The regen is basically an oscillator circuit with a gain control that allows the user to adjust the feedback to a point just below oscillation or, quite often, just above the critical level such that a small oscillation is present. The typical regen uses a tapped coil or additional windings to connect into the tuning tank and the tuning capacitor provides the total tank capacitance. The components are not critical and the values were pretty much the first ones found on the bench that were near the `right` value so don't hesitate to experiment. The transistor could be just about any small-signal NPN including the 2N4401, 2N3904, 2N2222, or others. The audio output is fairly weak and will need an amplifier to drive headphones or a speaker. See the audio amplifier page for suitable amps.
The advantage to this approach is that the tuning range is maximized since there are no fixed capacitors contributing to the tank. The disadvantage is that special, hand-made coils are required. The regens shown below use capacitive taps to achieve the re