This is a simple tubed mixer from the 50's. The circuit does not include the power supply and has 3 inputs with independent controls. The tubes are double and the inputs are high impedance.
This circuit provides a reference voltage for calibration. He uses a diode in a small thermal...
This circuit for model making was found in an English documentation. The SCR can be the TIC106, and...
The operational amplifier used in this circuit is no longer very common, but the configuration can be adapted...