Why is the control grid of a triode maintained at a negative potential?

The voltage on the control grid in a triode controls the current flowing from the cathode to the anode. When the grid goes negative, the tube is said to be cut off and no current will flow. When it goes positive, current starts to flow. As it goes more positive, more current flows. At some point, increasing the grid voltage won't cause any more cathode-to-anode current (plate current), and we call that saturation. We have to decide what to set the grid voltage at, and that is what we call biasing the tube. (We need to bias it.) Where do we want the tube operating with no signal applied? What kind of circuit is it in and how are we going to operate it? Class A, class B, class C or class AB? If a triode is set up with the grid negative all the time, no plate current will flow. If we set it up so that the grid is negative a little bit, then the input signal will have to go positive enough to overcome this voltage before plate current will flow. During the negative portion of the input signal, the tube will be cut off. Remember that it will only have plate current for a portion of the positive half of the input signal when the input voltage exceeds the "set" voltage on the grid (the bias voltage). This means that the amplifier will be operating class C. The class C amplifier operates for less than 180 degrees of the input signal.