What is biasing?
In relation to vacuum tubes, the term "biasing" refers to the act or process of setting the static or quiescient operating point of a tube by applying a predetermined voltage to one or more of its electrodes. The "quiescent operating point" of a tube refers to the amount of "idle current" in the plate circuit for a given applied plate voltage.

Why is biasing required?
Tubes require biasing because once plate voltage is applied (to a properly heated tube), current will flow in the plate circuit according to the characteristics of the tube. In some tubes, the resultant amount of plate current that will flow as the grid(s) drift towards their natural static values is greater than the design maximum value, which may cause overheating, permanent damage, &/or failure of the tube or related components.

Which tubes require biasing (& which ones don't)?
Theoretically all tubes require biasing for proper operation, however in a practical sense, rectifiers & regulator tubes never require any adjustments except when other components or circuit parameters have drifted out of design spec, in which case maintenance or repair is required, not simply adjustment. Small signal or "preamp" tubes typically don't require any adjustments to their bias point settings for a couple of reasons. First because they are ordinarily operated with high value resistors in the plate circuit, which will limit plate current to small values less than the design maximum. Second because the impedance to ground of the control grid is so high that electrons from the cathode collect on the grid & thereby develop a sufficient negative voltage to cut off the tube under static conditions. Some power tubes are also operated with cathode resistors so that a separate grid bias supply voltage isn't required. These circuits also have another current limiting effect on the plate circuit whereby increasing plate current causes larger voltage drop across the cathode resistor & hence a higher value of negative grid bias which reduces plate current.

When is biasing needed?
Ordinarily bias circuits which provide a fixed external negative voltage to the power tube grid(s) should be readjusted for optimim performance whenever the power tubes are changed. Tube characteristics including quiescent plate current vary sufficiently within a given tube type & even within a given manufacturer's lot to justify a routine calibration any time such a power tube is replaced.

Can I do my own biasing?

How is biasing done?