I'm trying to make my own speaker cabinet for my guitar. I was originally thinking of buying a subwoofer and a tweeter, however I noticed that a normal stack amp is 2x12" or 4x12", which leads me to believe that all of the speakers are the same. What type of speakers should I use? I already have a pair of subwoofers, can I use those? do they have enough frequency range (these have something like 40hz- 6khz), etc? What is normal for guitar amps?
You shouldn't use regular speakers for an amp cab. I learned this through experimentation while still in high school. We went the other direction though, and took an amp and tried to make car speakers. It will produce sound, so it works in that essence, but not the results you really want. The amp speakers are MUCH stiffer and not as much throw. Guitar tones are more simple than reproducing the entire audio spectrum at once (like a piece of music). So with a "tighter" speaker you get more control over the tones produced.
If you really wanted to experiment, you could use regular speakers, but you would need a woofer and tweeter as you originally guessed. A regular 12" woofer will fry over time if you try to push too much high frequency material to it. Because it has a large amount of travel and mass, it is difficult to make it move really fast (5000 times or more per second). Thats why a tweeter is better suited to that job (less mass, easier to move quickly). Consequently, the opposite is true as well, it takes a lot of air movement to hear the lower notes, and the small mass of the tweeter is unable to push enough air and will fry in trying to do so. So you'll need some kind of cross-over network, whether active or passive, to divide the audio into spectrum that is appropriate for each speaker.
the woofers that I got are stripped from an old cabinet, so thats all good. What I don't understand is:
Using a store bought 4x12" cab as am example, does this include woofers and tweeters? If so, how come the tweeter is the same size as the woofer? I'm actually having problems finding tweeters made for guitar speakers.
I also looked up the frequency ranges for amps, and I found that they usually don't go higher than 5khz anyway (the woofers I have can go a little past 6khz) so could I still do it with two woofers, taking into account the fact that they come from an old cabinet, and then have an appropriate frequency range?
Tom you are all good. You don't need separate tweeters. Most cabinets do not have split frequency drivers (separate woofers and tweeters). The speakers made for the amp are much stiffer and can handle higher frequencies better than a regular speaker. So your speakers that you got from the other amp are just perfect.
Now the fun part. You should match your speaker impedance to the recommended impedance of the amp. This will determine if you should wire your speakers in series or parallel. If you are unable to find the specs for the amp, I would wire in series. This will double your impedance of an individual speaker and will be less "strain" on the amp, making it less likely that you'll blow the amp.