What's the easiest way to understand/remember ohms, watts, etc. for speaker cabs?

jonnyglock9

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For some reason I am baffled by ohms, wattage, and compatibility between different heads, cabs, speakers etc. probably because I've mostly used combo amps in the past. I feel like this is pretty elementary stuff that I should know after playing guitar for 20+ years, but alas, I am at a loss!

Can ya help me out?

I just bought a Friedman JJ Jr. 20 watt amp, with an 8/16 ohms switch on the back.

I have an old Egnater 1x12 extension cab that has a pretty similar look, that has an 80 watt, 16 ohm speaker.

I also have a Celestion 12" 75W G12H-75 Creamback 8ohm speaker.

I want to take the Egnater speaker out of the cab, and put in the Creamback. Will this work without issue or risk of damage to any of the equipment?
 

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grumphh_the_banned_one

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One speaker matched to one amp is very easy.
Whatever ohm value is printed on the speaker is what you set your amp output ohm selector to.

(Or if your amp doesn't have a selector but multiple outputs, plug the speaker into the (clearly) marked output jack that corresponds to the speakers nominal ohm value.)

Watts - as long as your speaker is rated equal to, or higher than your amps output rating is, everything is fine.*

In your case, all you have to do is swap out the speaker, set your amp to 8 ohm and play your little heart out.
No problems whatsoever.



The less simple stuff (it's still fairly easy once you know how) is only if you want to connect several speakers simultaneously, but let that wait until the need arises... :D


* Note that tube amps, very often are capable of putting out far more power than their rated watt output, so that if you intend to dime your amp over prolonged periods (e.g. playing hard rock on a 20W amp with a heavy hitting drummer) you'll need a much higher rated speaker (twice/triple the amps rating) - but if you only play at home at moderate volumes this doesn't really matter, as you would be unlikely to ever need all the power the amp can deliver....
 
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SWeAT hOg

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Speaker and amp ohms should be matched for best performance. With many amps, a cab at double the ohm rating can be run safely, i.e. amp output is 8 ohms and cab is 16.
 
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grumphh_the_banned_one

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Speaker and amp ohms should be matched for best performance. But a cab at double the ohm rating can be run safely, i.e. amp output is 8 ohms and cab is 16.
As always, it depends.
Some (mostly vintage) amps had very badly specced transformers, and those could not handle mismatches very well.
On other amps it is safe to mismatch one step up or down.

And there are also conflicting opinions on whether mismatch is better/safer "up" or "down".

One thing i would say is that "infinite resistance" (as in no speaker connected at all) is very bad for the transformer, so that from that standpoint, having a lower ohm cab conncted to a higher ohm amp output should in theory be "better" than the other way around.


The safest to do is still just to match your amp impedance to your speaker/cab impedance.

Unless
the manufacturer explicitly mentions that their amp can tolerate a mismatch of one step up or down, in which case it is safe to experiment
 

efstop

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I would agree that very old amps should have a properly matched speaker with the correct ohms rating. A recent Marshall amp that says "16 OHMS ONLY" should not have a lower resistance speaker connected. My DSL1HR has such a rating. Other amps that have taps and a switch are obviously fine with lower rated cabs. Some amps have a minimum rating and no taps. They are also fine with higher rated cabs, but not ones below that. No 4 ohm cabs into an 8 or 16 ohm amp.

The issue is resistance. Lowering the resistance allows the transformer to push more power, possibly exceeding heat limits and allowing the output tubes also to overproduce. Like a runaway truck down a hill.

Increasing the resistance simply chokes the power output, and does no harm. Like a loaded truck going uphill. I have several amps that have a minimum 8 ohm load, and I've used them with 16 ohm cabs without issue.

The Class 5 (16 ohm only) was tested by the designer at 8 ohms, 4 ohms and no load, and survived. He doesn't recommend it, naturally. Suddenly switching the load by moving the switch, unplugging a speaker, or plugging in a speaker because you forgot while the power is on can have disastrous consequences.
 

ErictheRed

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The issue is resistance. Lowering the resistance allows the transformer to push more power, possibly exceeding heat limits and allowing the output tubes also to overproduce. Like a runaway truck down a hill.

Increasing the resistance simply chokes the power output, and does no harm. Like a loaded truck going uphill. I have several amps that have a minimum 8 ohm load, and I've used them with 16 ohm cabs without issue.

The Class 5 (16 ohm only) was tested by the designer at 8 ohms, 4 ohms and no load, and survived. He doesn't recommend it, naturally. Suddenly switching the load by moving the switch, unplugging a speaker, or plugging in a speaker because you forgot while the power is on can have disastrous consequences.

