A Peavey 6505 Amp Head But i want to make sure that before i invest that much money into it, i just wanna make sure on one thing. My Cab.

The Cab im using is the 4x12 120W cab that comes with the Crate Flexwave Head. Now i know enough to tell that as long as Wattage of Amp = Wattage of Cab, youll be fine, but is there anything i should really be wary of? (Other than the Cab not giving the "Best"?)

Just make sure to use the right ohm speaker out on the amp and you'll be fine.
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^+1. And NEVER EVER EVER turn you amp on without the cab connected, there's a good chance you'll be in the market for a new one shortly.
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I'm not an expert on these matters, but i think you might want to get speakers that can handle higher wattage than the head? that might just be an attenuator.
but the ohm thing is right for sure.
okie dokie, i think im good then

But heres a noob question though. I know how to match up the ohm (Thats basic math, make sure the number matches) But exactly IS the ohm?
Look it up on wiki, the only thing we guitarists need to know is that cab and amp must match.

Nevin: Speakers can handle more than their rated wattage, you're thinking of attenuators.
I don't give a shit if you listen to me or not
ok, now i know what ohm is, lol.

Quick Breakdown

Ohm is the electric resistance between two points of a conductor. So, patching that together with my small amount of brain power, Basically, if either the amp or the Cab has a different number, the signal would go either way frying either piece.

At least, thats what i was able to come up with.
The first point is right, but you can mismatch as long as the cab has the higher impedance. Mesa amps can take a fairly high mismatch and solid state amplifiers can take huge mismatches.
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get a speaker cable, don't use a patch cable.
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