The desk will explode. Then your amp. Then your head :wink:
Simply taking the speaker lead from an amp and plugging it into the line input on the mixer (God forbid you should plug into the MIC input!) may very well cause catostrophic failure!
There are several pieces of gear on the market that will assist you in hooking up. There is the Marshall Power Brake, and there is the THD Hot Plate, to name a couple of them. The reason that you don't want to hook the amp's speaker output directly to the desk is because of the "load mismatch" (OHMS!) and the fact that the amp is pushing AC CURRENT out the Speaker jack on the order of, say, many WATTS. A Marshall JCM800 (example) pumping 50 watts from a pair of toasty 6550 power tubes will be delivering 20 volts (+/-) into a line input on the board designed to handle, say 200 millivolts (that's a fifth of a volt!). OOOPS! That's the boards problem...
The AMPs' problem is that that line in on the board is presenting a fairly high impedance (50K ohms??) to the speaker out that is supposed to "see" 4-16 ohms. Big Problemo!
Do you have a "line output" on the amp? That can help with the interface, though the tone is often less than ideal. And you should STILL have a speaker load on the amp, anyway. If you want to really capture the tone of your amp CRANKED UP, you will definitely need something like the Hot Plate to take the amps output and convert it to heat instead of sound. Then you can plug THAT into your desk. And/or it will allow you to use a cab without blowing the doors off your studio...Or, just get a "modeler" and leave the amp in the garage... :wink:

Like moonbaby said, it will smoke then burn then melt. Don't try it!!!
Try a POD for ideas like that. Or something along those lines. (DI box)

Also, just to talk about recording guitars...the speaker cabinet is still a big part of how a quality guitar sound is produced. Things like corolation, saturation, eddie currents, flux capacitors...
O.K. I'm just joking about the last 2, but seriously...the cabinet (along with the type of speakers) is a big part of getting a good or great guitar track.
But a guitar DI (direct box) is a good alternative, for your idea.

Very true....
There is another alternative to your idea. There are speaker cabs on the market designed to "contain" the sound. In other words, they have a real 12" speaker in them, but they can be totally sealed with a mic (SM57) inside them. That way, you get the real tone from an amp/speaker combination without ripping your neighbors' eardrums! Randall makes one, and I have built one for my place and a couple for my buddies. Maybe I should market the "Moonbaby Amp Trap"...?

hmm

Thanks,

I thought the words 'smoke', 'melt' etc might well appear in yr. answers.
Thing is, I own an Orange OR80, stupidly powerful thing, can't record with it in my home studio, and I'm fed up with the guitar sound I'm getting just using non-valve equipment. I figured I might be able to use the Orange as a half-way job/pre-amp thingy. But there is no line out on it...
Obviously I'd get a better sound by cranking it up with a 4x12 and using a decent set of mics. But for recoring rough demos, it's just too much bother
I guess I could invest in a valve pre-amp/overdrive pedal...

Cheers for yr equipment-saving advice

I regularly record a local player who has an original Orange MATAMP (!). It is a BEAR to record...he just turns it up and lets it rip, but we're in a warehouse late at night. I can appreciate your dilemna. You should probably look at a POD or SansAmp to record with. But that Orange is a really great clean amp, eh?

Re: hmm

Jaike wrote: Thanks,

I thought the words 'smoke', 'melt' etc might well appear in yr. answers.
Thing is, I own an Orange OR80, stupidly powerful thing, can't record with it in my home studio, and I'm fed up with the guitar sound I'm getting just using non-valve equipment. I figured I might be able to use the Orange as a half-way job/pre-amp thingy. But there is no line out on it...
Obviously I'd get a better sound by cranking it up with a 4x12 and using a decent set of mics. But for recoring rough demos, it's just too much bother
I guess I could invest in a valve pre-amp/overdrive pedal...

Cheers for yr equipment-saving advice

Really no experience with this, But the Behringer DI's state that you can drive a speaker load into them, and they will balance it to mic level. And i dont think there really is much point in bypassing the cabnit. No 'cabnit only' emulating softwares come to mind.

With the DI100, you can rest assured that your sound source will reach your console balanced and noise-free. Go ahead and plug that guitar amp's speaker output into the ULTRA-DI—it’ll deal with ratings of up to 3,000 Watts and give you a perfect signal at the other end.

http://www.behringer.com/DI100/index.cfm?lang=ENG

1) POD: EWWWWWWWWWWWWWW screw line 6, their stuff sounds bad unless you buy one of their 100000FX model 1000$+ amps and its still not great. nothing beats real tubes. sansamp might work, i have the behringer knockoff which produces workable tones, but its tones are limited. a SA classic might have enough control though, worth a run at a store.

2) enclosed speaker: ive built one, works like a charm. build a 15X15" or larger box, 2/3 empty space and 1/3 enclosed with a speaker hole. i would link the pictures but theyre on my HD and i dont have DNS setup for it. basically its a shallow speaker box (or deep if you want, i was saving space) then an attatchable top (i used suitcase style clamps). it had foam all inside everywhere. to seal it i nailed some thin 3/4" wide strips on the speaker side and put weatherproofing on the rim of the outter case so when you clamp it it squishes together to make a nice seal. as for mic, i just screwed a mic clamp to the top of the top so it pointed directly at the speaker. i could have went fancier with the mic setup, but the speaker was solid, and could double for a normal extremely-mini cab.

DI's, as a whole, are NOT good on the speaker ouput of an amp, especially a TUBE amp that was designed to "see" a specific load.
A DI is designed mainly as a low-level circuit to match the pick-ups (or instrument) to a console input. I realize that there are models that CLAIM you can run an amp into them, but that is a cheesey compromise that will cause problems for the amp down the road. And if Behringer states that their product will do it, ask them this question:"Yeah? For how long?".Chances are they have simply put a big resistor in the circuit to handle the power, until....is there an emoticon for flames and smoke?

You can DI a head with either of these boxes. What you need to do it is both a dummy load and a speaker simulator. These two have both. Otherwise you could use dummy loads with line outs like the THD Hot Plate or Weber Mass then run it through a speaker simulator like the ADA MicroCab or Digital Music Corp. Cabtone, then into the mic pre.

The new Motherload:
http://www.motherloadusa.com/motherload_guitar_amplifier_loadbox_recording_directbox_home.asp

Or the good old Palmers:
http://www.palmergear.com/

A dummy load is a 8 or 16 ohm resistor to replace the load the speakers put on the amp. Without a load your output transformer will die in a matter of seconds.

A speaker simulator is a filter that cuts out the highs so the sound is more like a cabinet.
Without it, you get extreme fizz.

The sound is okay. Better than modeling. Nothing beats nice speakers / mics /pres.
Another benefit is that you can run the line out through effects (post amp!) and into another power amp. You can then crank the slave head and have a master volume with the power amp. Hope this helps.

Pat