At the moment I currently only use a basic guitar amp, but a few of my friends have 'heads' on top of their amps. What is the purpose of these? What do they do that the amp doesn't do?
A head is simply the name for an amplifier without a speaker. Your friends won't have heads on top of amps, they'll have heads on top of speakers (with the head driving the speaker). The "basic guitar amp" you have at the moment, is probably an amplifier and speaker combined in one cabinet. These are commonly called combos.
There are several reasons for using a separate amplifier and speaker cabinet (or speakers):
- you have more flexibility about what kind of amp and speaker you want to use (eg. Marshall head, Peavey speaker).
- you can use different amp and speaker combinations for different performance situations, or even just use your amp head connected to the PA.
The downside to this set-up (or upside to using a combo, I guess), is mainly that a combo is a lot easier to transport and set up.
Another couple of reasons why someone would choose head/speaker over combo : a comparable combo usually weighs more than one or the other, so is harder to hump around (poor old roady).Heads go wrong more frequently than speakers, so carrying a spare head to a gig is better than taking two combos.
In defence of the combo, one doesn't need to remember the lead that connects amp to speakers - it's built in.
Another, slightly off the subject point, is that while combos can be open or closed backs - with considerable sound differences, I rarely see an open back speaker cab.
It's maybe not common, but a head/speaker can be split, so the speaker is not under the head, meaning the sound is coming out of somewhere else, while the controls are to hand for the player - by using a longer speaker lead.
Heads are the amplifiers, the actual part of the signal chain that increases the power of a signal. Speakers convert that power to moving some physical apparatus (e.g. sheet of paper) that vibrates the air, making sound.
So like Bob said, people don't put heads on top of amps, but rather speakers. The amp itself doesn't make sound. In fact, if you're using a tube amp head without the speaker converting the energy output into sound, it is dangerous to plug in to just the head and play. Modern solid state amps generally have protection for this though.