Tube Biasing? Explain Please

I'm new to tubes and would like to know what the terms "Biasing and rolling means in regards to tubes? thanks Darren
Rolling means swapping or experimenting with different brands of tubes. Often times people take out the stock tubes and try different NOS tubes. Biasing allows you to adjust all the tubes so they all perform egually. Tubes may not age the same way. Biasing helps to fix that.
How to do biasing?
It depends on your amp. The manual should have instructions on how to do it. My amp you have to take a reading on each individual tube with a multimeter then adjust the pots with a plastic screwdriver until they all read the same.
What if your're using just a tubed pre amp? Is biasing necessary?
I don't think so. You might have to buy matched pairs of tubes though.
Tube preamps don't need biasing but power amps definitly do.If you have a Conrad Johnson they have red LEDs by each tube,you adjust accordinly.Some power amps are "Auto biasing" others you have to use a meter to bias.So have fun searching for that tube amp.Steve
Biasing is a DC voltage that is applied to the control element of a tube, ie the grid that sets up the nominal starting point on its characteristic curve when there is no signal imput. The incomming signal overrides the bias voltage increasing it on it's positive swing and decreasing it on it's negitive swing. The bias controls both the amplification and linearity of the tube stage. When you have an amplifier using two output tubes in a push pull configuration it is important that the bias on both stages is equal so they have the same amount of amplification to the incomming signal. This is what is meant by a balanced output and it reduces distortion caused by an unbalanced output. An explaination of a pushpull output stage is as follows. You have two tubes feeding the oposite ends of the primary of the center tapped output transformer. The secondary winding feeds the speakers. With no signal input to the output stage both tubes should be either drawing no current or equal current if the are properly balanced (biased). As the incomming signal goes positive one tube conducts more and the other tube conducts less. As the signal goes in a negitive direction the first tube now starts conducting less and the second tube more. As you can see the tube conduct inversely from each other. When one tube is increasing conduction the other is decreasing in conduction, and that is what is meant by a pushpull output stage. Even though the tubes seem to be subtractive, through the output transformer they are additive and greatly amplify the incomming signal.
In linear classes of operation like in hifi amps, the tubes will be drawing idle (no signal) current. If the tubes are matched they will draw the same current with the same bias.