Radio Broadcast (May 1929-Apr 1930)

Data on Its Design and Operation 111 ■ 1 1 1 1 1 141 1 1 1 1 1 1 1 ■ 1 1, AUTOMATIC VOLUME CONTROL By YIRGIL M. GRAHAM Radio Engineer9 Stromberg-Carlson Telephone Manufacturing Company Virgil M. Graham A screen-grid receiver is generally considered as one employing at least two screen-grid tetrodes in a radiofrequency amplifying system. The automatic volume control feature is a circuit arrangement which maintains the signal level at the detector practically constant, when, of course, the received signal is above a certain minimum value. The R. F. Amplifier The radio-frequency amplifying system of the Stromberg-Carlson No. 846 Receiver is very similar to that employed in the receiver described by the writer in August, 1929, Radio Rroadcast. Therefore, only the portions of the r.f. amplifier that have been changed or that have to do with the addition of the automatic volume control circuit will be discussed here. Fig. 9, the complete schematic circuit of the receiver, illustrates the arrangement of the r.f. amplifier. It will be noted that a grid capacitor and resistor are included in the control grid circuit of each radio stage. This arrangement allows the bias voltages to be supplied to these control grids without disturbing the ground connections on the "low" sides of the secondaries of the r.f. transformers. The control-grid biases of the first two r.f. tubes are obtained from two sources. The minimum values are determined by the IR drop in the resistors connected in series with the cathodes of these tubes. These minimum biases are connected in series with the voltages supplied by the automatic volume control circuit (which are, of course, zero when no signal is being received). The function of the automatic volume control circuit will be described in a later portion of this discussion. The control-grid bias for the third r.f. amplifier is obtained from the voltage drop across the cathode resistor only, as the automatic control does not operate on this stage. Automatic Volume Control The automatic volume control circuit consists of a 227-type tube with the necessary resistors and capacitors as shown in the schematic circuit. The function of this arrangement is to supply bias voltages to the first two r.f. amplifier tubes, the voltages being proportional to the strength of the received signal. These biases, increasing as the signal strength is increased, keep the r.f. voltage practically constant at the detector grid. The grid of the volume-control tube is coupled to the plate of the last r.f. amplifier tube through an 0.00025-microfarad capacitor. This arrangement allows the proper radio voltage to be supplied to the control tube without getting the effect of the whole input capacity and losses of this tube in the tuned secondary of the last r.f. transformer. The turns ratio of the transformer being close to 1:2, the effect of this capacity and loss will be re Fig. 1 — The three units coinposing the receiver mounted on the box-type baffle. This assembly forms a complete operating unit which is placed in the housing cabinet. duced to approximately one quarter when reflected into the tuned secondary circuit. This amount, of course, is negligible. Rias voltage is fed to the grid of the control tube through a 2-megohm resistor. The plate circuit of the control tube consists of two 100,000-ohm resistors in series, with a 0.5-microfarad capacitor between the plate and ground to bypass the radio-frequency current and to prevent it from feeding back to the control grids, of the r.f. amplifier. The voltage drops of the plate current in the two 100,000-ohrro resistors are used to supply the "controlling" biases to the grids of the first two r.famplifiers. The bias of the first r.f. tubeis secured from across both resistors andi that to the second r.f. tube is obtained! from across one resistor only. Thus, the "controlling" bias on the first r.f. amplifier is twice that of the second. This plan is followed because the signal voltages are greater on the second r.f. amplifier than on the first, so that the second cannot have as high a control-grid bias (when these biases approach the cut-off points of the tubes) if overloading is to be avoided. These "controlling" biases are, as mentioned above, in series with the minimum biases supplied by the cathode resistors. The grid bias of the control tube is fixed so that with no signal there is practically no plate current flowing. When a signal of the proper value to operate the automatic control is received, sufficient r.f. voltage is applied to the grid to cause plate current to flow, the amount of plate current being in proportion to the strength of the received signal. In other words, the action is similar to that of a peak vacuum-tube voltmeter. The plate current, flowing through the 100,000-ohm resistors and varying proportionally with signal strength, gives varying control-grid biases on the first two r.f. amplifiers. These varying biases regulate the gain of these two r.f. stages. These actions, reaching an equilibrium for any strength of signal, maintain the strength of the signal at the detector substantially constant, after a certain value of input is reached. Fig. 5. shows the relative output of the VOLUME CONTROL AND PHONOGRAPH SWITCH SENSITIVITY ON-OFF CONTROL SWITCH Fig. 2 — Front view of chassis illustrating location of controls and the arrangement of the visual tuning meter and selector dial in one escutcheon. PICKUP SWITCHBOARD SENSITIVITY JACK TYPE CABLE WIRING CONTROL _l / POTENTIOMETER VOLTAGE DIVIDER RESISTORS SHIELOS COVERING RADIO AMPLIFIER WIRING TERMINALS OF OUTPUT TRANSFORMER TERMINALS OF AUOIO TRANSFORMERS Fig. 3 — Bottom of chassis showing the location of the shielding for the radio transformers and other apparatus in the radio amplifier. • OCTOBER 1929 • • 355