I think this is the type of attitude that gets people in Matt's an my age bracket in serious financial trouble.
People who look at purchases from that perspective can find themselves in a lot of credit card debt very fast.
What's the APR on the card? Keep in mind that depending on what your payments are, on a big purchase you need to pay off MORE than the minimum payment, otherwise you could get stuck paying off the interest and not making any headway.
I would say that you should figure out what types of payments you would be making on the amp setup and instead of starting a credit card, open a savings account. Every month you make a deposit in that amount, and once you have the total you can purchase the amp.
With the credit card, you're paying out probably 10-15% (or more!) interest for the privelege of having the amp now. With the savings account, you get PAID a small amount of interest (perhaps 2%, I'm not sure what the going rate is), and you get your amp at the end of your savings period.
If you buy on credit, you're paying MORE than what it's worth. If you put your money in savings, you're not gaining anything to write home about, but you're not losing any money on the deal.
If you combine the hit you take of buying something new (it loses 15-20% of its value as soon as you take it out of the box) with the interest rates you'll be paying, you might be able to calculate how much money you'll be flushing down the toilet. So, if you buy a $1,500 amp at a generous rate of 10%/month and figuring you'll pay it off in a year, you're looking at roughly $1,600 for something that is worth $1,000 to $1,200 as soon as it comes out of the box. That's $400-600 you could be spending on something else, and a good chunk of change that you could have avoided spending.
Unless you positively, absolutely MUST have it NOW NOW NOW, I would wait and save for it. If you're willing to take the committment of monthly payments, why pay someone else to do so? Do it on your own terms! If times are tough, you can just give yourself a waiver on the month of July instead of letting your credit report (something to be considering at your budding young age) take the hit.
EDIT: As someone who has gone down both paths, let me add another thought: getting something immediately and paying it off over time for anything other than a house or a car just sucks. Five months later you've got your amp or bass or laptop or whatever, and it's getting dinged up and you love it but you still have to shell out $200 a month for it. Writing those checks gets painful! It's almost depressing. I started to feel spite towards by bass, like a pet that seemed like a good idea at the time but now eats more than you expected and you have to pay money for a rock that it can sit on.
On the other hand, when you can swagger into the store and plop down your wad o' cash and point to what you want and say "Wrap it, Tom", the only way to describe that is exultant joy. You feel like you really earned it (which you did!), and you get to take it home and ENJOY it, no strings attached, as opposed to worrying about whether or not you can make this month's payment for it.
Silly? Maybe. But I know how I'm going to make MY purchases.
