Radio Broadcast (May 1928-Apr 1929)

JUNE, 1928 RADIO BROADCAST 91 . 1. June, 1928* Radio Broadcast's Service Data Sheets on Manufactured Receivers A* C* 7 THE Amrad A. C. 7 is another representative of the a.c. electric receiver of the neutrodyne type. This receiver utilizes seven tubes, six of which are of the a.c. type while the seventh is a 171. A study of the wiring diagram of the complete receiver installation will bring to light several novel features in design. There are four stages of radiofrequency amplification, a non-regenerative detector, and two stages of transformer-coupled audiofrequency amplification. The power is obtained from a full-wave B power unit which simultaneously supplies the B voltage for the plates of the tubes and the a.c. voltages for the filaments. The r.f. system consists of an antenna coupling stage and three stages of Hazeltine-neutralized tuned radio-frequency amplification. The receiver is designed for a short antenna and the first r.f. stage is really a coupling stage, being untuned. The input circuit of this coupling tube consists of a radio-frequency choke, tapped for the antenna, in series with a fixed resistance, which is grounded. By utilizing this coupling tube the tuned settings of the other three stages are not disturbed by variations in antenna length or capacity. A very novel method of volume control for the entire receiver installation is incorporated in the plate circuit of this coupling tube. It consists of a variable resistance connected across the bifilar primary winding. Under normal circumstances a volumecontrol of this type would manifest an effect upon the grid circuit of the average tuned stage but since the grid circuit of this tube is untuned the effect of this variable resistance is that of only a volume control, without any detrimental effects upon other circuits. Each of the r.f. stages is contained in a separate can. The audio stages are not shielded and are of the conventional type, with an output transformer utilized to couple the loud speaker to the output tube. All four stages of radio-frequency amplification are neutralized, and the neutralizing condensers are designated as G> in the wiring diagram. The bifilar primaries utilized in this system of neutrali zation are marked Li in the drawing. The'receiver is a single-control unit, the four tuning condensers being ganged together and operated from one point. A filter system, consisting of a resistance and a capacity, is incorporated into the detector plate circuit, probably to keep the a.c. hum at a low value. The plate voltages for the r.f. tubes and the detector are obtained from one tap on the power unit, but voltage-reducing resistances located in the plate circuit of each radio-frequency THE SET IN ITS CABINET tube and in the plate circuit of the detector tube, reduce this voltage to the correct values for the individual tubes. It should be understood that volume control in this installation is not accomplished by plate-voltage reduction. At first glance one would assume this to be the case. Volumecontrol is obtained by a resistance which varies the a.c. voltage across the first-radio-frequency primary winding. Bypass condensers are used to bypass the filament and plate circuit in every stage: to further facilitate receiver stability, a radio frequency choke is wired into the plate-voltage system in the r.f. circuit. As was stated, the filament circuits are all a.c; and electrical balance is obtained by means of potentiometers. These are designated as Pi, P2, and P3 in the wiring diagram. The grid bias for the various tubes is obtained by means of a resistance through which the plate current flows and results in a predetermined voltage drop. These voltagedrop resistances are marked Rs and Re. As is evident from the drawing, the grid bias voltage is of like value for the r.f. and the first audio tube, while individual grid bias is obtained for the output tube. The power unit is of conventional type with twovariations. The rectifying tube is of the 280 fullwave type. As a contrast to other filter systems employed in rectifiers, this installation utilizes but one filter choke. The required filtering action is obtained by the use of large values of capacity. A Mershon condenser of several sections (each section being of relatively high capacity) is employed. -As is evident in the wiring diagram, two sections of this condenser are connected across taps of the voltage distributing resistance. This aids materially in the lowering of the effective resistance of the output circuit, and in the reduction of regeneration due to the action of this resistance as an impedance common to all circuits. The power transformer utilized consists of six windings. The primary winding is tapped for three values of line voltage. One secondary winding supplies the filament voltage for the rectifying tube, another supplies the plate voltage for this tube. The other three supply the a.c. filament voltages necessary for the various tubes in the receiver. Since a shunt potentiometer method of obtaining electrical balance is utilized, center taps on the a.c. filament windings are unnecessary. Control of the complete receiver is accomplished by means of a switch in series with the house supply circuit and the transformer primary. This switch is located on the face of the receiver panel. Coupling Tube 1st R.F .J' UX-171 CX-371 2nd A.F., THE CIRCUIT DIAGRAM OF THE AMRAD A. C. 7