Conversely, we can take an all-tube high voltage regulator and convert it into a hybrid design. The regulator shown below uses the 12B4 as the pass device, the 0A3 as the voltage reference, and the twin triode (12AX7 or 12AT7) as the feedback mechanism. Gain is scarified by the pre-filtering of the B+ voltage of the second triode to lower the output noise at the expense of a slightly higher output impedance. Like the previous all solid-state version, this is not a bad regulator, but it can be improved. Before we grow too nostalgic over the 0A3, let us remember that it is expensive, large, and relatively current hungry. Precise IC voltage references can be strung in series or even a zener diode can provide a more stable reference voltage. And the first triode is really outside of the feedback loop and is used only to buffer the reference voltage. In other words, replace the triode with a MOSFET.

  The hybrid high voltage regulator shown above also replaces the plate resistor with a depletion mode MOSFET that functions as a current source in this circuit. Thus we are able to realize all the gain possible from the 12AX7 (and thus lower the output impedance) and dramatically lower the noise level all at once. If current limiting is needed, it can be easily added to this circuit by spanning the 12B4's cathode and the plate with a 1N4007 reverse biased-diode; as the diode will only conduct when the 12B4's grid moves more than .7 volts more positive than the 12B4's cathode, the output tube cannot be driven too positively.
   We see that the triodes still do most the work, but much of the circuit housekeeping is preformed by the solid-state devices.

pg. 3

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