This circuit is from a 1975 RCA tube manual. It appears elsewhere on the site. R5 depends on the voltage of the amplifier with which it is used and can be changed.
The circuit shown is from an old European publication. In it we have the way to activate a relay...
We found this circuit in a European publication from the 1990s. Transistors are still common types...
This circuit was found in an American documentation and can be added to common amplifiers of any...