We discussed that a grid voltage, negative with respect to the cathode voltage, controls the current that traverses a vacuum tube and that the grid bias voltage determines the quiescent vacuum tube operating point. The input voltage signal is added to the grid bias voltage and is amplified.
There are two largely used techniques to provide a vacuum tube with a grid bias voltage, negative with respect to the cathode. The fixed bias technique requires a separate power supply that provides the wanted negative voltage. The cathode bias or self-bias technique connects the grid to ground and elevate the cathode voltage above ground. In this way the grid voltage is negative with respect to the cathode voltage.
These are discussed in the following: