. P.. July, 1936, p. 20.