Sorry, this is not correct at all. I've linked articles and written explanations about it in the past and don't want to go through it all again, but the general gist is that whenever you have an impedance mismatch (whether the load is higher or lower), you have reflected power, power that is not being transmitted into the speaker (load) but instead reflected backwards. Whenever you have an impedance mismatch, you deliver less power to the load, always. Lowering resistance does not "allow the transformer to push more power."

The other parts are also incorrect, sorry. There's a lot that I don't know about particular vintage amps or whatever, but I will unabashedly play my electrical engineer card here.

It is true that certain amps (many Mesas, for instance) have been designed to be protected against slightly larger impedance loads, such as using a 16 Ohm speaker with an 8 Ohm output transformer. I think that this is the reason why many people think that it is safe to use a higher impedance load, but unsafe to use a lower impedance load. When it comes to safety of mismatching your load, it all depends on the actual amp design, and I wouldn't use a rule of thumb for all amps. Even in those cases (like Mesa) where they may be safely run with a higher impedance speaker, the speaker receives significantly less power than if matched correctly.
 
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jonnyglock9

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Thanks, all!!

Isn't it also true that many modern amps (such as this Friedman JJ Jr.) can safely be on, with NO speaker connected, without any issues? I recall reading that somewhere.
 

ErictheRed

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Thanks, all!!

Isn't it also true that many modern amps (such as this Friedman JJ Jr.) can safely be on, with NO speaker connected, without any issues? I recall reading that somewhere.

It's true, but I couldn't tell you offhand which amps are safe and which are not. A great many number of amps can be seriously damaged by powering up without a load, especially if you start playing through them before you realize your mistake. It's not really worth it, best to be in the habit of having a load.
 

ErictheRed

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Match Ohms exactly, and make sure that your cab is rated for at least as much power (Watts) as the maximum output from your amp. Safe and easy!
 
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HogmanA

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SpekerImpedanceChart-848x656.jpg


Interesting chart from Hughes and Kettner, from this blog here:


I have never really worried about it myself, especially just 1 setting away.

I did blow a Carlsbro TC60 vintage valve head when I was 17 though. I wired 4 8ohm speakers in parallel, making a 2ohm load. But it lasted for months with daily use, as well as gigs!

When I opened it (I know, lucky I'm still here), there were three burnt resistors and burnt circuit board that I could see. It was never repaired.

In the blog, they say going in to a 2ohm load is the real danger area.
 

ErictheRed

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Also remember that all of those impedance values have tolerances, so any given 8 Ohm speaker could measure 7.1 Ohms or 8.6 Ohms, etc. So naturally you always have some amount of imbalance. Still, it should be relatively small if you take the printed values at face value.

I really wouldn't make it a habit of running mismatches unless you know that your amp was designed to handle them, though, no matter what an image from a particular website says. That might be correct for many amplifiers, but definitely not for all, and generally speaking the older the amp, the less likely they are to be able to handle a mismatch.
 

grumphh_the_banned_one

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View attachment 457317

Interesting chart from Hughes and Kettner, from this blog here:
If i owned that amp from that company, i'd think that the chart is correct.
After all, german engineers... They don't like spitzensparkenfunken!!!

The problem is that this chart is not universally applicable to every tube amp out there, as amps are designed differently.
 

HogmanA

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I wonder how much leeway there really is though. I suspect more than is being suggested in this thread.
The speaker impedance ratings are nominal, because it will change with frequency, amount of speakers, type of cabinet, etc etc. All that is factored in to how robust an amplifier must be, as it is understood by manufacturers that amps will be used with a vast array of speakers, combinations and cabinets. The amp settings may be more to do with the quality of sound than longevity of the amp. I said 'may' - purely speculative.

Excerpt from another blog, company called Aperion - I think they make Hifi amplifiers:

Speaker Impedance is Really Just an Average
Tweeter-0dB-3dB-Impedence-Difference-for-New-VGB-e1525314693594.jpg

The reality is that a speaker’s impedance will actually vary quite widely depending on which frequencies it is producing. There are a few reasons for this, but there are two main factors. First, every driver has a resonance frequency at which it moves freely once it is tapped or otherwise engaged. At that frequency the free movement of the driver generates what is known as “back electromotive force” which opposes the flow of current and creates a large spike in impedance. The other reason for impedance variance is an increase in inductance of the voice coil at higher frequencies which also causes the impedance to go up. To illustrate this, take a look at the impedance graph of our Verus II Grand Bookshelf. There are three big impedance peaks in the graph, which correspond to the resonance frequencies of the port, woofer and tweeter moving left to right. Then on the right hand side of the graph you can see a gradual rise as the frequencies get higher due to inductance. Looking at the graph one thing should be clear, there’s a wide range of impedance values across the speaker’s frequency response, which is true for nearly all speakers. So when we say that the speaker’s impedance is “6 ohms”, what do we mean? Well if you look closely, you can see that the impedance only dips below 6 ohms ever so briefly around 200 Hz. For the rest of the frequency response, the impedance is well above 6 ohms. Because of that, we can say the speaker has a “nominal” impedance of 6 ohms. That is, the speaker’s impedance will not be below 6 ohms for the vast majority of the time. For further reading, you can check out more technical details behind speaker impedance here.

