# Mismatching Impedances



## traynor_garnet (Feb 22, 2006)

Coming from the world of massive Traynor Bassmaster transformers, I have "mismatched" amp and speaker impedance without too much thought (one way up or down).

I have a Revolution 1 combo that I want to use at an upcoming gig but I don't like the stock speaker and would rather not buy a new speaker if at all possible. Right now, I am setting my amp for 8 ohms and using it with my external 2x12 cab (two 16 ohm speakers wired for 8 ohms). To save space, I would like to take out one of my 16 ohm speakers and install it in my Rev 1 Combo. This would mean I am running an 8 Ohm amp into a 16 ohm speaker.

I know this is a bit of a contentious issue but how dangerous is it to run my 8 ohm amp into a 16 ohm load? I very, very rarely play out and almost always have my Rev 1 hooked up to an external 8 ohm cab at home. 

It would be great if I didn't have to buy a new speaker I have little need for and I simply have no room in the car for my 2x12 cab. If safe, a brief mismatch/temp speaker install would solve a lot of problems.

Any thoughts would be appreciated. Are the Garnet trannies fairly robust?


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## greco (Jul 15, 2007)

traynor_garnet said:


> ......I want to use at an upcoming gig...... .


I remember many, many discussions here on GC about this topic. 

IIRC, the conclusion was that mismatch could be an issue if you are playing at higher volumes for extended periods of time.

However, I also remember many GC members stating that they had gigged with mismatched impedances on a regular basis and have not had any issues.

A coin toss?

Sorry, this post has likely been of no help at all...LOL.

Cheers

Dave


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## WCGill (Mar 27, 2009)

RG Keen says it's always better to drive a lower load than a higher one. That being said, if you're not pushing your amp flat out, a 100% mismatch either way probably won't hurt anything.


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## nonreverb (Sep 19, 2006)

Their output trannies are generally pretty good although I have replaced a couple on Garnet Pros. 
Tough call, you may never have a problem but then again if it does get dimed for some reason, the flyback voltage could send it into the netherworld. There's always a risk however slight it may be. 
Some amp's outputs can handle the mismatch well. The Fender Twin comes to mind.


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## traynor_garnet (Feb 22, 2006)

LOL, it seems this thread mirrors everything else I have read: you may be fine you may not be!

I wouldn't have the amp cranked and I wouldn't be playing very long (3-5 songs tops) but I don't want to be distracted by worry either. Strangely, I have tried to buy a speaker but every time I to to order or get one some strange delay creeps in. Argh, life . . .

TG


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## dcole (Oct 8, 2008)

I would say that you would probably be all right. A speakers impedance changes a lot anyways through out its bandwidth. Is it possible to throw an add up to see if someone has the same speaker / different impedance that they would trade you for, even for a short time?


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## sulphur (Jun 2, 2011)

I always thought that it was Ok to go from a lower impedance on the amp,
to a higher impedance on the speaker, not the other way around though.

You'll get a bit less volume that way though.
I have a 16 ohm in my YGL, on the back of the amp is says "4 ohm minimum", on the website, it says 8 ohms.

I found this...

Guitar amps are manufactured to operate at one or more specific impedance levels. For amps that only capable of running at the stated speaker impedance, issues will occur if the speaker is not matched correctly. An amp rated at 8 Ohms matched to a 16 Ohm speaker cabinet will result in volume loss due to the amplifier's inability to provide enough speaker voltage. In contrast, an 8 Ohm amplifier connected to a 4 Ohm speaker will work harder to provide the necessary voltage--usually causing amp failure. An amp's impedance statement can normally be found printed on the back of the amplifier near the speaker connectors.


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## WCGill (Mar 27, 2009)

sulphur said:


> Guitar amps are manufactured to operate at one or more specific impedance levels. For amps that only capable of running at the stated speaker impedance, issues will occur if the speaker is not matched correctly. An amp rated at 8 Ohms matched to a 16 Ohm speaker cabinet will result in volume loss due to the amplifier's inability to provide enough speaker voltage. In contrast, an 8 Ohm amplifier connected to a 4 Ohm speaker will work harder to provide the necessary voltage--usually causing amp failure. An amp's impedance statement can normally be found printed on the back of the amplifier near the speaker connectors.


Should be the other way 'round Jock. Driving a higher than rated load at high volumes may result in the aforementioned flyback voltage that will perforate the OPT's insulation and short the transformer.


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## Wild Bill (May 3, 2006)

sulphur said:


> I always thought that it was Ok to go from a lower impedance on the amp,
> to a higher impedance on the speaker, not the other way around though.
> 
> You'll get a bit less volume that way though.
> ...


With solid state amps, you're right! They don't really have a proper desired speaker load. As the load is lowered they simply put out more power! Raise the load and they put out less.

If you think about it, what happens if the speaker load gets TOO low? The transistors just can't handle the power they are handling and they burn out!

So transistor amps will have a MINIMUM speaker load! On the back you usually see a warning like "DO NOT OPERATE AT LESS THAN 4 OHMS", or whatever the engineer designed the amp to safely use as a minimum.

