4 Ohm/200 Watt Head to 8 Ohm/140 Watt Cab

[Lt. Bob]

Would the amp try (for lack of a better was to say it) to double but run out of supply or thermal capacity, so they derate it to reflect that?

well, they don't derate it ...... it actually only puts out that much power.
But the reason is what you said, the power supply can't provide enough current.

So when you halve the impedance, it does 'want' to double the output but can only do so if the power supply will provide enough current.
Some amps do manage to double power output when impedance is halved and that's because they have robust power supplies.
I've seen audiophile power amps that'll double output down to 1 ohm!
But these are big money amps with huge power supplies ...... some of them will weigh over 100lbs and cost tens of thousands of dollars.

But typically, on the more modest amps we all have I see figures more along the lines of that CS800.

 

[Farview]

I always figured that the CS800 did that because they were lying about actually putting out 400 watts/side in the first place. Maybe rolling down hill with a stiff wind behind it...

 

[Lt. Bob]

I always figured that the CS800 did that because they were lying about actually putting out 400 watts/side in the first place. Maybe rolling down hill with a stiff wind behind it...

Nah ..... it's typical of very many amps.

Here's specs for one of Crowns' amps just picked at random.
1800W into 2 ohms
1565W into 4 ohms
1120W into 8 ohms

Crown is hardly a POS and you can see that it doesn't come close to doubling power as the impedance halves.
I see that "power doubles when the impedance halves" thing spoken as gospel quite often but it's not the case much of the time.
And if you have selectable impedance on an amp, then it might not change much at all.
My Ampeg V-2 has taps for 2,4 and 8 ohms and it basically puts out around 60 watts regardless of which ohmage speaker you use.

 

[Walt-Dogg]

I'm confused now. Will pairing a 4 ohm 200 watt head (solid state) with an 8 ohm cab effectively cut the power output to around half of the original output? I really can't afford to blow out any cabs right now.

 

[Lt. Bob]

I'm confused now. Will pairing a 4 ohm 200 watt head (solid state) with an 8 ohm cab effectively cut the power output to around half of the original output? I really can't afford to blow out any cabs right now.

It'll probably cut it to around 125 watts which is within the rating of that cab so I wouldn't sweat it.

Plus, back in the 70's they tended to be somewhat generous with their power ratings.
I'd be surprised if that amp actually puts out 200 watts.

 

[lbanks]

Sunn's power ratings were fairly accurate. If anything, a little conservative.

 

[Walt-Dogg]

All sounds good to me.

 

[dan1072231]

ok i got similer problem i got a spider 4 hd150 half stack (solid state) and i need to get a smaller cabinet the one im looking at is Bugera 212V-BK 2x12 which is 140 watts and can be a 4/16 ohm or a 2 x 70 8 ohm will this work or am i ganna blow the cabinet or destroy my head?

 

[Lt. Bob]

ok i got similer problem i got a spider 4 hd150 half stack (solid state) and i need to get a smaller cabinet the one im looking at is Bugera 212V-BK 2x12 which is 140 watts and can be a 4/16 ohm or a 2 x 70 8 ohm will this work or am i ganna blow the cabinet or destroy my head?

well, you can't use the 2x70 part of that cab. Leave it on either 4 or 16. Check the ratings on your amp (I'm not gonna do it for you .... if you don't have the manual it's out there is Google-land somewhere) and see where it's rated. If it's rated to handle 4 ohms I'd go with that.
If it's only rated to 8 ohms then get a different cab .... one that's 8 ohms. And I'm guessing, since you didn't say, that that amp is 150 watts so as long as the ohmage matches up ok that cab will be fine.

 

[Farview]

You also have to take into consideration that you probably aren't going to be running the amp on 10 constantly. The amp will really only be putting out it's maximum output when you have the thing jacked to the moon, and the speakers will really only burn up if you do that constantly for a decent length of time.

The amp will sound like crap long before the speakers catch fire.

 

[Lt. Bob]

The amp will sound like crap long before the speakers catch fire.

that's a built in feature!

 

[soundchaser59]

......to make the impedance 4 ohms to get a full 200 watts coming out of each cab, am I correct?

......Also to get the full 200 watts I could use one 4 ohm cab.

Both statements cannot be true. If your amp is 200 watts at 4 ohms, you will not get 200 watts "coming out of each cab" no matter how many cabs you plug in to it. The wattage out divides among the cabs. You cannot afford an amp that will put out 200 watts to each cab no matter how many cabs you plug in.

If you plug a 100 watt amp into a 100 watt cab that has two 50 watt speakers in it, each speaker is going to dissipate 50 watts, the total out of the amp will never exceed 100 watts. (speaking hpothetically, of course). It's the same whether you are counting speakers or cabs, the watts divides evenly among all the speakers or all the cabs, assuming all the cabs are identical. A 100 watt 4 ohm amp plugged into two 100 watt 8 ohm cabs is only going to deliver 50 watts to each cab.

I have a 50 watt amp, and I've played 100 watt amps. After 39 years of playing guitar I STILL cannot find any justification for any guitar player needing more than 100 watts. If you are playing some place really big and you need 80,000 watt Marshall amps, then what you really need is a better sound contractor.

 

[lbanks]

No guitar player needs anything more than 30 watts. There. That's the law. You can all return to playing Minecraft.

 

[Sky Blue Lou]

No guitar player needs anything more than 30 watts.

Need and want are not the same thing. Need does not become want as easily as the converse.

 

[Lt. Bob]

No guitar player needs anything more than 30 watts. There. That's the law. You can all return to playing Minecraft.

yeah ..... you believe that.

My Mark V is 10/45/90 watts .
I need my clean channel set to 90 watts to get enough headroom for what I like in a clean.
I do play on 30 watt amps and like 'em but for clean I prefer more power.

 

[mixsit]

That 30 watts? Yeah. Except for out doors, larger higher volume stages, playing clean..
The doubling to 60 or 100 watts isn't giving a lot more volume, but fact of the matter is that clean classic Twin' starts to get crunchy in that kind of situtation.

I could add though, run a pair of D120's in that Twin.. not quite so crunchy'.

 

[Lt. Bob]

The doubling to 60 or 100 watts isn't giving a lot more volume,

that's the key.
Since it takes 10 times the power to get a perceived doubling of volume, the difference between 30 watts and 100 watts isn't as great as it might appear.
To get twice as loud you'd need 300 watts so 100 is definitely louder but not even twice as loud.

For me it's all about headroom.
To get a pristine clean with no breakup; 30 watts usually won't quite do it for me.

 

[lbanks]

yeah ..... you believe that.

My Mark V is 10/45/90 watts .
I need my clean channel set to 90 watts to get enough headroom for what I like in a clean.
I do play on 30 watt amps and like 'em but for clean I prefer more power.

30's plenty. I also think singers need to learn to move the equipment they use.

 

[mixsit]

30's plenty. ...

Yes. Exactly. Totally. 100%

Well, except when it isn't exactly true ..at all.

So... now what?
" Shure, but if you need more put a mic on it an.." etc.

Hmm. Yeah, '30.. Plus a few hundred on a floor monitor an...



(Hey and just so's you know... I still have the D120's but the Twin's long gone. Run a LoneStar Special Compact.

 

[lbanks]

And while I'm at it... Bassplayers should never have to carry more than 3 cabs. Any more is showing off.