8,720
edits
Changes
Tone
,→Pre-Amp
The ''pre-amp'' section of the amp is used to adjust the tone in a variety of ways, many of which you can control. The most obvious of these are the EQ controls, which are virtually universal to all amps. Even "bare bones" amps give a tone control that adjusts the relative darkness/brightness of the sound. Most amps have more: bass/treble, bass/midrange/treble, or bass/midrange/treble/presence. Some even have 5+ band graphic EQs. While presence is usually side by side with the other pre-amp controls, many times it actually controls the power section of the amp. Regardless, it will have the same impact on the tone either way; the only time you need to be aware of this is when running frequency-response-dependent effects in the effects loop or if bypassing the pre-amp.
The main job of the preamp is to transform, the extremely dynamic and bright signal of the pickups into a balanced, more full-range output. Every amp does this a bit differently, which is why they all sound different. By using different circuits, standard controls will not respond exactly the same way either. Different amps will have different cutoff or center frequencies for their EQ controls. Often, the EQ controls interact, so there are no static center/cutoff frequencies.
For older amps, this was all the pre-amp did; however, modern amps typically have a gain/drive knob to dial in your desired level of distortion. Having the gain in the pre-amp rather than relying on overdriving power tubes means you can get consistent distortion at many volume levels, rather than just when the amp is cranked up.