When you plug into the input jack of your amp, you are connecting to the preamplifier (preamp for short). The preamp boosts the level of your signal high enough to drive the power amplifier, and usually allows the musician to tailor the sound via equalization and gain. Some amplifiers offer more than one preamp channel. This may be accomplished by adding an extra gain stage or the amp may have a parallel channel depending upon design. On your amp, the channel controls operate the preamp functions.
On amplifiers with reverb or effects options, this module follows the preamplifier to allow reverb or effects to be added to the sound. It is usually designed to have the same signal level at the input as the output (known as unity gain) since you want to modify rather than change the signal. It is worth noting that reverb and effects usually sound best after any desired distortion is created, and it is for this reason that built-in gain and distortion circuits in guitar amps are located within the preamp. The reverb/effects loop has its own controls.
The power amplifier does just what the name implies; the power amp module takes the signal from the preamp/reverb modules and drives it high enough to power the speaker system. Since vintage guitar amplifiers rarely offered distortion they were traditionally run at maximum volume by guitarists (Hence Nigel Tufnel’s famous line, “But these go to eleven!”) which adds power amp distortion to the sound. Unfortunately this method resulted in dangerously high sound pressure levels which were as bad news for getting a good mix as they were for preserving your hearing (not to mention keeping a gig)! With the advancements in preamp design now available it is seldom necessary to drive your amplifier into power amp distortion but most guitar power amps still color the tone a fair amount. This coloration is the reason many guitarists choose to customize their sound by experimenting with using different brands of tubes in the power amplifier. When intentionally driving the power amp into distortion, try using less preamp distortion for the best results.
The speaker system converts the electrical signal from the power amp into mechanical energy in the form of a moving cone which recreates the original sound and any modifications to it added by the various amp modules. Most popular guitar speaker systems also add coloration to the sound of the guitar (notably they rarely have much frequency response above about 5-6kHz). The impedance of the speaker system should be matched to the output impedance of the amplifier and should never provide less than the output of the amp can safely furnish (usually 4 ohms for guitar, sometimes as low as 2 ohms for bass amps, indicated at the speaker output of the amp). The speaker system may be a separate cabinet attached via a speaker cable, or it can be part of an all-in-one combo amplifier.
Understanding the different components of your amplifier will help you make the most out of your equipment, from enabling you to optimize your signal chain to guiding your efforts to troubleshoot an amplifier that isn't performing to your expectations. In future articles we will explore each section in greater detail and offer specific tips you can follow in order to obtain the best performance possible from your equipment.