Tube and solid state amplifiers use the term bias in completely opposite ways. Unfortunately, this adds to the confusion in an area which is already misunderstood. The term bias refers to a negative voltage in tubes, whereas in transistors it is a positive voltage. This is the source of the confusion. Bias voltage controls current flow by applying a voltage across a grid.
Negative voltage reduces current flow.
Positive voltage increases current flow. The problem is in transistor amplifiers the term bias refers to current flow. Hence, when one uses the term bias, it is never clear whether voltage or current is being addressed. Since transistor amplifiers are more common, the term bias in general refers to current. To get high current in tubes, low negative voltage would have to be applied to the control grid.