Luna, the bias is supposed to be set so that the tube is being used in the middle of its operating range, in a region that is the most nearly linear for input-output tracking. Bias it too low or too high and it becomes increasing non-linear at either end of the operating range, respectively.
The actual value of bias depends on the type of tube. If you can refer to a tube manual, that may tell you the recommended bias. Or you can ask someone here who has a tube manual, listing the tubes you wish to have looked-up.
The actual value of bias depends on the type of tube. If you can refer to a tube manual, that may tell you the recommended bias. Or you can ask someone here who has a tube manual, listing the tubes you wish to have looked-up.