International projectionist (Jan-Dec 1935)

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10 INTERNATIONAL PROJECTIONIST April 1935 preventing a decline in the current through R-37 to match the falling off in Tube 7 space current. C-28 then, acts to steady the bias current through R-36 and R-37, and thereby to prevent any distortion that might occur if fluctuations in the space current of each tube were permitted to cause a corresponding change in the drop through the two grid bias resistors. Consequently, we may trace the a. c. components of the two tubes separately, as follows: Tube 6: from filament to plate; through the upper half of T-8 primary; from the mid-tap of that primary left, down, left, down, left through C-29; through the ground connection; then, branching, one part through R-34 and R-36 to the mid-tap of Tube 6 filament secondary, and the other part through R-35, R-37 and C-28 to the same destination. (The next speech alternation, of course, takes the opposite direction through C-28.) Tube 7: from filament to plate; thence down, right, up, right, up and right to the lower half of T-8 primary; from the mid-tap as before to the ground connection left of J-4; from the ground one part of the current goes directly through R-35 and R-37 to Tube 7 filament secondary, while the other part reaches the same destination through R-34, R-36 and C-28. At the next speech alternation the current is from filament of Tube 7 to filament transformer; from the mid-tap of that to the right-hand side of R-37, where the circuit branches, one line continuing through R-37 to ground, while the other is then traced upward through C-28, and then through R-36 and R-34 to ground. From ground back through C-29 and thence to the plate of Tube 7. Since the a. c. component of Tube Questions and Answers on Sound Projection Aaron Nadell Note: This department is launched in response to numerous requests by readers for such a service. The worth of this section, which will be a monthly feature, depends wholly upon the cooperation given by readers: the more numerous the inquiries, the more varied the department and the greater its value. Correspondents are requested to be specific and as brief as possible. Only initials will be appended to questions. — Editor. Q. What causes a black, tarry stuff to leak out of some parts of an amplifier, and does this do any damage? A. This black material is a sealing and insulating compound that has been softened by overheating. Line voltage may be too high. Loss of this compound, if not excessive, probably will do no harm. Q. My amplifier works almost as well with one of the tubes removed, if I raise the volume control. In case a tube goes bad, is it all right to operate this way in an emergency? A. Not unless you want to risk burning out the power transformer or the filter condenser. Stock enough spares. The tube you refer to is one of a pushpull pair, and the amplifier works without it at less than half volume and with inferior quality. P. A. System Amplifier Q. On amateur nights we use a microphone through the sound amplifier and speakers. How large an amplifier would I need to make a separate P. A. system, and how much should this cost? We have 400 seats. A. An amplifier with 5 watts output should be large enough. If you use only one "mike", a new amplifier and speaker will not prove expensive. Consult your local supply dealers for lowest prices. Q. My No. 1 projector accelerates to full speed in four seconds; but No. 2 requires ten seconds, although it runs alright once it is up to speed. What causes this, and how can it be corrected? A. Switch motors to find out whether trouble is in motor, in the projector head or in the sound drive. Examine motor for bad brush contact, ineffective lubrication caused by a clogged oil hole or seme similar condition, or any opencircuited starting winding, according to the type of motor. If trouble is in the projector, look for insufficiently lubricated shafts or bearings, evidence of wear, or improper lineup in the head, in the sound attachment or the driving gears or pulleys. Repair may involve overhaul of the projector or sound head. Q. I want to add another speaker. How can ' be sure that I have kept impedance match? A. Obtain from manufacturers the impedance of each speaker voice coil and the output impedance of the amplifier. Wire the speakers in series, parallel or series-parallel to preserve impedance match. If this cannot be done, change the output transformer of the amplifier for one that will match your speaker arrangement. 6 is at all times 180° out of phase with the a. c. component of Tube 7, the two a. c.'s always flow through C-28 in the same direction at the same moment, and never oppose each other in that condenser. The output circuit of Figure 1 is drawn from the secondary of T-8, and wired to terminals 1 to 5 of the connection block at the upper right-hand corner. Impedance match to the speakers or to a speaker-matching transformer is obtained by choosing a suitable combination of these terminals. There are 30 ohms between terminals 1 and 5, and three ohms between terminals 1 and 2. The test jack is connected across terminals 1 and 2. It will function regardless of whether or not those terminals are Used for output power, since an alternating voltage will still be generated in the portion of the transformer secondary that they bridge, and current will flow in a headset plugged in across them. New Forest Twin 50' for Two Arcs and Spot A new type copper-oxide rectifier, known as the Twin 50, has been introduced by Forest Manufacturing Corp. It is really a double rectifier in one housing. It uses two separate banks of copper-oxide rectifying units, each having a capacity of from 30 to 50 amperes, designed to supply two projectors. With one projector running, there is no reduction in current on this arc when the second arc is struck, as both rectifier elements are separate. One blower system is used for both rectifying elements. Automatic means are provided to protect the rectifier from any damage that might be caused by blower failure or shortcircuit. Individual Arc Adjustment Means are provided therein for adjusting the output voltage and current individually to each arc, so that the rectifier may be used for Suprex trims of either 30-55 amperes, or 40-50 amperes. Line taps are also provided to take care of line voltages from 200 to 245 volts. This rectifier is designed for use only on threephase A. C. supply, all threephases being rectified and thus giving a smooth output. For theatres using a spotlight requiring 50-55 volts at from 30 to 60 amperes, there is provided an attachment for the spot supply. Thus the Forest Twin 50 is a universal rectifier able to supply both Suprex projectors and a spot, a distinct advantage for theatres which, installing Suprex projection, heretofore had to provide other means for spotlamp supply, because of the higher voltage required. S. M. P. E. Spring Convention at Roosevelt Hotel, Hollywood, Calif., May 20-24, inclusive.