How Does a Mixer Vacuum Tube Work?

Vacuum tube mixers have been used to alter electrical signals since the invention of the vacuum tube. A mixer circuit can change the frequency of an electrical signal, or it can delay the signal to change the waveform (also known as changing phase).
How a Vacuum Tube Works
A vacuum tube is an active electronic device that uses electromagnetic energy to alter an AC input signal. Before 1947, vacuum tubes were commonly used in amplifiers and rectifier circuits.
When voltage is applied to a vacuum tube's terminals, electrons begin to flow from the vacuum tube's cathode to the plate terminal. This allows electrical current to pass through the vacuum tube. When a vacuum tube is used in conjunction with capacitors and resistors, the vacuum tube circuit can be used to modify the output signal in relation to the input signal.
Electronic Mixer Circuits
Audio mixers often employ hundreds of variable and fixed-value capacitors, inductors, and resistors. Audio mixers often employ hundreds of variable and fixed-value capacitors, inductors, and resistors.
A mixer circuit takes an AC input signal and changes the signal's frequency, phase, or both. A mixer often contains capacitors, resistors, and/or inductors to modify the output AC signal with respect to the input signal, to suit the user's needs. In the case of an audio mixer, for example, the AC signal can be modified to boost the bass (low-frequency) or treble (high-frequency) parts of the signal. The mixer can also be used to delay or reverberate (phase shift) the output signal.
Considerations
Two identical AC signals inverted with respect to each other can cancel each other out if the signals are combined.