This is an interesting configuration of the LM317 voltage regulator found in an EDN from the 90s. The circuit uses an optical coupler and is intended to supply constant current to a sensor resistor.
This amplifier circuit for the 7 MHz band, which corresponds to the 40-meter band of radio...
This high-power circuit from a Popular Electronics of the 80's makes use of common SCRs that admit...
This circuit is from an American documentation from 1975. The single junction transistors can be...