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Post subject: Fixed Bias ??
Posted: Fri Nov 21, 2008 2:13 pm
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what does bias AND fixed bias mean??? :)


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Posted: Fri Nov 21, 2008 3:16 pm
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biasing is the means of balancing the power of 2, 4 or sometimes more powervalved amps (class a/b). in the case of a 2 powervalved amp, one valve will push the power the other will pull. You have to balance them to get the amp to sound its best. Normaly on modern amps biasing is done via a trim pot on the circuitboard. Some older amps however have fixed bias which is set by a series of resistors wired onto the valve sockets so their bias is permanent.

This is very much a basic laymans explanation, others can expound on this much more for you.


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Posted: Sat Nov 22, 2008 9:43 am
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Actually, biasing is the setting of the idle current thru a tube or valve to get the desired operation. You can balance the bias of an amp if you have individual bias pots per side of the push-pull. Now we rely on balanced tubes. Fender used to put a bias balancing pot in their amps. It allowed you to balance the bias but not adjust. Fixed bias is a fixed value resistor in line with the negative bias supply voltage to give you a certain bias voltage. If you change brands of tubes and you have glowing plates on those tubes after you install them, a tech can change the value of that resistor to lower the current flowing to get the tubes to operate safely and correctly.


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Posted: Sat Nov 22, 2008 1:03 pm
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Great, thanks guys... I ask because I noticed that some valve amps with fixed biass cost a little cheaper than the regular valve ones... :)


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Posted: Sat Nov 22, 2008 1:25 pm
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Synkronized wrote:
Great, thanks guys... I ask because I noticed that some valve amps with fixed biass cost a little cheaper than the regular valve ones... :)


True...less components reduce the cost.


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