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My amp's highest output is 8 Ohm and the only cab I have right now is a 2 x 12 wired in series (I think), which is additive making it 16 Ohm (again, I think - Correct me if I'm wrong).

The manual for my amp says that the 8 Ohm output can be used with the 16 Ohm cab in what it calls a "Safe Mismatch".

What qualities can I expect in the sound when using a "Safe Mismatch" in this manner? The manual neglects to say what a "Safe Mismatch" actually means and how it will affect the sound produced.

As requested in some answers/comments:

It's a Mesa Boogie Mark V:25, tube amp, with selectable 25 or 10 W output for each channel.

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    A quick research suggests that this is different in tube vs. solid state amps. It would probably help to get a better answer if you can specify what amp you're using. – user13400 Dec 1 '14 at 11:21
  • the causes are slightly different, the results are similar – Tetsujin Dec 1 '14 at 11:24
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    With 8 ohms output from the amp., simple safe rule is any speaker >8 ohms is fine, any speaker <8 ohms is not. – Tim Dec 1 '14 at 13:38
  • I wouldn't push that rule too far, your 250Ω headphones are not going to do it much good at all ;) – Tetsujin Dec 1 '14 at 17:14
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    @Tetsujin: no, any solid-state amp is actually completely fine with headphone impedance. – leftaroundabout Dec 2 '14 at 13:21
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A tube power amplifier (like the final stage in your Mark V) has to have an output transformer to lower the output impedance (the "Ohms") to a level appropriate to drive a speaker. What does that mean? Well if you know what voltage and current and power are, then one way to look at impedance is how much of the power you are putting out is in voltage and how much is in current? That ratio is one interpretation of impedance.

The ratio of voltage to current coming out of power tubes is relatively high. But you need lots of current to make a speaker move, so you have to trade some of that voltage in for more current. That's what a transformer does. How much do you trade in?

The answer to that question is chosen by the amp designer and is your amp's rated output impedance. That means your amp is designed to put out its rated power with a specific ratio of voltage to current. The amp will pretty much put out how much voltage it's putting out in any situation, and it's designed to put out a certain amount of current along with that. Designed meaning that if it is not allowed to put out all the current it can, then you won't get the full rated power, meaning your sound will be quieter and maybe less clear. However, if your amp is allowed to put out more current than it is supposed to, then bad things can happen.

Current creates heat, more current is more heat, and too much heat can melt things. Like your output transformer. If your transformer isn't the first to go, then there are plenty of components in the amp that can be melted or just destroyed in other ways by passing too much current.

So you have to hold your amp back from running crazy with current. This is what a speaker's nominal load impedance number is meant to indicate. It's not a precise number, but it's good enough for making sure you're not letting your amp run away with itself and overheat.

Here's the short short version: If your speaker cabinet's nominal load impedance is equal to or greater than your amp's rated output impedance, you will be safe. Safe at least in terms of not overheating your amp. If the speaker's impedance is higher, as stated before you won't get the rated output power and there may be some tonal changes. There are lots of interesting interactions between a speaker cabinet and an amplifier, so how the sound will change isn't so easy to predict, but if the speaker impedance is higher, you can safely experiment and found out how it sounds.

One more thing to check is that the rated power dissipation of the speaker cabinet is greater than the rated output power of the amp. That way all the current coming out of the amp won't melt the speaker.

If you ever get into live sound PAs, just know that it's a different world because those amps are not tube amps. You still need to pay attention to power levels for sure, impedance not so much, and the rules of thumb will be different. But that's not relevant to your question. I just didn't want you to think back to this years later and think you can set up your PA the same way as your Mark V.

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If they're 2x8 Ohm in series, then yes that would give 16 - but guessing is no good.

3 ways to find out, check the maker's plate on the back, get a multimeter & test it, or take the back off & look.

For an annoyingly under-technical explanation, read on...

The reason 16 is 'safe' is that amps have a tolerance; 8 - 16 is pretty standard. Some can work down at 4 Ohms & a very few at 2. Running an amp into too low impedance will suck it dry (not a technical term ;-) & kill it pretty quickly at high volumes, through overheating.

Running into too high an impedance would really start to impact your output volume & eventually overload a valve amp's power stage.

If you really want brain-hurting detail, see How are speakers matched with audio amplifiers? (avoiding overloading either)

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16 ohm speakers are unlikely to be a problem. The term "safe mismatch" is really a misnomer. Amplifiers are not "matched" to the speakers. The amplifier output impedance is normally very low. But the amplifier has voltage, current and power limits that should not be exceeded. Additionally, valve amplifiers don't like very high impedance loads as stray inductance in the output transformer can result in very high voltages being generated. These can cause flash-over in the output tubes and/or breakdown of the output transformer insulation.

Your 16 ohm speakers will have one quite predictable effect - if the amplifier is rated at say 100W into 8 ohms, it will only deliver approximately 50W into 16 ohms.

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I am not a technical savy person per say! But through experience I know that running a 16ohm speaker with a Tube Amp rated at 8ohm will not blow up your amp, only thing I have noticed is volume difference, earlier breakup of the tubes, and some noise! I have run them mismatched for years with no real technical issues with transformers, or any other component other than Tubes! The Tubes really take the blunt end of it all, they may run a little hotter than normal! But in all my years of playing and experimenting with amps and cabs, I have never had an issue!

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    Just don't do it the other way round. – Ian Ringrose May 24 at 16:59
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Personally, I would never trust a "Safe Mismatch"... Obviously, systems are protected against those who don't care and don't understand impedances, but even though they say it is safe, it will reduce the life span of the amplifier. Tube amplifiers are more fragile to these changes mostly because the tubes heat a lot already when working like they're supposed, if you make them work like it isn't supposed to, they will heat even more...

Personally, if you really have no other choice, I would open the cab and connect only one speaker to the input, making it match the 8 ohm, you will lose the headroom and power will be half, but you'll be safe until you have another option.

For further response, the amp's type would help (tube/solid state)

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    It's not the valves that are damaged, it's the op transformer. – Tetsujin Dec 1 '14 at 11:53
  • Although you are right, and most common problems are in the transformers, I don't think it's too good for the valves either... – Antero Duarte Dec 1 '14 at 16:02
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    My experience: A lead from amp to speaker shorted out while setting up at a gig recently. It blew an internal fuse in my 1970 Marshall JMP. But before the fuse blew, one of the EL34 power valves glowed a really lovely and very bright purple, got incredibly hot, and stopped working. Running with too low resistance definitely can mess your valves up - although a short circuit is an extreme example. – user2808054 Dec 2 '14 at 12:33
  • @user2808054 my point exactly... – Antero Duarte Dec 2 '14 at 14:50
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    Too low a load impedance is not considered a safe mismatch for the reasons in the comment. Too high a load impedance is safe. – Todd Wilcox Mar 27 '15 at 1:35

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