International projectionist (Jan-Dec 1935)

November 1935 INTERNATIONAL PROJECTIONIST 17 hand end of that transformer secondary is positive with reference to the centertap, which is negative by comparison with that side of the winding. Hence, tracing as before, the circuit runs from the center-tap down to the ouput terminals at the bottom of the drawing, and back to the plate of V-2. That plate being more negative than the filament of V-2, electrons emitted in that tube are not attracted to the plate (closing the tube circuit) but are repelled by the plate and attracted back to the filament they have just left. Hence there is an open circuit in the right-hand side of this rectifier, and the only current flowing through the output terminals during the instant now under consideration derives from V-l. The direction of flow of that current, as seen, conforms to the polarities marked at the bottom output terminals of the drawing. The Next Alternation At another instant, when the a. c. polarity in the transformer secondary has reversed itself, the left-hand end of that winding will be positive. During that interval the plate of V-l will be negative with reference to the filament of V-l, electrons emitted by that filament will be attracted back to it and repelled by V-l plate, and the open-circuit just traced will have been transferred to the left-hand side of this rectifier. At the same time, current will flow through V-2, the plate of which (connected through the external load to the transformer center-tap) will be positive with reference to V-2 filament. Electrons emitted by the filament of V-2 will be drawn to the plate, continue down, left, down and out at the 24-volt negative terminal; in at the 24-volt positive terminal, and straight up to the transformer secondary. Tracing the same circuit in terms of current flow rather than of electron flow, it runs from the positive center-tap down, through the external load, up and PROJ #2 SYSTEM AMPLIFIER Cft =!= (41 OR 46 TYPE) right to the plate of V-2, thence (that plate being positive) to the filament of V-2, and through the fuse to the transformer. During this half of the cycle also the flow of current through the output terminals conforms to the polarities shown in the drawing. Consider a moment when the polarity of the transformer is in process of reversing itself, and the entire secondary is at neutral voltage. During that instant of time there is no flow of secondary current at all, and no charge upon the plate of either tube. Electrons emitted in the tubes are neither drawn to the plates nor repelled from them; some few may strike against the plates by chance, constituting a trifling flow of current across each tube too small to be considered. The large majority of emitted electrons are attracted back to the filaments from which they came, since each electron lost leaves a filament that much less negative (or that much more positive) and positive with respect to a free electron. Complete absence of electron flow across the tubes means no flow of current through the output terminals, inasmuch as those terminals are at that moment in series with two "open switches." At the next instant one of the plates becomes sufficiently positive to attract electrons, and current flow through the output circuit resumes. The amperage increases as the positive charge of the plate increases, attracting more and more electrons. After that charge has reached its maximum and begun to decline, the current through the output falls off, as fewer and fewer electrons reach the plate, until another moment is reached at which the transformer winding is at neutral voltage throughout. Again the output current falls to zero, to reappear again (still in the same polarity) as the plate of the other tube assumes a positive charge. The current through the terminals marked plus and minus 24 volts therefore varies from zero to maximum 120 times a second (or 100 times a second if the power supply is 50 instead of 60 cycles). Filtered Output Circuits Such current cannot be used for most sound reproduction purposes, and in Fig. 1 appears only at the output terminals previously mentioned. For the sake of simplicity in tracing the rectifying arrangements by which a. c. is converted to d. c, the other, parallel, output circuits have been temporarily ignored. The arrangements already considered indicate, however, that the tube circuits may be considered as a source of d. c. which, though it fluctuates in value from maximum to zero, never reverses its direction of flow. The positive terminal of this d. «. source is the center-tap of the transformer secondary, and the negative terminal the jumper between the plates of the two tubes. This fact is emphasized in the drawing by the addition of plus and minus signs midway between the two rheostats, R-l and R-2. The output terminals at the bottom of the drawing are connected directly to the d. c. source, without intervening filters, and hence are subjected, as seen, to the full fluctuation of the current. The other four outputs of Fig. 1 are distributed at the two sides of the drawing, and connect to the d. c. source through intermediate filters, consisting of inductance coils and condensers. The coils are in series with the line, the condensers connected across it. When the power switch of this rectifier's a. c. input is closed, and one or the other tube plates becomes positive, the condensers connected across the output line accumulate charges, robbing the line of current in doing so. The inductance coils build up fields, a process which opposes the flow of current. For these reasons current rise in the filtered circuits cannot keep full pace with the increase in the plate positive charge. But when the plate charge has passed through its maximum and begins to decline, and the voltage delivered to the d. c. circuits falls off, the condensers give up some of their charge, which returns to the line in the form of current flow, and the shrinkage of the magnetic fields around the inductive windings results in a reinforcing flow of induced current in the same line. Current fluctuation in the filtered circuits is greatly reduced, the amperage never falling entirely to zero at any time, and never at any time rising to the maximum that would be reached if the filters were not present. In the more thoroughly filtered branches of Fig. 1 the current flow is sufficiently steady to prevent even a sensitive sound equipment distinguishing between it and the pure d. c. drawn from a battery. Speaker Field Output Circuit As previously stated, the least thoroughly filtered branch is the speaker field output, which is shown at the lower left-hand corner of the drawing. The two terminals there shown are marked positive and negative. Tracing backward from the positive terminal, the circuit runs right through the three-ampere fuse, F-3; right through the filter inductance L-8, right and up to the positive d. c. source at the center-tap of the transformer secondary. Thence along whichever half of that secondary is negative at the moment to the filament of the associated rectifying tube, through vacuum to the plate of the same tube, along the jumper between the two plates