Explanation: The primary purpose of the suppressor grid in a pentode is to mitigate the effects of secondary emission from the plate. When electrons from the cathode strike the plate with high energy, they can dislodge other electrons from the plate material (secondary emission). If these secondary electrons are allowed to travel to other grids, it reduces the effective plate current and overall tube efficiency. The suppressor grid (G3) is typically held at or near cathode potential, making it significantly negative relative to the highly positive plate. This negative potential creates an electrostatic field that repels any secondary electrons emitted from the plate, forcing them back towards the plate. By returning these secondary electrons to the plate, the suppressor grid ensures that the plate current remains stable and high, increasing the tube's permissible gain and overall efficiency, especially at higher plate voltages.