So going back to the original question, why did we make a 6 ohm speaker when we could have just designed an 8 ohm speaker and had it draw less power? The honest truth is that we would have had to make sacrifices in terms of sound in order to get the impedance higher. Which is more important, a speaker that we feel delivers the signature Aperion sound that our customers know and love, or a speaker that is slightly easier to drive? Of course we chose sound as the most crucial attribute for our speakers. The thing is, there really aren’t any hard and fast rules when it comes to nominal impedance. I have no doubt that there are in fact speakers out there that are labeled as “8 ohms” that have impedance graphs with dips the same or more severe as our curve. Here at Aperion, we like to be honest and not overstate specs, but of course the same can’t be said for every speaker manufacturer out there. Fudging specs aside, even a speaker that is unquestionable 8 ohms would not draw that much less power than our Verus II Grand Bookshelf. Especially when you consider that listening volumes are a much larger factor. Johnny Rocker who listens to his 8 ohm speakers at 110 dB is absolutely taxing his amp more than someone listening to a 6 ohm speaker at a reasonable volume.
 

ErictheRed

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A transformer's impedance also varies with frequency. Transformers and speakers are both built on induction. A transformer is essentially two inductors physically placed side by side in a magnetic field so that they induce emf (electro motive force) in each other. A speaker is essentially just an induction motor: the current in the rotor (in the motor) or the voice coil (in the speaker) generates a magnetic field which applies a force. In a speaker, the force displaces the coil (which is attached to the cone) and in a motor, the force rotates a shaft.

The part of a transformer or speaker's impedance that varies by frequency is given by the equation X = 2*pi*freq*Inductance (typing on my phone). There is also the DC resistance of the speaker and transformer to take into account, but that piece doesn't vary.

Guitar transformers and guitar speakers are designed to be used together. Hence they have relatively similar impedance curves with respect to the audio frequency spectrum. So as the speaker's impedance varies, so does the transformer's.

I haven't made any kind of exhaustive study or comparison in the lab of various guitar amp transformers and speakers, but I'm sure that many people (amp and speaker manufacturers) have. Ideally speakers and output transformer's would have the same impedance so that they would vary in the same manner across a broad range of frequencies.

This is all to say that yes, you should impedance match your transformer to your speakers. The fact that a speaker's impedance varies is not very relevant, because a transformer's impedance also varies in a similar manner, and I'm sure that an engineer somewhere designed the devices to play well together. At least they should have been designed by an engineer somewhere, who knows what you have if you're talking about a kit thrown together by God knows who.

In fairness there probably is a decent amount of leeway for impedance mismatches in many amps, but really, why risk it? It's not hard to match impedances. The vast majority of amp heads out there can operate with 4, 8, or 16 Ohm outputs via a simple switch on the rear of the amp. If you're talking about a combo, just buy the appropriate speaker, don't risk damaging your amp because you want to save $100 and use a speaker that you already have. If you're carrying your amp head on stage somewhere and the venue only has a mismatched cab, then accept the risk for that one night or use a different amp or bring your own cab. It's not difficult.
 
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HogmanA

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" ...It is also worth pointing out that the speaker’s impedance changes with frequency, hence the term nominal impedance used by all speaker manufacturers. Therefore, it’s not necessary to worry about exact speaker impedances, as many musicians do..."

©Stewart Ward - January 2010 Designer since 1967 Session amps1979-2007 Award-Session Since 1991 Morley JD10, AP10 & MixMatch 1996-1999
 

ErictheRed

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Well what does he mean by "exact"? I wouldn't worry about an 8 Ohm output going into a 10 Ohm load, but I definitely wouldn't put a 4 Ohm output into a 16 Ohm load.

Not trying to have a pissing contest here, some actual data would be helpful. Maybe later I'll see if I can find some impedance vs frequency curves of guitar amp output transformers to see how they compare to guitar speakers, but the last time I looked for that info regarding a Mercury Magnetics transformer, I couldn't find it published anywhere.
 

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