A negative bias voltage is needed to prevent runaway current in a vacuum tube. The negative voltage can be applied to the cathode or to the grid.
1. Cathode bias: When applied to the cathode, it becomes self biasing, and the Anode voltage needs to be increased by the amount of voltage drop across the cathode resistor. So cathode bias circuits generally require a higher anode voltage to achieve the same power output.
2. Grid bias: when applied to the grid, a lower anode voltage can be used to achieve the same power. However Grid bias is not self adjusting and is referred to as “fixed bias”. In the case of grid bias an adjustment pot or some other means of controlling the grid voltage is needed because if the negative voltage on the grid is lost there will be runaway current through the tube. In the case of push pull amplifiers, we want the same anode current flowing through each tube on the “push” and the “pull” sides of the output transformer. As tubes warm up and age they tend to conduct at a different rate. This causes the DC current to flow through the output transformer which in turn, produces a greater or lesser amount of saturation. Bass frequencies are the first to go. So while we may manually adjust the Anode current to be In perfect balance, a short time later there may be some imbalance and the amplifier is then not performing at itsoptimum level and sonics are degraded.