The tetrode
A second grid is added between control grid and plate. This is a spiral of wire and is called the screen grid or screen. This grid normally carries positive direct current voltage, around 1/4 to 1/3 that of the plate voltage.The screen grid reduces capacitance between control grid and plate, minimizing the tendency of tube amplifier to oscillate. The screen grid can serve as a 2nd control grid, allowing 2 signals to be injected into a tube. This tube has 4 elements and is called as a tetrode.