# Urgent OHM question



## torndownunit (May 14, 2006)

Hey guys, quick question. There is an amp for sale locally I want try out. It has 2 4 ohm outs. I only have an 8 ohm cab. WOuld it be ok to plug in the 8 ohm cab just to run a quick test on it? 

I am planning on heading out soon, so any quick reply is appreciated lol.

Thanks


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## zurn (Oct 21, 2009)

There's no real danger, the amp wont sound as loud though.


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## hollowbody (Jan 15, 2008)

Yeah, if it's just to try it out, should be fine.


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## torndownunit (May 14, 2006)

Quick question again. It's not a topic I know a lot about. Any heads I have owned have had different inputs for different ohms.

There is 2 4 ohm outs on the this head. What are my options as far as speaker combinations? If I ran one of those outs to a 2x12 cab, would would the ohms of the 2 speakers need to be?

The head is a good deal, but I am just trying to figure out if it could be too much hassle/cost to get a speaker solution worked out. I only have a 1x12 cab with an 8 ohm speaker and one other 8 ohm speaker lying around that could go in an another cab.


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## greco (Jul 15, 2007)

If you ran the *2 x 8 ohm speakers wired in parallel* you would have 4 ohms total impedance. You could then run them from the 4 ohm setting on the amp.

Cheers

Dave


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## bluzfish (Mar 12, 2011)

It will all depend on the output transformer and how the output side is tapped. The labelling of the jacks on the back panel is subjective to how you think is the way someone else would logically think.
One thing for sure - it would be extremely unusual to have an output transformer to expect a total load of 2 ohms (2x4 ohm speakers in paralell). So if the manufacturer's logic is to plug one 4 ohm speaker into each jack, it would only be most likely to accomodate some special speaker line the company manufactures - a bad decision for the user.
More likely, the 4 ohm labelling on each jack means that one 8 ohm speaker is meant to be plugged into each jack for a total paralell load of 4 ohms.
Even that is unusual however because commonly, most output transformers are designed to accept a total load of 16 ohms with optional taps for 8 ohms and 4 ohms. These loads may be distributed between, usually, 1 or 2 jacks with labelling indicating what the total load would be when the speakers' individual loads are equal and divided by 2 (i.e. 2 x 8 ohm speakers in paralell would be 8 ohms divided by 2 to equal 4 ohms).
Although there is no industry standard, this is what you might expect. The only way to know for sure is to find a manufacturer's label on the actual output transformer and look up the specs.
Here is the safety information. You can always plug a higher impedence speaker into a lower rated impedence speaker output without any harm to the amplifier (i.e. an 8 ohm speaker connected to a 4 ohm rated jack). The tone and power output will change but the amplifier will not be harmed. To plug a lower impedence speaker into an output rated at a higher expected impedence will cause the tubes to work harder and burn out quicker at a rate depending on the quality of the other components and the circuit design. But you can't expect the amp to sound the same as when the impedences are matched.
As an aside - suppose you have an amp with 4, 8 and 16 ohm jacks. You can mix and match speakers of equal or different impedences by calculating the total load and plugging the individual speakers into the output jacks that equal that load.
i.e. you have 4, 8 and 16 ohm output transformer taps on the amp and you have one 8 ohm speaker and one 16 ohm speaker. You would add the speaker impedences together and divide by 2 for a total load of ((8+16)/2) which equals 12 ohms. Plugging the 8 ohm speaker into the 4 ohm jack and the 16 ohm speaker into the 8 ohm jack will normally be tapping the output transformer at 12 ohms (4 ohms + 8 ohms) which is an accurate impedence match.
I know this is a little long winded. Sorry for being a great big tech nerd...


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## greco (Jul 15, 2007)

bluzfish said:


> ..... you have 4, 8 and 16 ohm output transformer taps on the amp and you have one 8 ohm speaker and one 16 ohm speaker. You would add the speaker impedences together and divide by 2 for a total load of ((8+16)/2) which equals *12 ohms*.


Doesn't an 8 ohm speaker wired in parallel to a 16 ohm speaker calculate out to a total of 5.33 ohms?

If the OP used *one 4 ohm output jack on the amp* and *wired his 2 speakers of 8 ohms each in parallel* he should be fine....*Yes? .....NO?*

Cheers

Dave


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## WCGill (Mar 27, 2009)

Yes it will be 4 ohms. 5.3 ohms is the correct impedance of an 8 and 16 ohm speaker combination paralleled. Tube amps are generally very forgiving of impedance mismatch so trying out the amp with an 8 ohm load is no big deal. In differing with an earlier post and several before that I've seen here, it's easier (generally) on an amplifier to drive a lighter load than a heavier one, ie a 2 ohm speaker load into a 4 ohm tap is preferable to an 8 ohm load.


