# 4 ohm into 8 ohm cabinet...?



## 23cicero (Mar 25, 2008)

Hi, excuse my ignorance but I have a question: Can I run a 4 ohm amp into a cabinet with an 8 ohm speaker without any problems to either?

I have an old Fender Vibro-Champ (4 ohm) and I'd like to see what it sounds like through a bigger speaker and I only have one cab with an 8 ohm speaker.

Has anyone done something like this before? Is it worthwhile to do so?

Also, assuming that it's okay, do I have to disconnect the speaker on the amp first or do anything else in particular?


Thanks for any help you can give me.


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## Jeff Flowerday (Jan 23, 2006)

I've moved this to the tech forum, you'll most likely get a response from our experts faster here.

Unfortunately I've burned the brain cells which used to know in which way the mismatch would be allright for a short period and by how much.

Either way wait and see what the experts say.


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## Ripper (Jul 1, 2006)

Tube amps are very forgiving about impedance mismatching. Putting your 4ohm amp into an 8Ohm speaker is fine. It's better to go up than down, as long as you don't go too far with it. I remember an old tech told me one time that going double was a good rule of thumb for tube amps, IE: 4x2=8 so you are okay with 8 ohms. I run some 4 ohm heads into 8 ohm cabs all the time and have never had a problem. Don't however do this with solid state amps, they won't take it. This only works for tubes.


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## james on bass (Feb 4, 2006)

I don't know much about tube amps, but a head that is 4 ohm stable will be fine with an 8 ohm cabinet.

A 4 ohm stable amp will not like any impedance lower than 4 ohms.
I.E. 8ohm cab + 8 ohm cab = 4 ohm load (what the amp sees).
4ohm+4 ohm = 2 ohms. 4 + 8 ohm = 2.67 ohms approx.


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## keto (May 23, 2006)

The way I remember it is "head goes INTO cab" and do the math - if the result is less than 1, I don't do it. So if head is 8 and cab is 4 that's a NO GO, but if head is 4 and cab is 8 that's (GENERALLY, YMMV, THE RISK IS YOURS, ETC ETC) OK.

A further illustration of this is that the Ho/Ultimate attenuator runs at 30 or 32 ohms and any head 4ohms and up can be run into it. They give a firm warning not to use a 2 ohm amp (old Fenders can be, Super Reverb for example iirc) and they build a special unit for that instance. Even with a 4ohm head I'd think twice about using it, but my Traynor is 8 and works fine, my Hiwatt has a tap for 16 and that's about ideal.


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## nonreverb (Sep 19, 2006)

Ripper said:


> Tube amps are very forgiving about impedance mismatching. Putting your 4ohm amp into an 8Ohm speaker is fine. It's better to go up than down, as long as you don't go too far with it. I remember an old tech told me one time that going double was a good rule of thumb for tube amps, IE: 4x2=8 so you are okay with 8 ohms. I run some 4 ohm heads into 8 ohm cabs all the time and have never had a problem. Don't however do this with solid state amps, they won't take it. This only works for tubes.


Yer close but the rule for solid state amps is the opposite. Solid state amps will deliver a certain amount of current into a load they are designed for. If the amp wants to see an 4 ohm load, it will be stable with a 4 ohm, 8 ohm or 16 ohm load. The only cost will be less output wattage as the impedance increases. It will not be stable with a 2 ohm load...this is when things go boom. This is because the transistors will try and deliver the extra current to the 2 ohm load which is beyond their design capability...next stop...repair shop:smile:


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## 23cicero (Mar 25, 2008)

Hey, thanks for all your help guys. I was pretty sure I was okay but, you know, I had to be sure.

The second issue is: the old Vibro-Champ has an RCA jack for the speaker not the typical 1/4 (I don't know what you call them -- you know, the 'normal' ones). Can I get some sort of an adapter that will allow me to plug in my speaker cable or how should I go about connecting my cab to the RCA jack?

Thanks again, Scott


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## Gunny (Feb 21, 2006)

Yes. I have one of those adapters from a few years back. Got it at Radio Shack. If the "Source" ie Circuit City nearest to you is still open for business, you can get one there. Not an expensive item.
If you're really stuck, let me know and I'll send you mine.


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## 23cicero (Mar 25, 2008)

Hey Gunny, thanks for that. I did a little more digging online and found it out. I'm sure I can find one this weekend here in Calgary. If, for some crazy reason I can't I'll let you know.

I'll also send a note and let everyone know what I think of the Vibro-Champ through a 112...

Thanks again everyone. Problem solved.


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