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.
This circuit is from an Electronics World of 1964. The circuit can be made with modern...
This circuit is from an old American documentation from the 70s. Transistor 2N3904 can be replaced by...
We found this circuit in the March 1980 Radio Electronics magazine. The circuit used is the HA2400...