Why do most guitar amps have a preamp and a power amp section?

Why is it difficult to build a guitar amp with a single amplifier section that goes from the very weak signal from the guitar, to the very strong signal that drives the speaker?

An audio power amplifier uses negative feedback to obtain pretty constant gain and low distortion. Trying to build tone controls into this amplifier will severely affect the negative feedback process and, it might become positive feedback. Then your power amplifier turns into a power oscillator.

That is a certain reason why you don't incorporate tone controls into the power amplifier section.

An audio power amplifier has to produce large currents in order to drive a loudspeaker and, the wiring to the power amplifier (if mixed with the preamplifier) will create large problems; speaker drive currents can leach into the input circuits and will create unwanted problems turning your "amplifier" into an oscillator (again). Look up star-point wiring for amplifiers to see why.

This is another reason why we keep low power signal amplification away from the power amplifier.

A guitar requires a high input impedance amplifier such as from a JFET circuit. This type of circuit needs to be largely free of power supply variations that are caused by driving high currents into a loudspeaker.

This is another reason why the front-end (or pre-amp) is kept separate from the power amplifier. Usually a secondary power supply stage is used that has much more filtering on the power rails.

It then makes sense to split a guitar amplifier into three stages: -

• High impedance front-end amplifier (specific to normal guitars) that outputs a line-input level signal
• Tone control section with some amplification (processing regular line input level signals)
• Power amplifier (line input level to speaker)

But, it's fairly uncontroversial to join the front-end with the tone control section so it usually boils down to two physical sections.

Your thoughts are quite alright. In addition, it makes sense to have two stages since

• To get both high current and voltage gain, one stage is usually too weak.
• The input is single-ended, while the output is often push-pull.
• EQing and tone control are easier to implement single-ended.
• Pre and power amp use distinct voltage/current regimes, so you'd want to choose different transistor/tube types for each.

sound effects (e.g. equalizers, delays, overdrives, ...) are usually applied on a line level audio signal. applying them on a weak signal (a signal straight out of guitar pickup or a microphone) may corrupt or weaken the signal further and applying these effects on a high power signal (right before a 100W speaker) needs massive, powerful components. so :

weak_signal ----preamp----> line_level_signal_and_many_effects ----poweramp---->speakers

many guitar amplifiers have both preamp and power amp on board, so for you to apply effects on the line level signal, either they have their own effects or they give you a FX_OUT, FX_IN terminals which are output of preamp, and input of power amp and you can plug your effects chain to this terminals.