Mesa 2:90 inst./line switch question . . . - Jemsite
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post #1 of 6 (permalink) Old 06-02-2006, 08:20 PM Thread Starter
 
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Mesa 2:90 inst./line switch question . . .

Yesterday I was practicing before rehearsal and after about half an hour my cab started smokin!!. I turned off the amp and unplugged everything immediately. The amp was hotter than normal. Even 2 minutes after I shut off there was still some faint smoke coming out of the speaker with a burnt plastic smell. I was pissed off. when I got home and pluged back my 2:90 in my other recto cab I noticed the Line/Instrument switch was left on the Instrument position. Is it possible that the fact that I have left the switch to the inst. position while driving the power amp with my preamp at full may have killed the speakers? Did this harm the amp? When I flipped back to the line position and played through it everything was fine. I would really appreciate some help here!!

Thanx guys!
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post #2 of 6 (permalink) Old 06-02-2006, 09:03 PM
 
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Re: Mesa 2:90 inst./line switch question . . .

Man, I wrote an extensive reply earlier to this but, I realized it wasn't much help...

I have my 2:90 running with a preamp through two cabs, and one thing I've noticed is that it gets pretty hot.

But I've had it retubed and rebiased, and it runs like an absolute champ now. I even believe I have it running instrument, not line.

So I'm not sure, that sounds pretty crazy. I don't know if that could be an impedance problem or not. Have you checked the cabinet impedance level on it?

If you find out what it is, let us know; the more knowledege, the merrier.
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post #3 of 6 (permalink) Old 06-02-2006, 09:10 PM
 
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Re: Mesa 2:90 inst./line switch question . . .

You can use either the line OR instrument options. It doesn't matter. In fact, the manual for the 2:90 claims that "line" can be used if your preamp doesn't have enough output.

Right now, I'm using a crappy Digitech Twin Tube preamp from the early 90's (until I get a Triaxis) and it DEFINITELY doesn't have enough output (even with the 2:90 cranked). Originally, I had the switch set to 'instrument' and it wasn't loud enough. Then, I read the manual, about how 'line' can be used to increase output for weak preamps and the difference in volume was pretty significant. So, to answer at least one of your questions -- having the switch on 'instrument' definitely shouldn't harm the amp. And, if it did, something is wrong with the amp.

You said you were "driving the power amp with your preamp on full." That's actually backwards. You're supposed to have the output on the AMP CRANKED and you're supposed to control your overall volume with the output of your preamp...

I don't know why your cab was smoking, though...
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post #4 of 6 (permalink) Old 06-02-2006, 09:15 PM
 
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Re: Mesa 2:90 inst./line switch question . . .

Oh yeah, to the original poster, this is VERY important...

With the 2:90, if you're only using ONE cabinet, you have to TURN DOWN THE VOLUME on the channel you aren't using and, according to the manual, you have to TURN THE PRESENCE ALL THE WAY UP on the channel you're not using, also.

You have to remember that the switch on the left of the amp is "on" and the one on the right is "standby" for BOTH channels. You can't turn one channel on and off, so they're both 'active' once you turn the amp on. Therefore, if there was no speaker handling the load from the channel you weren't using and you didn't have the volume all the way down, you might have fried something...
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post #5 of 6 (permalink) Old 06-04-2006, 11:07 PM
 
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Re: Mesa 2:90 inst./line switch question . . .

Quote:
Originally Posted by Exhale
Yesterday I was practicing before rehearsal and after about half an hour my cab started smokin!!. I turned off the amp and unplugged everything immediately. The amp was hotter than normal. Even 2 minutes after I shut off there was still some faint smoke coming out of the speaker with a burnt plastic smell. I was pissed off. when I got home and pluged back my 2:90 in my other recto cab I noticed the Line/Instrument switch was left on the Instrument position. Is it possible that the fact that I have left the switch to the inst. position while driving the power amp with my preamp at full may have killed the speakers? Did this harm the amp? When I flipped back to the line position and played through it everything was fine. I would really appreciate some help here!!

Thanx guys!
Normally you use the 290 power amp in the "line" setting. "INstrument" only changes the input sesnsitivity for the one channel anyway.Atleast I'm 95% sure of that having owned the amp and checking it out up to about 2 years ago.

Putting the selector switch into "instrument" increases the sensitivity of channel B. This obviously makes the output of that channel louder as well. But running a preamp into it in the "instrument" setting should hurt anything.Volume is volume and if you weren't louder than usual, you shouldn't have done anything to your speaker.

If your speaker is smoking, then perhaps it is failing or even might be electrically out of phase.If for some odd reason it is out of phase (what kind of cabinet is it ?) it could run hot and sound lousy

At any rate, run your 290 in the "line" setting and have your speakers checked out.Occasionally, a speaker cabinet with a wiring issue will pop up and you might have one of them. I don't know what cabinets you have but in a 2x12 or 4x12, it is possible for one or more speakers to be wired backwards and be electrically out of phase with the others
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post #6 of 6 (permalink) Old 06-06-2006, 07:17 PM
 
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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.
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