than its source to conduct, thus the use of a depletion mode MOSFET to shift the output voltage of the Op-Amp to level greater than its own B+ voltage. All in all, this is not a bad high voltage regulator. (In actual use, it will require more protection diodes as the Op-Amp can only survive while it is safely nested in the limited voltage span provided by the zener diode.) Replacing the MOSFET with a triode actually simplifies the circuit, as the tube is a depletion mode device that can conduct current with a negative grid voltage.
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Hybrid high voltage regulator
Furthermore, this hybrid voltage regulator does not require the second zener that the solid-state version needed to limit the maximum current draw, as this version is intrinsically current limited by the limited voltage swing of the Op-Amp (no greater than its positive power supply rail). This hybrid version also enjoys a slower turn-on ramping up to output voltage, as the triode's cathode must slowly come up to heat to conduct.