First – biasing is for the output tubes only on your amp – that’s basic but some owners have asked about how to bias the pre-amp tubes. The intent is to set them at a proper idle current so that they swing signal positive and negative above and below their operating range, clipping symetrically (or nearly) when you reach the extreme limits of the output stage.
Anytime you put new output tubes in and amp. the bias MUST BE checked. This is especially important when installing current production tubes in a vintage amp. I have found gross differences in bias between vintage tube and new tubes. Most likely this is due to differences in materials and construction but whatever it is, YOU MUST ALWAYS CHECK THE BIAS WHEN INSTALLING NEW OUTPUT TUBES.
