The motion picture projectionist (Nov 1931-Jan 1933)

Motion Picture Projectionist May, 1932 A New Amplifier Output Tube In view of the widespread interest which has been accorded the recent introduction of the Speed "TripleTwin Tube," the Motion Picture Projectionist is particularly fortunate in being able to present the following article on the subject from the pen of Mr. A. Ernest Lyle, Chief Engineer of the Cable Radio Tube Company, manufacturers of this new departure in tube design. — The Editor. CONTEMPORARY developments in the art of sound reproduction,, together with present-day economic conditions, have necessitated a complete revision of ideas concerning the magnitude of audio power necessary for satisfactory theatre and auditorium sound. The high values of power initially used have been continually and sharply reduced. Theatres formerly using audio outputs of hundreds of watts and from twelve to eighteen speakers now find equal or superior coverage possible with one or two directional reproducers requiring approximately 10 to 15 watts of audio energy. Where special acoustical conditions have necessitated additional speakers for coverage of difficult locations, these are being satisfactorily supplied from another small 10 to 15 watt amplifier. In general these amplifiers derive their output power from the use of type 250 tubes operating in push-pull. The object of this paper is to describe a new amplifier tube of greatly increased efficiency, the Speed "TripleTwin" tube, with particular reference to its application in the theatre and public address field. Introduction The "Triple-Twin" is a new output tube capable of delivering high audio powers from relatively low input signal. It is at present available in three types. Type 291 designed for i d.c. amplifiers, where the total avail By A. Ernest Lyle j£—*r "^ *i i • ^_3 ["•i> Fig. 1. (Upper) Single-Tube Circuit Fig. 2. (Lower) Push-Pull Circuit able voltage is limited to that of the d.c. 120-volt line; Type 293 for automobile use where economy of power consumption is a prime requisite; and the 295 for general amplifiers utilizing a.c. as their power source. The Type 295 "Triple-Twin" only will be considered here. This "TripleTwin" tube operates at a plate potential of 250 volts and delivers in excess of 4.5 watts undistorted power for an applied grid signal of 5 volts R.M.S. Even this rating is very conservative, as 6 watts may readily be obtained without the introduction of objectionable harmonic distortion. (Fig. 3). For commercial sound work, however, the push-pull application will generally be indicated. Two "Triple-Twin" 295 tubes operated in this manner, and at 250-volt plate potential, are capable of delivering 14 watts of undistorted power for an applied grid signal of 7.5 volts R.M.S. The characteristics of the Speed Type 295 tube are shown in Table I. The "Triple Twin" tube consists Table 1: Triple-Twin Tube Characteristics Output Tube Heater Potential Volts 2.5 Heater Current Amperes 4.0 Input Amplifier Section Plate Potential Volts 250 Grid Potential Volts —14.0 Plate Current M. A. 4.0 Plate Impedance Ohms 12,000 Amplification Factor 14.4 Mutual Conductance Micromhos 1,200 Load Impedance Ohms 12,500 Undistorted Power Output Watts Grid Signal (for full power) Volts RMS 5.0 Output Section 250 —3.0 52.0 3,000 13.0 4,350 4,000 4.5 essentially of two sets of three elements each, directly coupled within the glass envelope. The first set of elements constitutes a driver section whose purpose is to present at all times a high impedance to the incoming signal and to maintain the plate characteristics necessary to supply power as required by the output grid. This section employs an indirectly heated cathode which is directly connected to the grid of the output section. This indirectly heated cathode electrically isolates the input emitter from the output filament. The second set of elements constitutes a driven section in which the grid operates essentially at zero potential swinging equally into the positive and negative portion of its grid-voltage, plate-current characteristics. Efficiency of Tube The efficiency of an output tube is customarily considered as the ratio of its a.c. power output to the d.c. power input. Amplifier tubes for receiving purposes are normally operated at a high value of negative grid bias, and input signal swings are so restricted that their positive peaks do not extend out of the biased portion of the grid-voltage plate-current characteristics as this would cause grid current flow with consequent serious harmonic distortion. However, for a given power output the anode voltage necessary to overcome the field of the negatively biased grid and to produce that output must be considerably higher than would be necessary to produce the same power output from a tube whose grid was not negatively biased. Thus the "Triple-Twin" tube operating at zero grid bias and accommodating both positive and negative grid swings achieves a notable increase in efficiency. This efficiency is still further enhanced by the fact that this tube attains its maximum power output and minimum distortion for the approximate same value of load impedance and thus is able to operate into a load nearly equal to its own impedance rather than double this value, as is common practice in triode operation. In analyzing the overall efficiency of an amplifier tube it is, of course, not sufficient to compare only the ratios of power output and power input, as these give no indication of the sensitivity of the tube. Where the sensitivity of one tube is sufficient to allow the elimination of one or more stages, then the effective efficiency naturally becomes much greater. Table II compares the Speed "Triple-Twin" tube with two popular tubes of its own class having similar