International projectionist (July-Dec 1934)

October 1934 INTERNATIONAL PROJECTIONIST 17 ary winding of T-6, the speech input transformer of the monitor speaker amplifier. The a. c. voltage generated in the secondary of T-6 causes current to flow through the grid resistors R-24 and R-25. The voltage-drop developed across each of these resistors is connected across the grid and filament of the corresponding tube. The connection to the filament is from the junction between the resistors downward, right and down to the top of R-26, and through R-26 to the center-tap of the filament winding of the transformer. The a. c. component of the plate circuit may be traced from the plates of the tubes through the plate transformer primary mid-tap and downward about an inch to the right-hand side of condenser C-15; through that condenser and upward, right and then down to the top of R-26; through R-26 to the filament secondary and thus back to the filaments of the tubes. The output circuit is, of course, from the plate transformer secondary through L-4, the 3.8-ohm voice coil of the loud speaker. So much for the monitor amplifier. We are now ready for its big brother. The System Amplifier The "power pack" portion of the main amplifier is shown at the bottom center of the drawing. Line a. c. is supplied to the primary of a power transformer. T-5, that has four secondaries. Just to the left of the longest, or plate secondary, are two 280-type tubes, tubes 7 and 8, in a full-wave rectifying circuit. Their fila WOTO CELL CIRCWT SOUNOHEWI "»•• ments are lighted by the topmost secondary of the power transformer. The two bottom secondaries of T-5 are the ones that light the filaments of the amplifying tubes in the system amplifier. The lower of the two supplies the '56 and '57 tube heaters; the upper of the two lights the four filaments of the 245 power stage. These four tubes are shown directly above the two 280's. The '56 tube, a heater-type triode, is about an inch to the left of them; the '57, a heater-type pentode, is an inch again to the left of that. All these filaments terminate in arrowheads, indicating connection to the appropriate winding of the power transformer. The photo-electric cells, of course, have no filaments to be heated; their cathodes emit electrons under impact of light. The two 280 tubes shown just to the left of the power transformer provide plate current for the system amplifier and its two photo-electric cells. A 280 tube is designed to serve as a full-wave rectifier all by itself, and we see one so wired in the power supply of the small monitor amplifier that we analyzed a moment ago. But in the case of this larger power pack one 280 tube isn't enough, so two are used. Each tube serves as a simple half-wave rectifier, because its two plates are connected together, and the two together constitute a full-wave rectifying circuit. This circuit is in every way identical with the full-wave rectifying arrangements in amplifiers discussed earlier in this series. If the reader does not at once recognize it as such, that is merely r@-kF* due to the trifling difference of two connected plates inside of each tube, instead of a single plate. As a matter of fact, a 280 tube also has two filaments, one inside each plate, but the drawing does not show them. The two filaments act electrically as one filament. Two plates connected together act electrically as one plate. The plate of the upper 280 (the two connected plates being one), is wired to the upper end of the plate secondary of the power transformer. The plate of the lower 280 tube is wired to the lower end of that secondary. The center-tap connection of that secondary constitutes the negative lead of the rectifier's d. c. output. We may trace that lead downward, left to the right-hand side of choke-coil L-8, and thence upward to the place where the power is used. The positive line of the d. c. output leaves the filament supply secondary (the top-most secondary), and branches two ways. One branch runs upward to supply the four 245 tubes. The other runs leftward through choke-coil L-7 to supply the '56 and '57 tube, and the photo-electric cells. We can trace this rectifier circuit in detail, beginning with the branch that supplies the 245's. The Rectifier Circuit Starting at the negative terminal, which is the center-tap of the plate secondary, the circuit runs downward to the right-hand end of L-8, thence upward two inches, right two inches and upward an inch to the right-hand end of L-l, which is both a filter choke and a speaker field coil. Through this coil downward a half inch FIGURE 1 *IC www svpptT sine* EXCITER UMP SSt 11