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.
This circuit was found in a 1999 English magazine, but can be easily assembled with BC548...
This sound relay or Vox was found in 70s RCA documentation. SCRs can be common types today and...
This telegraph microtransmitter for a frequency determined by the coil and the crystal was found...