MOSFETs can also be used, but not as the input to a hybrid circuit. A better place for the MOSFET might be as a grounded-source amplifier that an input triode could cascade into. If a P-channel MOSFET is used, the MOSFET can be DC coupled to the triode's plate resistor. The complete circuit will not invert the phase at the output. Thus, a feedback loop could be returned to the triode's cathode, which would give the triode control over the MOSFET's output.   

Hybrid Phono Circuits?
     Hybrid circuits were mentioned in that distant first half of this article, but giving this topic a fair hearing would require more pages than I can write before we enter a new year. So what follows are a few schematics that offered as starting points for some new projects and articles.

              Hybrid Loftin-White first stage

          Hybrid triode-MOSFET first stage

       Even the lowly IC Op-Amp could be used as the first stage of two-gain stage passive equalization preamp. The IC can be quiet and it allows DC coupling with the tube, if a small DC offset is put in place by using an additional resistor in the IC's feedback loop. This resistor applies a small amount of current from the IC's positive power supply rail, which will cause the Op-Amp's output to establish a negative DC offset at the output. Unfortunately, we cannot connect the resistor to the triode's plate, as this would create a positive feedback loop, not a negative feedback loop. (A second IC could be used to invert the DC signal from the plate, but this would complicate the circuit.) An AC feedback loop could wrap around the triode, but I would not want the Op-Amp's near infinite gain, and thus its near infinite potential

    FETs are marvelously quiet. The low noise from the best FETs make the quietest tube sound thunderous in comparison. I have always thought that the FET should first handle the delicate input signal from a phono cartridge and then give its amplified output to a tube, thus utilizing the best features of both technologies. But how to proceed? We can make a cascode with the FET receiving the input signal or we can cascade the FET into the tube in a Loftin-White like configuration, which may result in lower noise and a more tube-like sound.
    Note the DC coupling between FET and triode  and how the triode and bypass capacitor shield the FET from the power supply noise.
   Another avenue I haven't had time to investigate is using a FET only in its triode region of operation, wherein it exhibits rp!

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