Clipping occurs when you ask an amplifier to put out more power than it can. As Timoteus said, it basically "clips" the top and bottom bottom curves off the sound waves, leaving very sharp corners from where the wave is rising or falling to the flat top or bottom where the amp hits its limit. These sharp corners are loaded with high-frequency energy, which is why it blows the tweeters.
A lot of more recent power amp designs have a built-in limiter (Peavey calls it DDT) to prevent this, but many of them let you switch it off (I don't know why). Look for a switch, probably on the back of the amplifier, labelled DDT or LIMITER or something similar and make sure it's on.
Heavy feedback is often at fairly high frequencies. Besides that, feedback can also push an amplifier into clipping very easily - and once again your tweeters get the brunt of it.
A neat trick is to put a lightbulb in series with your tweeters. An 1156 (used as a back-up light on a lot of cars and available at most auto-parts stores) works well.
Tungsten (the metal the filament is made of) has a very low resistance when it's cold, and won't interfere with the signal at normal levels. However, its resistance rises exponentially as it heats up. Power levels large enough to damage the tweeter are also large enough to light the bulb, raising its resistance. Then the bulb (under a dollar) takes the brunt of the power surge and protects the speaker. Note that this is just for the tweeter, not the whole speaker system. If you mount the bulb where it can be seen from the booth, the sound tech can watch and pull back the gain if the light comes on. In four years of running these tweeter lights in four cabinets I've burned out one bulb and NO tweeters.
John
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