Intercom (CB21660E)

This good gain circuit was found in a Popular Electronics from April 1967. The transistors are germanium of the time.

Blocking oscillator (CB6921E)
This blocking oscillator configuration was found in a 1964 American book. The circuit can be...
Tunnel Diode Listening Transmitter II (CB19426E)
This low power circuit for the FM band was found in a spy book from the 90s. The tunnel diode is...
FM AC Transmitter (CB20347E)
This transmitter, indicated for espionage applications, is permanently powered by the power grid....