We found this circuit in a 1960 Radioarama magazine. The instrument tests common general-purpose transistors indicating their gain on a moving coil instrument. The loop serves to adjust the circuit.
The circuit is simple, as it uses only one transistor and a relay that can be MC2RC1 or MC2RC2,...
We found this circuit in a 1990 documentation. It works with the power of the digital circuit...
The filter shown in the figure can operate with frequencies up to several tens of megahertz thanks...