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Your standard-issue acoustic guitar works because the strings vibrate at a certain frequency, and those vibrations are transferred to the hollow body of the guitar. The sound waves bounce around in there and get the whole body vibrating. Since a big piece of wood vibrating at 440 Hz (or whatever) will make more noise than a thin piece of wire doing the same, the guitar is naturally amplified, and the sound is loud enough to fill a small room.
This is great for playing alone, but what if you need to be heard in a concert hall over, say, a drummer, bass player, and horn section? You could theoretically build a HUGE guitar, but that would be impractical. Instead, what some genius in the 1930s realized is that you could use electricity and magnets to amplify the sound. The important parts of an electric guitar, then, are the neck, the strings, and a few electromagnets underneath the strings called the "pickups." Nothing else is vital--not the shape of the body, not the paint, nothing. That's why you can have electric guitars that look like this but most acoustic guitars look something like this
The pickups are the little rectangle or oval shaped things underneath the strings you'll see on any electric guitar. As stated earlier, they're usually not much more (or at least don't have to be) than a magnet wrapped in wire. When a guitar string (made out of a magnetic metal) vibrates at a certain frequency, it disturbs the magnetic field, creating an electrical current at that frequency. This current travels down the wire from the guitar and heads into the amplifier, where the signal is amplified (the waves are just made bigger) and this current then causes the speaker of the amplifier to vibrate at exactly the same frequency as the string(s) were.
Simple, right? Yes, in principle. But then it gets complicated.
To start out with, there are two basic types of devices you can put in an amplifier to actually do the grunt work of making the signal stronger. In the olden days, they used vacuum tubes. Then, in the 60s and 70s, somebody discovered that you can use transistors to do the same thing. As you might be able to tell from the picture, the transistors have the advantage of being a hell of a lot smaller. Solid state amps (as ones based on transistors are called) are also cheaper to make and have less chance of distorting the signal. Perfect, right? Technology is on the march and tomorrow is brighter than today!
Except no. Most amplifiers for things like microphones and audio systems try to reproduce the signal exactly as it came in. Early guitar amps did this as well, until somebody figured out they sounded damn cool if they got turned up really loud. What was happening was that the level of the electrical signal was too high for the electronic components in the amp, so the top part of the wave got "clipped."
Visual aid:
In most audio amplification, this is VERY BAD, because it sounds like crap. For a fun experiment, turn your computer speakers up all the way and play a song. It will probably sound a bit like diarrhea. For whatever reason, however, when you do this to a guitar, it sounds a bit like this. There are, of course, levels of distortion. In the clip above the amp is pushed into a moderate level. If you listen to someone like B.B. King, he has the amp set so that it's just a smidgen over its maximum, and you end up with a very smooth, mild, almost unnoticeable distortion. If you listen to a heavy metal band, however, they've got the things turned up so high that the waves end up looking square.Why exactly most people enjoy the sound of an "overdriven" or "distorted" guitar (the two terms mean roughly the same thing) is hard to answer. Certainly many people who grew up in an era in which the sound was to be as avoided in guitar amps as it would be in any amplifier say that it sounds like "noise" to them. For whatever reason, though, most people who have grown up listening to music since the 60s enjoy it and find it sounds "gritty."
So what does this all have to do with the tube vs. solid state issue? It turns out that a tube amp (also called a valve amp in England, for some reason) sounds better overall, but ESPECIALLY when it is distorting. This is because generally tube amps will have a much more graded scale, from creamy smooth blues sounds at around 3/4 on the volume dial to pure, balls-to-the-wall rock if you turn it up all the way. The transistors in solid state amps tend to handle the signal differently, by keeping it as clean and undistorted for as long as possible before suddenly breaking into very harsh, unpleasant-sounding distortion. There are also other factors, such as the supposed "warm" sound of a tube amp versus the "lifeless" sound of a solid state, which do have some validity--but to the majority of untrained listeners they're pretty damn similar. Even virgin ears, however, can usually tell the difference between "cranked" (guitar talk for turning it up to the point of distortion) tube and solid state amps.
Because they sound better, they're more delicate, and vacuum tubes aren't used in much else nowadays, tube amps tend to be more expensive. A smallish (usually around 15 watts or so) solid state amp will tend to run about $100-$200 dollars. A tube amp of a similar size will cost you minimum $300-400, and sometimes up into the thousands. For this reason, most beginner-level amps are solid state, and the vast majority of pro guitarists (and, indeed, almost anyone who is playing actual shows for a paying audience) bite the bullet and upgrade to a tube amp at some point. Occasionally guitarists (often jazz guitarists) will play with high-end solid state amps if they want a very clean sound, and some pretty big-name guitarists have heavily used solid-state amps in the studio (Brian May!)--but in the vast majority of times tube amps are the way to go.
So now you know the basics about guitar amps. We've only scratched the surface...
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