The resulting audio signal extracted by the diode from the RF signal is coupled back into the grid circuit by audio transformer T1, T2 whose iron core serves as a choke to help prevent RF from getting back into the grid circuit and causing feedback. The capacitor C4 provides more protection against feedback, blocking the pulses of RF from the diode, but is usually not needed since the transformer's winding T1 normally has enough parasitic capacitance.
The audio signal captured by the diode from the RF signal is reintroduced into the grid circuit through audio transformers T1 and T2. These transformers act as chokes to prevent RF from re-entering and causing feedback. Capacitor C4 provides extra protection against feedback, blocking RF pulses; however, it is usually not required since the transformer's winding T1 typically has sufficient parasitic capacitance.