Radio Broadcast (May 1928-Apr 1929)

160 RADIO BROADCAST JULY, 1928 July, 1928. Radio Broadcast's Service Data Sheets on Manufactured Receivers A. G npHE Kolster 6K A.C. receiver is an example of conventional a.c. receiver design utilizing the standard types of a.c. tube. The receiver consists of three stages of tuned radio-frequency amplification, a non-regenerative detector and two stages of transformer-coupled audio. The tuned radio-frequency stages are of two types. The input stage consisting of a tapped primary inductance coupled to a variometer which in turn is tuned by means of a shunt variable condenser. The other three radio frequency stages are of the conventional type, utilizing fixed primary and secondary inductances and tuned by variable condensers. Stabilization of the radio-frequency system is effected by employing resistances in the grid circuits of the radio-frequency tubes. These grid suppressor resistances are of the order of 800 ohms each. The three radio-frequency and first audio tubes are cx-326. The detector is a C-327 and the second audio is a cx-371. The detector utilizes the grid condenser and leak arrangement; the grid condenser being of .00025 mfd. capacity and the grid leak of 2 megohms. The power supply for this receiver is a full-wave rectifying system employing a cx-380 tube. The power transformer consists of five secondary windings and a tapped primary winding suitable for various line voltages operating at 60 cycles a.c. The five secondary windings supply five volts for the cx-380, 300 volts for each plate of the rectifying tube, 2\ volts for the detector, 5 volts for the CX-371 power tube, and \\ volts for the cx-326. A centertapped transformer winding is employed in order to obtain the electrical balance in the filament circuit of the c-327 tube. The electrical balance in the A VIEW BEHIND THE PANEL 326 tube circuit is obtained by means of a potentiometer shunted across the filament circuit with the center tap connected to the B-minus. The filter system of the rectifier consists of a two section "brute force" choke and condenser arrangement. The condensers used in the filter are of 2 mfd. each. A four-sect ion voltage divider resistance isj employed to obtain the various output voltages from the rectifier. The 180 volts necessary for the power tube are obtained by tapping the mid-section of the rectifier filter. The voltage drop in the rectifier output occasioned by the drain across the voltage divider resistance reduces the output voltage to a value suitable to the application of the plate of the three r.f. amplifiers and the first a.f. amplifier. These four tubes receive like values of plate voltage. A 3000-ohm resistance is employed to reduce the rectifier output voltage to a value suitable for the detector plate voltage. A 220-ohm resistance is employed to produce a C voltage which is applied to all tubes other than the output power tube. The C-bias for this tube is obtained by means of another resistance of 910 ohms located within the power unit. Control of the r.f. and first a.f. filament is made possible by means of a rheostat rated at 0.2 ohm in series with the TJ-volt filament winding. The speaker is coupled to the output tube by means of a choke condenser system where the condenser is of 2 mfd capacity. The panel light mounted on the front panel is operated by means of the power switch and the filament potential is obtained from the five-volt winding which supplies the power tube filament. The filament leads for the powe' tub, and the cx 326 tubes and the B and C voltage leads for all tubes other than the power tube are contained in one cable. The photograph shows the parts lay-out of the receiver exclusive of the power system. The receiver is equipped with a single tuning control. Supplementing this control, however, is another in the form of a vernier for the r.f. input system.