Radio Broadcast (May 1928-Apr 1929)

164 RADIO BROADCAST JULY, 1908 FIG. I The resistance, R5, and the2.o-mfd. condenser on the receiver side of it, C2, is a trap into which motorboating tendencies fall. It is applicable to resistance-and impedance-coupled audio amplifiers and was first described in April Radio Broadcast. The one-mfd. condenser, C4, from the 135-volt tap to ground may not be necessary. The Thordarson engineers state that a 0.02 mfd. condenser at C3 will cause the circuit to amplify still more at 90 cycles without decreasing the high frequencies using a screen-grid connection, is sometimes produced by a microphonic feedback from the loud speaker. USES OF THIS AMPLIFIER THIS amplifier has been developed primarily as a replacement unit for use with a radiofrequency system which, with its original audio amplifier, does not produce sufficient output to deliver good loud speaker volume on weak incoming signals. The conaplete device includes a two-stage audio-frequency amplifier and also a B powersupply unit capable of handling not only the audio-frequency requirements but several radiofrequency tubes and a detector as well. It will take the place of the audio-frequency system in any existing receiver when connected to the output or plate terminal of the detector. The spacecharge tube takes only one eighth ampere of filament current, which may be furnished by an A-battery of either the storage or dry cell type. The audio-frequency amplifier uses transformer coupling between the detector tube and the space-charge tube. The Z-coupIer is used between the space-charge tube and the power tube. Direct current is kept out of the speaker by a coupling transformer which follows the power tube. The power unit for this amplifier includes a "compact," P, having a transformer, two filter chokes and two buffer condensers for the rectifier tube, all within one case. In addition to the high voltage winding on the transformer there is also a center-tapped filament winding for the power tube. The condenser block contains filter condensers and also the two 90 and 45-voIt tap bypass condensers, Q and C3. An additional 1.0 mfd. condenser may be necessary across the 135 volt top as indicated on Fig. 1 and if the 25,000 ohm anti-motorboat resistor is used, it needs a 2.0-mfd. condenser. These are not shown on the photographs. All plate voltages are secured from a single voltage-dividing resistance unit, Ri, having taps at the proper places. Grid bias for the spacecharge tube is secured by the drop across the filament resistor, R2. Grid bias for the power tube is secured from a part of the voltagedividing resistance, Ri. The inner, or spacecharge grid, is supplied with a positive charge of approximately twenty-two volts through the 10,000-ohm resistor, R4, connected to the fortyfive volt line. There are six binding posts. One is for a connection to the detector tube plate, two are for A-battery connections and the remaining three provide connections at 135 volts, 90 volts and 45 volts for a radio-frequency amplifier of any type. In utilizing this amplifier with a previously built radio-frequency system no additional controls are required. Volume is controlled from the radio frequency end of the combination. PARTS LIST Ti i Thordarson Z-Coupler, T-2909 T2 1 Thordarson A. F. transformer, R-300 T3 1 Thordarson speaker coupling transformer, R-76 P 1 Thordarson power compact, R-171 Ri 1 Thordarson-Ward Leonard resistance unit R-508-1 Ci 1 Dubilier B-block, R-171 C2 1 Dubilier fixed condenser 2 mfd. (optional) C3 1 Dubiler fixed condenser, \ mfd. C4 1 Dubilier fixed condenser, 1 mfd. (optional) R2 1 Yaxley 10-ohm filament resistor R3 1 Yaxley 1 5-ohm filament resistor R4 1 Ward Leonard 10,000-ohm resistor R6 2 Ward Leonard Resistance 25,000-ohms (optional) 3 Benjamin cushion sockets, 9044 Sw 1 Yaxley switch s. p. s. t. 6 X-L bakelite top push posts (Input, A + B— , A—, B + 135, B + 90, B + 45) VT2 1 Power tube, UX-171A or CX-371A VTi 1 Screen grid tube, ux-222 or cx-322 VT3 1 Raytheon bh rectifier tube [The only special part in the above list is the Thordarson Z-coupler. Naturally, any dependable input and output transformer or condensers may be used, provided they are electrically similar to what the author specifies. The resistors must have the correct resistance and be able to dissipate the proper amount of heat. Readers may obtain the manufacturer's literature in this amplifier from Radio Broadcast or from the manufacturer direct — Editor.l