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2,665 Posts
It's just like this only different!

If you're not meaning to become a techie James then maybe this analogy will help:

When we talk about biasing an amp we're talking only about the output tubes.These tubes need to have a minimum idling current which is calculated from the tube specs and the voltages applied to the tube within the specific amp.

It's like adjusting the idle speed in an old car with a carburetor. Too fast and you waste fuel and prematurely wear out the engine. Too slow and performance really suffers.

Too little idle current with most tubes means less power and a thinner sound. Too much warms up the tone but hurts tube life, sometimes down to minutes if something is REALLY screwed up!

Unless as Elcabong pointed out the amp is cathode, or self-biased (different biasing method that means less power but more harmonics. Think Neil Young's "Rockin' in the Free World" guitar tone) there normally is an adjustment control inside for a tech to set the proper idling current.

Some mfgrs hard wire the bias point, telling you they've chosen the best "sweet spot" and since different tubes can vary in bias sensitivity don't worry about the lack of adjustment. Just buy your tubes only from them! And of course, they've handpicked only the finest tubes!

To most techies this is just crap. These amp makers just set the bias on the low side so that they're not likely to have any failures during warranty that will cost them replacements. The tubes they supply are just whatever they got a deal on that they check for a particular bias sensitivity. They're not likely to be any better sounding. Besides, better sound is personal taste. Maybe a low bias in a Booger sounds good to someone who only wants to play "Enter Sandman" for the rest of his life but a rockin' bluesman nearly always wants to pick his own tubes and have his tech bias them up to the sweet spot in HIS amp!

The help and references suggested in the other replies are excellent but I thought I'd try to give you a "not too techie" reply.
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