The Cine Technician (1939)

66 I 1 1 I. I I X E T E C H N I ( I A X June-July, i Details of New Mirrophonic Sound Heads Two new sound heads of an entirely different design to the I-A sound unit fitted on the Universal Base, and the 206 Reproducer set, are available. These new sound heads, the 12 1 1 type and the TA-7400 do not employ constant speed sprockets of the 707 type, but depend on a kinetic scanner for pulling the film at a uniform speed past the scanning light. The kinetic scanner is an inertia controlled drum maintained at constant speed by means of a special double flywheel. The drum is mounted on ball bearings so as to reduce friction to a minimum. The double flywheel incorporates a mechanical filter, which suppresses any oscillation that might be transmitted to the scanning drum by the uneven passage of the film. On the 21 1 sound head, the lens tube focuses the image of the exciting lamp filament on the sound track, thereby dispensing with the usual mechanical slit. The Photoelectric ("ell compartment on the 211 type is located on the door of the sound head compartment. No P.E.C. amplifier is employed on either the 211 or TA-7400 sound heads, the output from the photo-electric cell being taken to the main amplifer via an output transformer mounted in the sound head and a combined volume control and change over control cabinet. On the TA-7400 Sound Head the optical system is different from that employed on the 211 type. This system is termed the "Optical Bridge." It differs from the usual scanning methods, inasmuch as the beam of light from the exciting lamp is applied to the film on the celluloid side of the film. A microscope is focussed on to the sound track on the emulsion side of the film, which magnifies the image on the track and projects this on to a large masking slit and collimating lens, which in turn focusses the light passing through the slit on to the photoelectric cell. This method of scanning results in low ground noises and a greater efficiency at high frequencies. These new sound heads are driven by 110 volt, 50 cycle motors. Amplifiers A new range of amplifiers of advanced design are used on Mirrophonic systems. These amplifiers — types 86, 87 and 91 — are all A.C. operated, and in addition to an excellent frequency characteristic and high degree of reliability which have come to be taken for granted in Western electric amplifiers, there is employed the important new device termed a "Harmonic Suppressor" (described in the previous issue of the Journal). All parts of the circuits in the new amplifiers requiring adjustments can be checked by means of a selector switch associated with a "Percentage Meter," that is a meter graduated to read percentages of the normal or correct value, which is taken as 100%. All that the operator need do when clucking valves is to turn the selector switch to whichever valve he requires to test and read the percentage meter, which should show a reading of 100% for a valve with satisfactory emission. The 01 A amplifer is used on the smaller Mirrophonic systems. It has a gain ol 02 db. and .in output of 8 watts. Western Electric Equipment Contrary to usual Western Electric practice, this amplifer does not employ a push-pull final stage. The amplifer has three stages, resistance capacity coupled, the first two valves are 310-A pentodes and the last stage a 300-A. The 300-A valve is a new valve of improved efficiency. It has an oxide-coated filament rated at 5 volts, 12 amps. The operating plate voltage is 325 volts, and current 60 milliamps. At this rating the valve is capable of delivering 8 watts speech output. It is of interest to compare the 300-A with the 242-A type now used in 43-A amplifiers. Although the 91-A amplifer only employs one 300-A valve in the output stage, the harmonic content of the audio output is negligible. This is accomplished by employing reverse feedback in the amplifier. Compensation is also made in the amplifier for higher frequency scanning losses. The 86 type amplifier is used on intermediate size Mirrophonic systems. This amplifier has four stages, the first three employ 262-A valves resistance capacity coupled, and the output stage is push-pull, utilising two 300-A valves. The output of the 86 amplifier is 15 watts, the gain of the amplifier being 98 db. The gain frequency characteristic of the 86 amplifier is flat from 50 to 12,000 cycles. The amplifier has its own power supply built in. The 91 -and 86-type amplifiers differ in appearance from the series of 40-type Western Electric amplifiers. They are built on shallow chassis and the complete units are housed in strong metal cabinets. The 87 amplifier is a high-power, single-stage, pushpull amplifier and is used with an 86-type amplifier in large theatres. It has a gain of 10 db. and an output of 50 watts. If more than 50 watts output are required, additional 87 amplifiers can be added in parallel. Two 284-D valves are used in this amplifier and the plate voltage is supplied by a built-in rectifier, which has two half-wave rectifiers. The rectifier valves are 249-B high voltage mercury type valves. These three amplifiers are designed to operate from an A.C. supply of 105-115 volts, 50 or 60 cycles. Horn System. The Di-phonic loud-speaker low frequency unit can consist of from one to four cone units according to the si/e of system, mounted at the base of a shallow cavity, which is in turn surrounded by a very rigid battle. In the cavity there are fitted "deflectors," which improve the frequency response of the whole unit and also result in a better distribution of the higher frequencies radiated by this unit. The cellular horn which reproduces the frequency range above 300 cycles is composed of a number of separate horns of approximately the same exponential rate of taper. The wide ends or "mouths" oi these horns are all joined together to form a large common mouth which has a cellular appearance. The narrow ends ol the horns are brought together to form a common throat. Either one or two dynamic units of an improved type can be fitted to the throal according to the si/e of the horn and the auditorium to be c<>\ ered,