In days of old, when tubes glowed bold
(Transistors weren't invented),
A 6B4 and not much more
Would leave your ears contented.

   The simplest use of solid-state and tube technologies might be the circuit shown above. Here a silicon diode is used to bias a 12AX7. The diode's cathode connects to ground and its anode attaches to the tube's cathode, which allows it to conduct current. In this regard it functions much like a cathode resistor, but it differs in that the voltage across the diode is fixed (about 0.7 volts). Multiple diodes yield multiples of  0.7 volts, such as 1.4, 2.1, 2.8, 3.5, and 4.2 volts.
    (Although maybe not strictly necessary, the addition of a small valued bypass capacitor lessens our worries that the diodes will harm the sound by providing a path for the higher frequencies to take from ground to the triode's cathode. The sonic test would be to build and compare two simple line stages: one with a cathode resistor bypassed by a large electrolytic capacitor and one with a diode string bypassed by a small film capacitor.)
   The next move up in complexity is to replace the diode string with a two-lead current source. These devices look like an ordinary solid-state diode, but actually comprise a FET, which has its gate tied to its source. The FET's IDSS sets the current flow. Replacing the cathode resistor or diode string results in a Grounded Cathode amplifier stage that always idles at the same current draw, in spite of tube aging and B+ voltage drifts. Unlike the diode string, these devices must be heavily bypassed in this application, as otherwise no gain will result. A constant current flow means a constant voltage developed across a resistor; only the idle current should be constant.

   The next rung up the complexity ladder is the use of a transistor to regulate the idle current. The transistor's emitter goes to ground and its collector goes to the tube's cathode, which is shunted by a large electrolytic capacitor. The transistor's base conduction sets the amount of current flow through the device, which in turn sets the current flow through the tube. This arrangement can make DC coupling between tubes much more stable. Below is an example of a Lofton-White amplifier with a twist.

Lofton White output stage with DC feedback

    The twist is the use of an auto-bias circuit made up of one transistor and an extra resistor. A DC feedback loop keeps the driver tube's plate at the correct DC value to maintain a preset idle current flowing through the output tube. In this example, the defined idle current is 70 mA (about 17 watts of dissipation).

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