Guitar Speaker Cabinet Ohm setting question

[Uladine]

I'm going to get a Marshall 1960A 412 cab. There are three different ways I can run it.

1. In stereo at 8 ohms per side

2. In Mono at 4 ohms

3. In Mono at 16 ohms

I was thinking the cab was 8hms mono, but apperently it isnt.

The power handling is 300 watts.

I was wondering what some of the advantages/disadvantages were to running the cab each way.

My power amp delivers 200 watts per channel at 8 ohms, 300 watts per channel at 4 ohms, Bridged mono at 8 ohms it delivers 600 watts, and mono at 4 ohms is 830 watts.

I don't want to blow the speakers pushing 830 watts through them at 4 ohms. Should I just get another speaker cable and run it in stereo? Or just run one channel from the power amp to the 4 ohm mono setting on the cab?

Wattage and impedence confuse me. It reminds me of math. When I try to learn more about it, the more confused I get.

 

[charger]

It's pretty simple: the higher the impedance number, the smaller the load the speakers draw, and the smaller the load the amp is putting out. Although you should always try to match impedances (if your amp says 4 ohm, the cab should be 4 ohm) it is safe to have a higher impedance on the cabinet than on the amp. So for example, an 8 ohm cabinet will be fine with a 4 ohm amp output, and a 16 ohm cabinet will be fine with an 8 ohm amp output. But an 8 ohm cabinet will not be fine with a 16 ohm amp output. When I say fine, remember to take that with a grain of salt... really with a tube amp you want to keep these loads pretty close to recommended values. With solid state there may be more room for error, though in general, a transformer is a a transformer.

So in your case, I would runthe cabinet at 8 ohms per side stereo, and run your rig in stereo.
If you don't want to do stereo, run it mono at 4 ohms, and keep your volume down. There's a chance you will blow your speakers... but just keep your volume down.

One question... why such a powerful amp? This article is good reading on the subject of some of the myths of amp power.

 

[Uladine]

The amp I have is solid state, so I dont want to crank it like a tube amp to get "sweet tone". I pretty much want lots of headroom. I also keep hearing that under powering your speakers is just as bad as pushing too much power through them. It accepts pretty much any load, but it won't accept a 2 ohm load in bridged mono, which isnt something I plan on ever doing anyway. I think I'll probably run one channel of the amp into the cab in mono. Then maybe our other guitarist can use the other channel for his cab (hes getting the same cab and we both have POD Pros). Thanks for the help.

 

[charger]

Yeah, I figured it was solid state. The assortment of tubes required for that much wattage could keep a house warm. Just remember, it's a lot easier to blow speakers with a solid state amp, and try to keep your power down.

 

[PFDarkside]

Hey bro,
I'd just run it in full stereo. Is the POD stereo? You might be able to get some cool chorus and delay tones by keeping it stereo.
You just have to be careful with the upper limits of the volume knob.

If you want a perfect match:
Amp running in bridged mono (600W @ 8 ohms)
Cab set to 16 ohms mono.
The amp's output is 600W at 8 ohms, but at 16 ohms it will become 300W. The power handling of your cab is 300W.
I'd do this if you didn't want stereo.

 

[ad0lescnts]

"It's pretty simple: the higher the impedance number, the smaller the load the speakers draw, and the smaller the load the amp is putting out. Although you should always try to match impedances (if your amp says 4 ohm, the cab should be 4 ohm) it is safe to have a higher impedance on the cabinet than on the amp. So for example, an 8 ohm cabinet will be fine with a 4 ohm amp output, and a 16 ohm cabinet will be fine with an 8 ohm amp output. But an 8 ohm cabinet will not be fine with a 16 ohm amp output. When I say fine, remember to take that with a grain of salt... really with a tube amp you want to keep these loads pretty close to recommended values. With solid state there may be more room for error, though in general, a transformer is a a transformer. "

Does this mean that if you hook up an 8 ohm cabinet to a 16 ohm head, the load isnt big enough for the cabinet and that is bad?? does this mean that underpowering is bad? or am i totally off? thanks

T

 

[Lt. Bob]

It's more complex than that. All amps are rated into various loads. We've all seen how amps wiil be rated at (I'm just making these up but it's typical) 100 watts into 8 ohms and say 150 watts into 4 ohms. Thats because the lower resistance allows more current through and as long as the amp's power supply is able, the amp will put out more wattage. To some extent, it's only limited by the amount of current the power supply can furnish. That's why something like a 28 watt Matchless will blow a cheap 100watt amp like a crate out of the room; it has a beefier power supply.
It's also frquency related. Frequency changes the impedance of the load and impedance; not resistance, is what an amplifier is "seeing" while operating. Some amps freak out when asked to run into shifting phase angles; which is going on as the frequency changes although how much can be very different between different brands of speaker.
So it depends a lot on the amp. I haven't seen many amps that were set up for a 16 ohm load except for the ones that have a variable setting. It's usually safe to assume an amp is ok with 8 ohms.
In your case....I'd run stereo into 8 ohms. I dislike bridging amps. Many of them exhibit wierd behaviour and sounds when bridged plus they get more suseptible to impedance effects when bridged.

There, I guess I just totally confused things huh?