Tubes Asylum

Biasing

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Why do power tubes need to be biased and why is it that pre-amp tubes are ready to go once installed in the socket?

Yes this is an admission of my limited knowledge of basic electronics!

Thanks!

Follow Ups:

RE: Biasing - 6bq5 13:09:10 06/01/23 (0)
the manufacturing variances inherent in Power tubes cause output variations by adjusting the bias, the tube can be operated within its sweet spot, a more linear part of its gain curve-
when using tubes in a push/pull circuit, having balanced tubes means that the amplification is more symmetrical on the push/pull part of the cycle
Happy Listening
RE: Biasing - Tom Bavis 07:52:35 06/01/23 (0)
Preamp tubes use self-bias - no adjustment needed. They operate at a small fraction of their power rating and signal voltage, so small deviations from optimum are insignificant.

Power tubes can use self-bias, but most can deliver more power using "fixed" bias (which is usually adjustable...). Since the operating point of power tubes affects power output, distortion and tube life, having an adjustment allows the stage to be optimized despite variations in individual tubes.
RE: Biasi - Ian L 06:32:02 05/31/23 (0)
Typically, ALL the tubes will be biased but the method used to bias them can be different. It's often the case that the output tubes use a method (ie. circuit) that makes the bias user adjustable from outside the chassis, whereas the input and driver tubes are most often biased with a circuit that is not.

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