International projectionist (Jan-Dec 1936)

New W. E. 300A Power Tube for Theatre Sets Compared With the 242A By J. O. McNALLY MEMBER OF THE TECHNICAL STAFF, BELL TELEPHONE LABORATORIES Data given in the appended article requires supplementary explanatory comment. It will be noted that Table 1 gives the output of two 300A's in push-pull, with 325 volts plate current, as 15 watts. The last paragraph of the article cites the output of two 300A's in push-pull, with 450 volts on the plate and a plate current of 60 milliamperes, as 25 watts. This distinction is important in figuring operating efficiency. Any comparison relative to efficiency and cost between this 300A tube and RCA's 6L6, described herein last month, should allow for different bases of engineering calculation with respect to harmonic content, tube life and overall type of amplifier used. — Editor. THROUGH the development of a new power vacuum tube — the 300A — improved amplifiers for sound picture and public address systems are now available at lower cost. One of the factors contributing to this cost reduction is a decrease in the operating plate voltage, which has made possible savings in the rectifier equipment contained in the amplifiers. This lower plate voltage has also made possible economies in the construction of the tube. This results in lower maintenance cost due to the lower cost of tube replacements. Improvements in the electrical behavior of the 300A tube are indicated by comparing its operating characteristics with those of the 242A tube, employed in the 43A amplifier, used at present in the majority of theatres equipped with W. E. sound systems. The new 300A tube is employed in the 86A amplifier, which is designed for similar purposes. The comparison is given in Table 1. The 242A tube requires a plate voltage of 800 volts, or more than twice that of the 300A tube. The power output of the 43A amplifier is 12 watts ; whereas 15 watts is obtained from the 86A amplifier. The filament power required for the 242A tube is five times that for the 300A tube. Comparative sizes of these two tubes is illustrated in Fig. 1. Tube Operating Characteristics Like the previous tubes, the 300A is a triode. It has an oxide-coated filament which may be operated on 5 volts a. c.„ normal filament current being 1.2 amperes. The plate is formed from sheet nickel, blackened by a carbonizing process to increase its heat-radiating ability. Molybdenum wire, wound flat and welded to two nickel support wires, forms the grid. TABLE I Type Tube 242.4 300,4 Operating plate voltage 800 325 Operating plate current — milliamperes per tube 67.5 60 Power output in watts of two tubes in pushpull ...,..:... 12 15 Filament power in watts —per tube 32.5 6 The general appearance and construction of the tube are shown in Fig. 2, where the base with filament and grid structure are in the left hand, and the plate structure in the right. With a plate potential of 350 volts and a plate current of 60 milliamperes, the normal grid FIGURE 3 Cross-section of 300 A tube, shown in dashed lines, superimposed on a similar cross-section of the 252A tube in solid lines, indicates one method employed to decrease plate resistance FIGURE 2 Disassembled elements of 300A tube bias is— 74 volts. A single frequency input of 74 peak volts will deliver 8.5 watts to a 3000-ohm resistance load in the plate circuit. The ability to obtain large power outputs from the 300A tube with low plate voltages is made possible by the low plate resistance. This reduction in plate resistance has been accomplished by decreasing the amplification factor, by decreasing the spacing between the elements— particularly that between filament and grid — and by increasing the area of the elements. The amplification factor has been reduced to the lowest practical limit, which is set by the point at which the varying current in the plate /g£} ft..i.mmuiuuni»iuu|luiuii.u«H.um.mi»»uil ^ i FIGURE 1 Comparative sizes of 300A and 242A W. E. tubes -ffT i11 [20]