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 circuit was found in English documentation from the 90's. Transistors are general purpose types....
This instrumentation circuit is from the June 1986 issue of Radio Electronics magazine. Equivalent...
This is an interesting idea that we found in the May 1971 issue of Radio Electronics magazine. It can...