The variation of the voltage, applied to the grid of an electronic vacuum tube, produces a variation of the current from the anode to the cathode. In vacuum tubes used in input stages (for instance 12AX7 vacuum tubes), current is generally limited in a range from 0 up to a few milliamperes. In vacuum tubes used in power stages (for instance EL34 vacuum tubes), current ranges from 0 up to a few hundreds of milliamperes.
With an appropriate configuration, this current variation can be exploited to obtain voltage and/or power amplifiers, where the voltage signal applied to the grid is conveniently amplified.
