This circuit is from an old Japanese publication of 1962. The circuit uses two batteries, one for the filaments of the tubes and the other one of high voltage for the plate circuits.
This circuit was found in a 90's espionage publication and serves for analog lines. The circuit is...
This Audio System circuit was found in 1965 documentation. It is a 50 ohm line circuit with 22 dB...
This Wien bridge oscillator circuit uses a Texas Instruments 709 amplifier. It was found in an...