The motion picture projectionist (Nov 1929-Oct 1930)

32 The Motion Picture Projectionist January, 1930 The Three-Element Vacuum Tube By Edward W. Kellogg* MANY factors essential to a successful talking motion picture owe their existence largely to the vacuum tube amplifier. I have in mind in particular high quality loud speakers. These owe their existence to the amplifier tube; first, because without the ample power and high quality voice currents for their operation which amplifiers can furnish, the loud speaker development would have been extremely difficult; and secondly, because the vacuum tube is the foundation of popular radio reception, and without the stimulus of this field of application loudspeaker development would not be near where it is today. Figure 1 illustrates the manner in which a tube is connected when it is used for amplification. It is customary to measure the plate voltage and grid voltage, using the negative filament terminal as the reference point. The hot filament gives off electrons which are negative charges of electricity. These are repelled by the grid which is negatively charged compared with the filament, but they are attracted by the plate which is positively charged. Owing to the proximity of the grid, the electrons surrounding the filament are more strongly affected by the grid potential or voltage than by that of the plate, or, in other words, a small change in grid voltage has as much effect on the escape of the electrons as a much larger change of plate voltage. Nevertheless, owing to the holes through the grid the plate voltage does have an effect on the movement of the electrons within the space between filament and grid. All of the electrons which pass through the meshes of the grid are carried to the plate, but the number which thus escape to the plate depends jointly on the voltage of the grid and that of the plate. Grid and Plate Potentials In a tube of given construction, a change of one volt in grid potential has as much effect on the number of electrons reaching the plate as a con * S.M.P.E. Trans., Vol. XII, No. 36. Fie. 2. Portion of resistance — capacity coupled amplifier. + 100 VOLTS -PLATE -battery: siderably larger number of volts change in plate potential. For example, referring to Fig. 1, if the grid potential were changed from — 4.5 to — 5.5 volts, the plate current would be reduced, but if the plate potential is now raised from 90 to 97 volts, the plate current will be brought back to its original value. We should then say that 7 volts change of plate voltage is equivalent to 1 volt change of grid voltage. This ratio is known as the amplification factor and is designated by the symbol u. If the current is supplied to the plate through a very high resistance, so that the voltage consumed in the resistance is large compared with the tube plate voltage, the plate current can be held practically constant. Under these circumstances any change of grid potential results automatically in the compensating change in plate voltage. Figure 2 shows the connections of a resistance-capacity coupled amplifier. In applying the change in plate voltage of the first tube to the grid of the second tube it is customary to interpose a capacity C. This makes it possible to connect all of the filaments to the same set of leads, but since the first plate is at a potential of say +73 volts with respect to the negative filament and it is desired to have the grid of the second tube at an average of — 4.5 volts, it is necessary to interpose a device across which a constant voltage 77.5 volts can be maintained. For certain special purposes a 77.5 volt battery INPUT J VOLTAGE \ Fig. 1. Conventional tube circuit diagram METER might be used in place of the condenser shown, but if the voltage of the plate of tube No. 1 goes through a rapid change and returns to its original value, the size of condenser C can be so chosen that its charge will not change appreciably during this interval, and under these conditions it accomplishes just what a battery would accomplish; that is to say, it keeps the plate of No. 1 tube and the grid of No. 2 tube at a fixed voltage difference, thus transmitting all of the fluctuations while permitting grid No. 2 to vary above and below the desired average of — 4.5. The grid leak resistance R2 serves to keep the grid at the desired average potential. This must be a high resistance for two reasons: (1) If it is a low resistance, it will constitute a useless load and reduce the amplification obtained, and (2) a low resistance will permit too much change to take place in the charge on the condenser C. Transformer Coupling Figure 3 shows schematically two tubes connected by means of a transformer. When a voltage is supplied to one winding of a transformer, a corresponding voltage is developed in the other winding. This "secondary voltage" may be greater or less than the supplied or primary voltage, depending on whether the second winding has more or less turns than the primary winding. The transformer provides a very convenient method of connecting tubes in cascade for amplification, and by providing more secondary turns on the transformer than primary turns, or "stepping up" the voltage through the transformer, a greater amplification per stage is obtained than that corresponding to the amplification factor or u of the tube itself. We have shown so far simply how the vacuum tube can amplify voltage. It is now in order to consider how this may be done without causing distortion. The condition for distortion