With reference to this circuit, the passing radio waves induce a voltage on the antenna.
Induced currents flow through L2, which magnetically couples the RF energy to the L1-
C1 tuned circuit. The grid circuit elements, Cg =100pF and Rg = 1M couple the tuned
radio signal to the triode’s input. Amplified RF currents flow in the plate circuit, setting
up a magnetic field around L3 which couples energy back into the tank circuit in phase
with that imposed by the radio wave, reinforcing it. Now we get stronger RF currents
across L3, more energy fed back in phase, again an amplification……

If enough energy is fed back and enough amplification is obtained from the triode, the
circuit will break into oscillation. For DSBC (double side band with carrier) AM
demodulation, we don´t want the receiver oscillating. The mission of the throttle
capacitor C2 is to limit the amount of current flowing through L3 so that the circuit
won´t oscillate. C2 is adjusted so we get maximum amplification of the incoming radio
signal. Usually, this occurs when the circuit is in the threshold of oscillation.
Regenerative receivers need rather small operating-point currents, and it is not unusual
for them to operate satisfactorily with low plate voltages. Low currents also make
regeneration control smoother.

AM demodulation is accomplished through grid-leak detection. Fig shows the key to
understanding this type of detection. First, the triode’s input is modelled as a diode
pointing downwards. When the grid turns positive with respect to the tube’s cathode,
due to the presence of the positive half cycle of the carrier, some electrons emitted by
the cathode are attracted by the grid, flowing in the external circuit to the grid capacitor.
As a result, the grid capacitor replenishes its charge. During the carrier’s negative half
cycle, conduction between cathode and grid stops and charge leaks from the capacitor
through the external circuit (L1 and the grid resistor). The next incoming carrier cycle
the phenomenon repeats itself. The combination of Cg = 100pF, Rg = 1M and the
equivalent diode act as a voltage-clamp circuit.