International projectionist (Jan-Dec 1935)

GWN CONTROL POTENTIOMETER UNO SW'TCM ARE MECHANICAL* CONNECTED SO that ,\f oorCNTiOMCTER t& MOVED rRQM'H tqI INPUT FKOM R£PROPUCER EQUALIZER MODERN AMPLIFIERS REFLECT VAST OUT PUT TO 9 A OR 10A AMPUFIER 2v g .iaov IMPROVEMENT IN THE ART I HE full extent of the changes that have taken place in the design of theatre amplifiers during the last few years cannot be appreciated until the old and new are placed side by side, as in Figures 1 and 2. Only by carrying the comparison down to small details can the projectionist realize what these changes mean to his work and to the nature of his responsibility. Figure 1 represents one of the very first amplifiers used for theatre sound. Some of these amplifiers are giving satisfactory service even today in large deluxe theatres. The filament circuits of this amplifier begin at the bottom center of the drawing, with two terminals marked " — 12V" and "+12V". The line from the positive terminal runs up, right, up, right, up an inch, right an inch and up an inch to the positive terminal of the filament of the extreme right-hand tube. Through this filament, then down, left and up to the filament of the center tube. Through this and down, left, down and right to "Filament 2 & 3 Jack", which is an arrangement for plugging an ammeter in series with the filament circuit to read the filament current. From this jack to the double winding of the retardation coil. Thence down and left to "Filament 2 & 3 Rheostat." Thence to the negative terminal at the bottom of the drawing, through a threeampere fuse. This filament circuit has a branch line Leroy W. Chadbourne that supplies the extreme left-hand tube. From the positive terminal up, right, up, right, up an inch and then, instead of turning right, follow the other branch left about two inches, thence up an inch to the positive side of the filament. Through the filament, left through the grid bias resistor, and down to "Filament 1 Jack". Through this, through the retardation coil, through "Filament 1 Rheostat", and then down and right to the 3-ampere fuse and negative. The source of power for this circuit is external, and consists of 12-volt storage batteries, although at present it is common practice to use a rectifier. The source of plate power originates, as far as this amplifier is concerned, with the other three terminals at the bottom of the drawing, marked "G", " + 130V." and " + 350V." Beginning at the high voltage end, the line may be traced upward through the quarter-ampere fuse to "Plate 3 Jack", thence through the retardation coil to the plate of the righthand tube. From the filament of that tube back to the positive terminal of the NOTE: The drawings in this article are intended only to aid the reader in contrasting new and old amplU fiers, and in no wise are they indicative of the present relative merits of various sound equipments. — Editor. [13] filament circuit, which is shorted to "G". The 130-volt line for the other two tubes has two branches. From the 130volt terminal up, right, up, right and up past the filter condenser, then left, and up to "Plate 2 Jack". Thence up through the retardation coil to the plate of the middle tube, and back to "G" through the filament circuit. Instead of turning upward at "Plate 2 Jack" the other branch of this line may be traced leftward from the junction point below that jack and into "Plate 1 Jack". Thence upward through the coil marked "Input Transformer" to the plate of the left-hand tube. In the lower right-hand corner of the drawing, just above the plate power input terminals are four terminal posts, "A", "B", "C", and "D". Just below these are three resistors. This arrangement is included to provide for a second type of plate power input. The plate power for this amplifier was provided in two different ways. One was to use a 350-volt bank of glass-case storage "B" batteries. In that case the full 144 cells were connected to the " + 350V" tap, and a branch connection was taken at the sixtieth cell for the " + 130V" tap. In other installations, plate power was derived from a vacuum-tube rectifier mounted on the amplifier rack alongside of Figure 1. In that case the " + 130V" and "+350V" terminals were not used. Two wires, + and — 350 volts d.c. led from the rectifier. The negative line