This circuit was found in a 1968 US publication. Transistors used are uncommon today. The circuit can be adapted to operate with modern equivalent components.
This circuit is from a 90s American documentation. The circuit can use complementary pairs...
Found in a magazine 73 for radio amateurs, in a 90's edition, this circuit uses a general-purpose FET...
Found in a 1960 Sylvania transistor manual, this circuit makes use of transistors from that era....