Grid bias

The (control) grid in an electron tube is used to control the electron flow from the heated cathode to the positive charged anode. A small change in grid voltage will result in dramatic change in anode current, thus creating an amplifying effect. When the grid votlage reached a certain level and even becomes positive, all the electrons from the cathode will flow to the grid and burn it out. To prevent this happening, a correct negative grid bias must be added to the grid in order to keep the grid voltage negative.