Extension cab question....

[gunslinger_burrito]

How do I know if/what kind of extension cab I can use on an amp? Is it just a question of having enough wattage to drive them? I'd like to eventually get something that's 212 or 410, but I'd ALSO like to be able to add something onto that, for the purpose of having a more immersive sound in a live setting. Is there a lowest wattage I'd be looking at to power two 410s or two 412s?

[benjuro]

Your amp should tell you what the minimum impedance it will accept is (don't ignore this, and research if you don't know how impedance is calculated), and how much power it puts out.
According to the techs I've spoken with at Eminence, guitar speakers are typically quite efficient and don't require a ton of power to produce sound...that being said, getting decent tone out of them depends on a number of things, including how powerful your amp is/how much headroom you have, that your impedance is matched up correctly, and that your cabinet(s) are designed and constructed well. How many watts you "need" to get there depends on the venue, your specific volume needs, the efficiency of the cab design...and the competition with your bandmates.
My 4 watt Vox could technically drive my fullstack if I wanted it to...but I don't.

[gunslinger_burrito]

I guess I should have rephrased my question. Is there a certain amount of power recommended to drive a half/full stack? I know it's pretty common to pair a 50-60 watt amp with a 410 or 412, and I think it seems like 100 watts is typically used to power a full stack....

[Gunner Recall]

The signal will divide up the wattage evenly...so in a 90w amp with a 2x12, each speaker would be fed 45w. Running two 2x12s (four total speakers), each speaker would see 22.5 ...and so on.
As long as the extension speakers are powerful enough to handle the amp, you're ok...it's not an issue of not having enough watts.

OHM will be the thing you really need to watch out for. That's where things get a little tricky.
Your amp is going to want to see a particular ohm, usually 4 or 8 or 16. This is determined by the speaker type and how the speakers are wired.
This is a quick primer to series, parallel, and series/parallel wiring http://www.usspeaker.com/speaker%20wiring-1.htm
Using different wiring you can turn two 8 ohm speakers into a 4, 8, or 16ohm load.

Some amps have multiple outputs or switches to select ohm, others are fixed.
You'll want to look for an amp with an impedance (ohm) selector toggle or multiple ext. outs to give you the flexibility to run 2 (or more) cabs.

[gunslinger_burrito]

So the amount of "watt power handling" is just how much the speakers can take before getting overloaded, right?
And if you had two 8 Ohm cabinets, in parallel, would your amp need to be set to 4ohms? Or what? I always get lost doing this, because I know that ohms are units of resistance and so the higher you set your amp's OHMS to, the less power it lets out, right?

[benjuro]

You are right that two 8 ohm cabs in parallel will give you a 4 ohm load. Lower number (ohms)=higher amount of resistance, which in solid state amps typically will produce more power. Tube amps are not subject to this bump in output and will put out the same watts regardless. SS amps are okay with mismatched impedance (say, one 8 ohm and one 16 ohm cab plugged into your amp) while tube amps are not as forgiving. Do not go below your amp's minimum impedance rating.
As long as you don't go over the speaker's rating, you should be good to go. I'm not aware of any guidelines/"rules" around how many speakers to match your watts; many Mesa Boogie combos pump 100 watts into a 1x12. I use 100 watts to drive a 410, a 412, and a 215 all at the same time. Those are probably close to the extremes.

I know it's pretty common to pair a 50-60 watt amp with a 410 or 412, and I think it seems like 100 watts is typically used to power a full stack....

I think this is more the norm.

[madmax1012]

UGHHHH MATH

[gunslinger_burrito]

UGHHHH MATH

This^

I thought that higher ohm rating meant higher resistance?

[theavondon]

Guys, it's more than math. It's physics.

[gunslinger_burrito]

Every time I go through this, I not only forget it all as soon as I don't immediately need it, but I feel like I make it more complicated than it needs to be.