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Figure 32 STANDARD VR-TUBE REGULATOR CIRCUIT

The VR-tube regulator will maintain the voltage across its terminals constant within a few volts for moderate variations in RL or E8. See text for discussion of the use of VR tubes in various circuit applications.

ance is to remove the load and vary the series resistor until the VR tube draws about 30 ma. Then connect the load, and that is all there is to it. This method is particularly recommended when the load is a heater type vacuum tube, which may not draw current for several seconds after the power supply is turned on. Under these conditions, the current through the VR tube will never exceed 40 ma. even when it is running unloaded (while the heater tube is warming up and the power supply rectifier has already reached operating temperature ).

Figure 32 illustrates the standard glow discharge regulator tube circuit. The tube will maintain the voltage across Rl constant to within 1 or 2 volts for moderate variations in Rl or Es.

Voltage Regulated When it is desired to sta-Power Supplies bilize the potential across a circuit drawing more than a few milliamperes it is advisable to use a voltage-regulated power supply of the type illustrated in figure 33 rather than glow discharge tubes.

A 6AS7-G is employed as the series control element, and type 816 mercury vapor rectifiers are used in the power supply section. The 6AS7-G acts as a variable series resistance which is controlled by a separate regulator tube much in the manner of a-v-c circuits or inverse feedback as used in receivers and a-f amplifiers. A 6SH7 controls the operating bias on the 6AS7-G, and therefore controls the internal resistance of the 6AS7-G. This, in turn, controls the output voltage of the supply, which controls the plate current of the 6SH7, thus completing the cycle of regulation. It is apparent that under these conditions any change in the output voltage will tend to "resist itself," much as the a-v-c system of a receiver resists any change in signal strength delivered to the detector.

Because it is necessary that there always be a moderate voltage drop through the 6AS7-G in order for it to have proper control, the rest of the power supply is designed to deliver as much output voltage as possible. This calls for a low resistance full-wave rectifier, a high capacitance output capacitor in the filter system and a low resistance choke.

Reference voltage in the power supply is obtained from a VR-150 gaseous regulator. Note that the 6.3-volt heater winding for the 6SH7 and the 6AS7-G tubes is operated at a potential of plus 150 volts by connecting the winding to the plate of the VR-150. This procedure causes the heater-cathode voltage of the 6SH7 to be zero, and permits an output voltage of up to 450 since the 300-volt heater-to-cathode rating of the 6AS7-G is not exceeded with an output voltage of 450 from the power supply.

The 6SH7 tube was used in place of the more standard 6SJ7 after it was found that the regulation of the power supply could be improved by a factor of two with the 6SH7 in place of the 6SJ7. The original version of the power supply used a 5R4-GY rectifier tube in place of the 816's which now are used. The excessive drop of the 5R4-GY resulted in loss of control by the regulator por-

Figure 33 SCHEMATIC OF VOLTAGE REGULATED POWER SUPPLY Ti—615 or 520 volts each side of c.t., 300

ma. Stancor P-8041. T-—5 volts at 3 amp., 6.3 volts at 6 amp.

Stancor P-5009. CH—t henry at 250 ma. Stancor C-1412.

Figure 33 SCHEMATIC OF VOLTAGE REGULATED POWER SUPPLY Ti—615 or 520 volts each side of c.t., 300

ma. Stancor P-8041. T-—5 volts at 3 amp., 6.3 volts at 6 amp.

Stancor P-5009. CH—t henry at 250 ma. Stancor C-1412.

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