do you have to bias guitar amp preamp tubes

Note that the behavior of this type is similar to the algorithms used in profiling modelers and other products. Therefore the Comp Type parameter allows you to choose an idealized cathode follower with different distortion characteristics (Comp Type = Ideal). It's great for vintage Plexi and other high gain sounds but its clipping behavior is not ideally suited to certain tones. The cathode follower is a bit of an imperfect design though. The good amp designers understand the interaction between the last stage and the cathode follower and tune the bias point for the desired distortion characteristics. Too negative, however, and the sound can get indistinct. Reduce the Preamp Bias a bit and you'll hear the bass clean up. If you play lightly you'll hear the bass is kind of stuffy and tubby. For example, the Dizzy Blue models are biased near zero (0.08 IIRC). The cathode follower interacts with the last stage and slight adjustments to the bias point can cause major changes in the distortion characteristics. You can tell if an amp has a cathode follower if the Preamp Comp parameter is not zero. Things get especially interesting when a cathode follower is involved. Experiment with the bias point to find your optimum tone. Getting the right balance of even and odd harmonics is one of the keys to achieving 'edge of breakup' tones. Adding even harmonics gives a warmer sound but too much and things can get muddy. Odd harmonics give clarity and a more aggressive, open tone but this can be cold and harsh. The more asymmetrical the distortion the more even harmonics are introduced. Symmetrical distortion has lots of odd harmonics and very little even harmonics. Negative values of Preamp Bias bias the virtual tube towards cutoff and positive values toward saturation. In general cutoff is smoother than saturation but it depends on the external circuitry.

If it is increased it will be biased more towards saturation. If the quiescent current is reduced the tube will biased more towards cutoff. If the tube is biased exactly halfway between the supply voltage and ground then it will clip symmetrically (greatly simplified). The operating point of a tube determines the symmetry of the clipping. The earlier stages are important but generally not nearly as much and the bias points are not exposed to the user. This is the most important stage wrt to the feel and texture of distortion. The Preamp Bias control in the Amp block controls the operating point of the last virtual triode stage in the preamp.

Do you have to bias guitar amp preamp tubes