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What would happen when you under Bias your tubes?

Hi, I am a tech noob and I always wanted to ask if it is safe to under biasing tubes and especially the power tubes? Will that affect performance or will that do damage to the amp? I am asking because my power amp which is the Lamm ML 2.1 has manual bias and when I left the bias unchecked for awhile. It somethings jumps much too high then originally set and those 6c33c doesn't last! I mainly set the bias to 70%. Is that OK? Thanks.
luna

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barjohn
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The bias voltage is set to have a class A amplifier operating in the linear portion of the range.  The amplification across the frequency range will only be linear over s relatively small portion of the tube's amplification range.  You can easily go out of class A operation and introduce non-linearity to your signal thus providing sound that was not in the original audio source.

(Old Navy Engineer from vacuum tube days)
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