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 is from the 1994 Linear Applications Handbook. The operational circuit can be...
This circuit is suggested by National Semiconductor (now Texas) in an old manual. The circuit...
With this circuit, found in a 80s Popular Electronics, it is possible to use two ordinary phones in a...