as Current Sources
Cheap and readily available, the three-pin regulator finds a home in almost all solid-state audio circuit boards, but is relatively rare in tube gear other than as a heater regulator. The problem, once again, is that solid-state and high voltage seldom go together for long. Most three-pin regulators are fixed at low voltage and seldom more than 20 volts. High voltage versions of adjustable three-pin regulators are usually only safe up to 60 volts, which is less than the bias voltage used for most 300B based amplifiers. Three-pin regulators, however, are capable of more than just voltage regulation.
All three-pin regulators contain a voltage reference, a pass device (losser element), an Op-Amp, and protection circuitry. These are all the requirements for a good regulator, but not just regulators, as they are useful for other circuit types as well. For example, the three-pin regulator can be even used (ala Cathode Follower like role) as the output pass device in a single-ended solid-state buffer amplifier, as its intrinsic current limiting and output shorting protection come in very handy.
In addition, a three-pin regulator, both the fixed and the adjustable, can be configured as a current source. An active current source also requires a voltage reference, a pass device (losser element), and a control circuit. One extra resistor is all that is needed to convert a three-pin regulator into a current source.