This oscillator uses a small IF transformer taken from an old transistor radio. FET admits equivalents. The circuit can be used to calibrate radios.
This SCR circuit can operate on frequencies from 1 to 20 kHz. We found it in an American...
Here is a very useful circuit that can be expanded, including the use of more integrated circuits...
The same basic audio oscillator as the CIR15855E can be modulated automatically by a relaxation...