Biasing an amp refers to the POWER tubes only,. and has nothing whatever to do with the smaller Preamp tubes,....
It is basically an adjustment for the voltage in the power tubes,and is VERY important for several reasons, one is for the life of the tubes,... too hot, and they burn right up,.... too cold, and the amp sounds like shit,....too far out of sync,...(unbalanced tubes), and there is all kinds of noise issues,... hums, crackles, static, etc,... and is why it is a neccessity to have fairly balanced power tubes,..... (usually in sets of two at a time, altho there are some matched quads if you need them,....)they work as a pair in amps that use two or more power tubes,...
It is usually not needed when using Fender amps, as they are built for all levels of skill, aand as such are set up so anyone can trade power tubes with no ill effects to the amp.....
This is not true of all amps,... some of the 'botique' types are EXTREMELY sensetive to changing power tubes, and will not react the way you want,.. and can even be damaged by tubes that are too different from the old ones you are replacing.
If you are sort of confused by it all, take your amp to a skilled person to do this for you,....
better safe than sorry would be a key phrase to remember ....
By the way,... I do have some used bottle shaped RCA 6L6G tubes if you need them,.... much cheaper than new ones,... and they sound extremely nice compared to the cheap stuff they build now....
Email me for details....
Steve