This 4-tube circuit was found on a Radio Craft from December 1945. The circuit has excellent sensitivity. Power comes from a power supply with a transformer.
The sensor in this circuit is a common transistor like the BC548. The circuit is from a 1979...
With this circuit it is possible to control a relay up to 100 mA with an extremely low input...
Found in an Electronic Design of 1964, this circuit is intended to the signal amplification in the 88 to 108...