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(Should the output voltage drop instead, the TL783's conduction eases up and the regulator's output voltage climbs with the decreased voltage drop across resistor Rs.)
(The output of the TL783 cannot be sent directly ground, as the adjustment pin must see a voltage 1.27 volts lower than its output. Thus the role of the 47 volt zener. It brings the TL783's output up to 47 volts above ground. A further advantage to using the zener is that it allows for higher voltage dropping resistors that relay the regulator's output voltage to the adjustment pin. The two 1N4370 zeners are there to protect the TL783's adjustment pin from high-voltage spikes.
The 1µF capacitor might not be a good idea, as the zeners might prove too slow to save the TL783. Actually, in this circuit and all the other hybrid regulators, avoid over designing. For example, do not eliminate the 2 ohm resistor at the output or decide to shunt the adjustment pin to ground in the series regulators by adding a capacitor. High-voltages kill solid-state devices easily.
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Conclusion
The 300B has a potentially wider use than just a single-ended output tube. High-voltage regulator pass device is just one possibility.
Of course other tubes could be used, such as a triode connected 6550. But having a 300B based regulator would be just too delicious to pass up.
If anyone tries any of these circuits, please send the results in to share with all of us.
//JRB
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