Re: Mesa 2:90 inst./line switch question . . .
Short answer: YES! You were clipping the input of the power amp. The speakers were forced to hold larger than normal square waves at full output, meaning they did not have time to cool during the excursions. You'll probably want to replace those speakers if you notice any difference in sound. If not, for the time being they should be okay but their life has been severely shortened.
There seem to be some misconceptions about the signal levels. Line level is around 1.2 volts (+4dB). This is the level coming out of your preamp (most likely) and the level that the input is matched to calibrate to. If it's set to instrument level (-10dB to -20dB) that is signal range it will calibrate as 'normal', hitting the input with that +4 signal is going to be way too hot and distort the input stage of the amplifier (in an undesirable way).
When that happens the distortion is reproduced by the amp and overloading its circuitry along the way. Then that distortion gets blasted down to the speakers. Speakers produce noise by the exact opposite way your pickups do; the voltage comes into the voice coil and forces it to move against the magnet (as opposed to the string moving against the magnet and creating the voltage). Along with the mechanical energy that is released at this point there is also thermal energy (heat) that is released. However the constant motion of the speaker and varying of the voltage across the voice coil create air current and periods of little or no electricity being passed through in which the coil can cool off.
When that signal from the amplifier is hard-clipped at the input, it is being forced to produce large squared off waveforms which means that there are longer periods of time where the speaker is not in-motion as it's holding the flat-line where the signal is clipped but still being fed a lot of electrical current. The voice coil heats like an electric stove in these cases because there is not enough dynamic to the voltage and not enough air-flow to cool it. As a result it heats things touching it as well; i.e. your paper/polymer speaker cones, hence the smoke you saw.
It's not something that happens instantly and is more associated with lower frequencies (since they have the longest wave excursions they will cause the longest clips). But if you play for a while, you're going to over-heat those speakers not to mention the potential damage to your amplifier. Ask any professional live-sound guy; they'll tell you that you NEVER clip the input on the power amp because that's how you start a fire as you experienced.
Best of luck, and be more careful in the future. Also, make sure to match impedance to avoid trouble as well.
Last edited by TarktoneRecordings; 06-06-2006 at 07:32 PM.