What's a preamp?

Originally Posted by: Lordathestrings

The preamp is the first part of the amp that gets your guitar signal. It boosts the level enough to give the power amp something to work with, and also provides some tone or EQ adjustment. There may also be effects loops built in, which provide places to plug in external effects.

You will hear, and read, a lot of arguments about the sound qualities of tubes versus transistors. Generaly speaking, tubes tend to have a 'warm' sound, while transistor amps are 'accurate', which doesn't necessarily sound good. Accuracy is important in stereos, but guitar amps are usually chosen based on the 'character' they add to your sound. Accurate amps, by definition, have no character of their own - what enters the input, comes out the output sounding the same, only louder.

With tubes in the preamp, your guitar's signal is given the 'tube' sound that most of us expect to hear. If the power amp, that boosts the preamp output to drive the speakers, is made with transistors, it will accurately reproduce the preamp output, including the 'tube' sound.

The big difference gets noticed if the power amp is pushed hard enough to saturate. A tube power stage has distortion that starts out as a subtle change in the harmonic content, and gets progressively stronger as the amp is pushed harder. Transistor amps stay clean right up until they run out of power, and then they get very dirty very quickly. These types of distortion sound different, and your choice comes down to personal taste.

The important thing to remember is that once the sound has been 'warmed up' by a tube preamp, a transistor power amp will not 'sterilize' that sound.

For clean playing, a transistor power amp may be a better choice than tubes. It would definitely be cheaper, smaller, and lighter than a tube amp with the same output rating. And since a lot of recent music, especially metal, uses pedals and the preamp for distortion, the type of power stage becomes less important.