Explanation

(c) For any fixed value of the grid bias, the plate current increases as the plate voltage is increased, because more of electrons are drawn towards the anode. Also, for any fixed value of the plate voltage, more plate current flows when the grid is positive. As grid is made more and more negative, electrons are repelled back and very few reach the anode, when the grid becomes highly negative no electrons reach the plate. Thus, for a fixed plate voltage, it is possible to cut out anode current completely by making the grid suitably negative. This is called the cut-off voltage, hence, plate current is reduced.