#2
Amps get biased, not tubes.

It is not needed on all amps, mainly just Fixed bias amps. What amp do you have?

I'm not going to get into all the technical jargon but it has to do with the amount of electrical current that is allowed across the tube plate (plate dissipation).

I was going to be a prick and tell you to Google it.

but, this is a good start:

http://www.google.com/#hl=en&source=hp&q=what+is+amp+biasing&aq=f&aqi=g1&aql=&oq=&gs_rfai=CKCLu8sSBTMelPI-GNKSh2bAGAAAAqgQFT9AqHEw&fp=af44e22a6bb12c87

Also, I have a Blog that walks through how I biased my amp.