I need help, for my birthday I want an electric guitar, but I cannot get a amp so I looked at the store, and Preamp tubes are $18 but what are they? I need a cheap solution to an amp without paying too much over $20
Oh god...

You cant find an amp for under $20 dollars. Sorry. A pre-amp tube is used in tube amps. You cant use it on its own to do anything really.

What you can do is buy a Vox amPlug. Then you can play, but you can only use headphones. It costs about $40
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DigiTech Bad Monkey
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haha you're probly going to want to invest a lot more than $20 in an amp.
as mike said, pre-amp tubes are used in tube amps.
just a warning though, when buying your first amp, i wouldnt reccomend a line6 spider at all.
i got one for my first, completely regret it.
Everyone here might shun the idea, but if you get one guitar item for your birthday, why not just ask for a guitar pack that comes with a crappy guitar and a crappy amp, instead of just buying a crappy guitar by itself, and then trying to some way to amplify it for $20?