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Automatic Volume Control Page 2

THE DISTRIBUTION SYSTEM
The distribution system consists of the resistor capacity network necessary to apply the d-c voltage developed across resistor R1 to the grids of the various tubes being controlled. There are four points to consider in checking the a-v-c distribution system of a receiver. These are: (1) Voltage distribution, (2) Filtering, (3) Time factor and (4) Cost. We will consider these in the order named.

1- Voltage Distribution. Referring to Figure 3, we will assume that the second detector is a Type 75 tube in which the diode plates are paralleled in the conventional half-wave circuit and used for developing both a-f and a-v-c voltages. The triode section of the Type 75 is used as a first audio stage, and to provide grid bias the cathode is raised above ground potential by 1.5 volts with resistor R8. This makes the cathode 1.5 volts positive with respect to ground and, when no signal is being received, the control grids of the first three tubes are biased 1.5 volts positive also. To give the first three tubes a negative grid bias, a cathode resistor is placed in each cathode circuit - resistor R5, R6 and R7. The value of this resistor will, of course depend upon the plate and screen grid currents of these tubes. We can figure the voltage required, however, by allowing 1.5 volts to make up for the 1.5 volts positive potential applied initially, then 1 volt for minimum grid bias and 2.0 volts to take care of any gas and/or emission present which would buck out an equal amount of grid bias. This gives us a required 4.5 volts that each that each cathode should be raised above ground potential. For maximum sensitivity, a lower bias is sometimes used and in some cases to eliminate excess gain, a higher bias is applied.

2 - Filtering. In addition to the d-c a-v-c voltage appearing across R1 (which is ordinarily about 1/2 megohm), an a-f voltage is also present. This a-f voltage is coupled to the grid of the Type 75 tube through condenser C9 and is applied across resistor R9, (which is also ordinarily about 1/2 megohm). To prevent this a-f voltage from feeding back to the preceding grids and causing distortion, particularly at low levels, resistor R2 is made large with respect to R.9. If Resistor R2 is made at least 1 megohm, little or no trouble will result from a-f or i-f coupling between the second detector and the preceding tubes.

Resistors R3 and R4 may be on the order of 100,000 ohms and in conjunction with condensers C2, C3 and C4 will prevent r-f and i-f voltages from feeding back to the preceding stage. Since condensers C2, C3 and C4 complete the tuned circuits to ground, they must not too small or some of the tuning range will be sacrificed. These condensers may be from .01 to .05 mfd. Condensers C5, C6 and C7 bypass r-f and i-f voltages across the cathode resistors and are ordinarily about .1 mfd. Condenser C8 is by-passing a-f voltages and so must be about 2 to 5 mfd.