I'm really not an expert on tube technology. I understand that BIAS is a way of restricting current flow. Tubes need to be biased to run cooler and to remain in the linear region.
1- When a tube is biased, is it completely turned off or you just partially reduce the current flow ?
2- if the tube is not completely turned off, how can you get rid of the noise when there is no signal at the grid ?
thanks in advance
It's apparent that you're thinking of audio power output tubes, although bias (properly, grid bias
) applies to any and all amplifier tubes, voltage and power amplifier tubes alike.
Audio output stages may fall into three classes:
In a Class A amplifier, the tube is biased so that plate current always flows in the tube, even with the largest signal cycle to be applied to the control grid, which is never driven to cutoff. Single tubes or pairs of tubes in push-pull may be operated in Class A.
In a Class B amplifier, the tube is biased to cutoff and amplifies only half of a signal cycle. This requires a pair of tubes in push-pull for satisfactory (i.e., distortion-free) results. One tube amplifies the top half of the signal; the other tube amplifies the bottom half, and the two halves are "married" in the output transformer.
In a Class AB amplifier, the tube is biased somewhere between Class A and Class B. At low signal levels, it behaves like a Class A stage; at higher levels, it behaves like a Class B; thus, like Class B, a pair of push-pull tubes is required.
Class A is the most inefficient mode of output tube operation (although typically the most distortion-free). Bias is comparatively low, and the output tubes run hot as hell.
Class B is far more efficient, and capable of much greater output with the same tubes than Class A. With the tubes biased to cutoff, they run cool. On the other hand, a Class B stage is harder to design than a Class A stage, and far less common. It is touchier to handle, and much less forgiving of parts deterioration due to age; furthermore, it is totally unforgiving of abuse.
Again, Class AB falls between the two. Most tube-type stage amps and many high-fidelity amps and receivers are built to Class AB designs (particularly, class AB1, in which control grids are NEVER positive with respect to the cathodes).
I can't guess what sort of "noise" you're referring to. There are (too) many different kinds of noise, with more potential sources than kinds. Chances are, however, that the source has nothing to do with your output stage, and thus represents an entirely different question to be dealt with in a different context.
Hope this helps.