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What is tube biasing and why do you have to do it to tube amps?

I'm just a simple bass player and have not used tubes other than for a pre-amp and see lots of talk here about biasing tubes when you change them out.

Just curious.
 

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Roughly: tube biasing is setting the tube range of operation. There are two ways to do it, cathode bias and fixed bias.

With cathode bias, the cathode resistor determines the point of operation; you do not need to "rebias" when changing tubes.

In fixed bias, a negative voltage is applied to the grid. There is usually a pot to adjust this voltage. Some amps, like Mesa DR for example, set it to a given value and you do not have to rebias. But they also tell you to buy their tubes:confused-smiley-010
 

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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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Discussion Starter #5
That helps a bit. At least now I know what people mean when this comes up.

thanks fellas. For a bunch of thin-stringers, you're okay!:tongue:
 

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Peavey Wolfgang EVH Wolfgang Charvel Style 2
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To add... biasing also applies to preamp tubes but since they are self biasing there is no cause for manual adjustment.
If there is a desire to learn more about biasing and/or a desire to do it yourself I recommend Aspen Pitmans Tube Amp Bias kit.
I do all my own biasing and for this I use an oscilloscope and a frequency generator... truly this is the way biasing should be done... but with a little math and a multimeter you can get in pretty close... some guys also do it by ear which I recommend you use a multimeter with to get you in the ballpark and within safe operating levels and only do it this way if you "know" what the amp should sound like in the sweet spot.

KHINGPYNN
 
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