Preamps help guitar amplifiers make the sound they do by boosting the original signal from the guitar before it reaches the main tubes and power of the amplifier. The main use for this, as musicians discovered, was to add distortion and feedback to the signal going into the amp itself. This is a great help to guitarists who need heavy distortion at lower volumes.
When the first guitar amplifiers were made in the 1930s and 1940s, the designers, including Fender, Gibson and Rickenbacker, realized the tone for the amp could be hard to control with just the regular tube amplifier. Because guitar amps were specifically designed for the tonal range of the guitar, the designers needed to build preamps that could control tone. This means the preamps added controls for the high-, middle- and low-range tones needed for guitars. The original purpose was not to use distortion since few musicians until the 1960s found a use for it.
The first, and original, form of preamp was built into the amplifier itself. Most musicians were unconcerned with the idea of the preamp. It just provided tone and was part of the whole package. In a combo amp, one in which the preamp, amp and speakers are in the same unit, the preamp essentially was invisible to musicians. They used the preamp tone controls in combination with the tone controls on the guitars. While the preamp controls provided treble, midrange and bass, the guitar controls also contributed to these three sounds. A Fender Stratocaster, for example, has three control knobs. One is for volume and the other two are for tone.
The original preamp could provide a clean sound that was either sharp or heavier on the base. It also could be pushed to the top of its gain level to make the guitar’s signals distort. When the gain levels are put all the way up on the preamp and on the amp itself, distortion results. Prime examples of this are the Marshall and Mesa/Boogie amps. Marshall was one of the first preamp/amp combinations specifically designed for distortion. It was used by such powerhouse bands as Led Zeppelin and The Who.
The second form of preamp, a more recent development, is the amp modeling preamp. These preamps were pioneered by Line 6 with its AxSys 212 amplifier in 1996. Realizing the average guitarist might want the amp modeling technology without buying a whole new amplifier, Line 6 then introduced the Pod. This is a standalone unit that can be plugged into any amp, into a recording console or, now, a computer via a universal serial bus.
Amp modeling, as used in either the standalone preamp units or now built into many amps, is designed to emulate the sounds of particular, usually famous, amplifiers. For example, the guitar sound on "Whole Lotta Love" by Led Zeppelin and "Money for Nothing" by Dire Straits is very distinct. Both of these can be re-created by amplifier modeling on the Line 6 Pod. Along with amp modeling, the Pod preamp also has many sound effects built in. These include specific distortion models based on “stomp boxes” by Electro-Harmonix and other companies that no longer make them. Other effects include most of the famous ones from wah-wah and delay to compression and phase-shifting.