There's probably more misunderstanding and opinions about how to properly
bias a tube amp than any other area of tube lore. Everybody seems to find a
"secret" shortcut that magically turns a "dead" sounding
amp into the sweetest sounding amp they're ever heard.
In fact, a reverse analog is more true. Bias is a little like setting the
idle on a car. You can't make a Ford into a Ferrari, but you sure can make a
Ferrari run like a Ford.
Remember that the Brits call these firebottles "Valves" with good
reason. The tube's control grid is like a faucet, with the bias voltage like
the handle. As you open the faucet by applying more and more positive (Bias)
voltage, more electrons flow, and ultimately you hear music. It's simply about
control, that's all.
A lot of other problem areas can exist in an amplifier that no amount of
bias adjustment can fix. But we still tend to focus on the biasing because
it's where the "rubber meets the road."
Biasing may result in the following: