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Tone
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== 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.
Some amps have two sets of EQ's. These are for pre and post distortion. Pre-EQ changes the character of the distortion, whereas post-EQ influences the ultimate frequency response of the sound with no change the distortion character. Some amps have additional, rare features like voicing switches. These may affect pre-amp EQ or other operation, but also may control rectifiers or feedback in the power section. Like the presence control, what section of the amp the controls affect only really matters for certain effects in the effects loop or when bypassing the preamp.
The pre-amp is also where the different ''channels'' of the amp reside. Effectively, each channel is a different circuit, although some may share similar components.
== Effects ==