This battery-powered 3-valve circuit was found in American documentation from 1947; Two batteries are used, one high voltage and one low voltage for the filaments.
This audio circuit was found in the February 1983 issue of Radio Electronics magazine. The...
This circuit was obtained from an English magazine from 1981. The magazine no longer exists, but...
This circuit was found in a 1979 documentation. The transistors can be the BD136 and BD135,...