While the three-pin regulator proved useful for providing the correct cathode bias voltage, it was bypassed by a large valued capacitor. Thus it really did not see the AC aspect of the amplifier's performance. In the following circuits, the solid-state devices are very much in the signal path.

High Voltage Current Sources
   Using a current source to load a triode works very well, as it buffers the tube from the power supply noise while achieving high gain and low distortion. In many ways a current source is the opposite of a voltage regulator. Where the current source offers a nearly infinite output impedance, the voltage regulator offers an ultra-low output impedance. Where the regulator maintains a fixed output voltage independent of current draw; the current source, a fixed current independent of voltage. Although wildly different in function, both can be used to achieve the same result: lower noise at the output of a tube circuit. The voltage regulator irons out the ripple and the current source ignores it. 
   The analogy for the voltage regulator might be a hugely heavy car that uses its mass's inertia to smooth the road's surface, while the analogy for the current source might be supremely compliant coils and soft tires that contract and expand to the road's irregularities.

   Making a solid-state current source requires as little as one FET or one depletion mode MOSFET. With a few more parts, a current source can be made from one transistor and three resistors. At the other extreme, a current source can comprise a floating power supply, a precision voltage reference, a discrete Op-Amp, and a high voltage pass device (usually, a MOSFET, as the MOSFET is a voltage driven device. Making a MOSFET conduct does not require a gate current, unlike the transistor, which does require a base current. This base current flow adds to the current through the collector, making the precise regulation of the current out of the current source difficult.)

pg. 7

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