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As everyone knows, or soon finds out, tetrode linear amplifiers require "stiff' screen-voltage supplies for lowest distortion. Earl Weaver, W2AZW, uses a pair of 813e in his output amplifier, and devised the circuit in Fig. 1 to stabilize the screen voltage. It is a shunt-type regulator that derives a regulated voltage from the high-voltage supply. Since the high-voltage supply will usually need a bleeder resistance for regulation purposes, the shunt regulator also takes care of that requirement.
Fig 1. The regulated screen supply used with a pair of 813s at W2AZW.
| C1 | 10 nF, 2000 V. |
| C2 | 10 nF, 400 V, if needed to prevent oscillation. |
| R1 | Adjustable wire-wound, resistance and wattage as required. |
| R2 | 22 kΩ, 2 watt. |
| R3,R4,R5 | 100 kΩ, 2 watt. |
| MA | Milliammeter required for original adjustment. |
A zero-bias tube is used, and the grid is always conducting, unless the source voltage drops so low that the VR tubes extinguish. The output voltage is equal to the sum of the VR drops plus the grid-to-ground voltage of the 811-A. This grid-to-ground voltage is the regulating potential, of course, and varies from 5 to 20 volts between full load and no load.
The initial adjustment is made by placing a milliammeter in the circuit as shown and adjusting R1 for 15 to 20 mA higher than the normal peak screen current. This adjustment should be made with the amplifier connected but with no excitation, so that the idling plate current will be drawn. After the adjustment is completed, the meter can be removed from the circuit and the filament center-tap wired directly to ground. Since R1 is in a high-voltage circuit, it must be treated with full safety precautions, and all adjustments should be made only after the power is turned off and the high-voltage terminal has been grounded.
Any number of VR tubes may be used to provide a regulated voltage near the desired value. VR tubes with various operating voltages can be connected in series, if the current ratings are the same. Two 811-As can be connected in parallel if higher current capacity is required. The maximum current through the 811-A should be such that the manufacturer's plate-dissipation rating is not exceeded. It may be necessary to adjust R1 for a slightly higher current under minimum load than is first expected, to compensate for full-load voltage drops in the high-voltage supply.
At W2AZW, the 813 screen current varies from about 5 to 60 mA, and the shunt regulator holds the screen voltage constant to within 10 or 15 volt.