This circuit was found in a 80s American documentation. The zener diodes determine the points of action of each stage and the transistors can be general purpose types.
We found this circuit in the October 1980 issue of Radio Electronics magazine. The circuit uses...
Found in an English publication from the 1980s, this circuit uses components that are still common...
With two cascading tunnel diodes, this circuit found in a 196 General Electric documentation...