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.
This conventional version with a transistor, already explored many times on our website, was found...
This circuit came out in an English magazine from 1977, but it can be easily assembled today, as...
This simple radio for listening to a headphone with two CK722 transistors was found in Radio...