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## bluzfish (Mar 12, 2011)

greco said:


> Doesn't an 8 ohm speaker wired in parallel to a 16 ohm speaker calculate out to a total of 5.33 ohms?
> 
> If the OP used *one 4 ohm output jack on the amp* and *wired his 2 speakers of 8 ohms each in parallel* he should be fine....*Yes? .....NO?*
> 
> ...


 Oops. I should have proof read my response a little more diligently. You are right. The transformer winding taps are in series so you add the output winding tap ratings you are connecting to together for the total expected load. For speakers in paralell, you add the speaker impedences and divide by the number of speakers to determine the total actual speaker load. For speakers in series, you simply add the individual impedences to determine the total load. In any combination, the lower impedence speakers will sound propotionately louder than the higher impedence speakers.

I would guess the labelling on the jacks to mean they are wired to a 4 ohm tap and then wired in paralell to accept a total load of 4 ohms (2 x 8 ohm speakers in paralell = 4 ohms, i.e. one 8 ohm speaker connected to each jack). Anyway, that has been my experience for the last 45 or so years on guitar amps.

However, again, there are no standards, so unless you know the OP xformer specs, you can only assume that logically, transformers with a 2 ohm tap are extremely rare in guitar amps. At least since the 50's guitar amp speakers have been almost exclusively 4, 8 or 16 ohms and guitar multiple speaker cabinets designed to accomodate 2 x 4 ohm speakers in series or 2 x 16 ohm speakers in paralell. Guitar amps typically are designed for an 8 ohm load with accommodation for adding another 8 ohm extension speaker in paralell.

In any case, plugging an 8 ohm speaker into an expected 4 ohm load is not harmful to the amp.

Thanks for calling me on that.

Eric


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## bluzfish (Mar 12, 2011)

BTW you can find the same information on speaker impedence matching in any Mesa Boogie amp manual.

Eric


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## keto (May 23, 2006)

WCGill said:


> Yes it will be 4 ohms. 5.3 ohms is the correct impedance of an 8 and 16 ohm speaker combination paralleled. Tube amps are generally very forgiving of impedance mismatch so trying out the amp with an 8 ohm load is no big deal. In deference to an earlier post and several before that I've seen here, it's easier (generally) on an amplifier to drive a lighter load than a heavier one, ie a 2 ohm speaker load into a 4 ohm tap is preferable to an 8 ohm load.


Wow, you're the amp maker and so I defer to your knowledge, but for 15 years I've been reading the opposite. I always thought it was safer for 8 ohm amp to run 16 ohm speaker, but never a 2 ohm speaker. Wasn't this why some of the old Fender amps, with 2 ohm outputs, were virtually indestructible? You could plug them into anything and never risk blowing them? And old Marshalls, often 16ohm amp tap only, are risky to plug into something 'smaller'? I've always remembered it as 'amp goes into' as in, a math calculation. 8 (ohm amp) goes into 16 but does not go into 4 (or 2).


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## greco (Jul 15, 2007)

keto said:


> ........... I've always remembered it as 'amp goes into' as in, a math calculation. 8 (ohm amp) goes into 16 but does not go into 4 (or 2).


I have concluded (rightly or wrongly....I am not totally sure now...LOL) from having read several threads on this topic in the past (many in this forum) that a 100% difference in mismatch of impedance is OK (either way) and this is what I have remembered/used. 

I remember Wild Bill saying that the 100% mismatch only really becomes a concern (re: being hard tubes and output transformers) if you are playing at very high volumes for extended periods of time.

I'm old fashioned......I always try to match impedance...it helps me sleep better at night...LOL

If you really want your brain to hurt......start talking about how impedance varies with frequency (not now, not in this thread, not ever...please....LOL).

Cheers

Dave


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## WCGill (Mar 27, 2009)

As a tube amplifier drives a higher load than designed to, the risk of flyback voltages in the output transformer is increased, especially if the amp is being pushed, has severely mismatched output tubes, or anything else that compromises the design. There is less chance of this with a lower impedance. Fenders it seems, had less OPT trouble than Marshalls (my blonde Bandmaster being an exception), possibly a combination of better transformers and less rock 'n roll. :rockon2:


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