This crystal or galena radio is one of the many versions available on this website. This is from a documentation from the 70s. The coil is formed by 50 + 50 turns of wire 28 in a ferrite rod.
In the figure we have a simple amplifier stage for BC547 series transistors. The gain is...
Found in a 1980s American documentation, this circuit can use a SCR and a general-purpose transistor....
This circuit was found in a 1973 publication. The operational ones can be exchanged for...