If you look at the data sheets for output tubes, you will find curves for power output vs output load (Output transformer primary load, of course, which will then translate it to whatever speaker load is necessary) along with the same for distortion. The two curves are rarely the same. Max power out will not be the same as lowest distortion figure.

If you are building a hifi power amp, you go for least distortion. If you are building a guitar amp you will probably go for max power.

The curves also usually do not have a definite, easy to see peak point. Instead, they have broad curves with a range of useful values.

That's why tube amps are not really that fussy about the speaker load, although as WC pointed out, a mistake too much on the high side might lead to arcing inside the output tranny.

Wild Bill/Busen Amps


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## sulphur (Jun 2, 2011)

Ah, I was waiting for some info from you amp guys.

Is it Ok to be running that 16 ohm speaker, with another 16 ohm in a seperate cab on the speaker out?
Is that parallel? Does it bring the ohm rating down doing that, or am I still running it all at 16 ohm?

Sorry, trying not to highjack this thread.


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## keto (May 23, 2006)

sulphur said:


> Ah, I was waiting for some info from you amp guys.
> 
> Is it Ok to be running that 16 ohm speaker, with another 16 ohm in a seperate cab on the speaker out?
> Is that parallel? Does it bring the ohm rating down doing that, or am I still running it all at 16 ohm?
> ...


Rough calculations:

16 + 16 = 8

8 + 8 = 4

4 + 4 = 2

These are not exact but close enough for rock and roll and any tube amp. So in your scenario, you're at 8 ohms, not exactly but close-ish.


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## traynor_garnet (Feb 22, 2006)

Well, I found out that the speaker out and external out on my Garnet are wired in parallel. So . . .

I put a 16ohm 12" speaker in my combo. Since I have a 16ohm 12" Cab at my jam space I will run that cab on the external out. Viola! An 8 ohm load and all is well.

Just so I know that the world is against me, however, my amp started making some noise this week. Sounds like a bad tube. Time to problem solve and start tube swapping . . .

TG


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## ampsplus (May 7, 2010)

Always go HIGHER than lower, this makes the impedance that the output tubes "see" higher, and they will work a bit less hard. If you go low, keep the volume levels lower than normal, it will run, but they will burn up faster at high levels, and/or short out quicker. It is all about reflected impedance, what the input of the transformer sees as it's mate on the secondary. The tubes and primary make 1 circuit, the speaker and secondary makes the other circuit.


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## WCGill (Mar 27, 2009)

It's just about impossible to "burn up tubes faster" with lower reflected impedances. The lower impedance places the output tubes on a point on the curve where they simply don't produce enough power to over-dissipate. With a higher-than-rated load, output power is also limited but flyback voltages generated by this mismatch can slowly or quickly degrade the insulation on the primaries, resulting in sound deterioration over time or complete transformer failure. Poorly designed transformers (some Marshall, just one example) may be more prone to this and factors such as playing at extreme volumes or intermittent output jack to speaker contact will certainly exacerbate the situation. So no, it's not better to go higher.


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## nonreverb (Sep 19, 2006)

Agreed, I worked for a number of years on computer monitors (CRT) and televisions. The high voltge is generated by a "flyback" transformer. Essentially designed to deliberately generate voltages beyond 25kV in some cases. Those voltages could easily punch holes right through the housing when things went wrong. You can imagine what's happening in your output transformer with a high load mismatch or worse, an open condition with the amp dimed.

As WCGill rightly points out, Marshall has a reputation for grenading output transformers due to high load mismatch and I always warn my Marshall customers to pay attention to impedance settings.



WCGill said:


> It's just about impossible to "burn up tubes faster" with lower reflected impedances. The lower impedance places the output tubes on a point on the curve where they simply don't produce enough power to over-dissipate. With a higher-than-rated load, output power is also limited but flyback voltages generated by this mismatch can slowly or quickly degrade the insulation on the primaries, resulting in sound deterioration over time or complete transformer failure. Poorly designed transformers (some Marshall, just one example) may be more prone to this and factors such as playing at extreme volumes or intermittent output jack to speaker contact will certainly exacerbate the situation. So no, it's not better to go higher.


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## doriangrey (Mar 29, 2011)

the easiest way for me to remember how impedance works over the years is a saying that goes, "low into high will fly but high into low might blow" ... I learned the hard way in that I had my 100 watt head set at 8 ohms and ran it into a 4 ohm speaker cab and my amp lit up like a Christmas tree and I had to take it in for repair.


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## WCGill (Mar 27, 2009)

doriangrey said:


> "low into high will fly but high into low might blow"


"Into" is the key word here, not sure if you mean into the head, if so, then your saying is correct. Your loading of 4 ohms into the 8 ohm setting should not have caused any damage under normal circumstances.


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## doriangrey (Mar 29, 2011)

WCGill said:


> "Into" is the key word here, not sure if you mean into the head, if so, then your saying is correct. Your loading of 4 ohms into the 8 ohm setting should not have caused any damage under normal circumstances.


hmmm...not sure what you are referring to when you say 'loading"...we are probably saying the same thing...it's my understanding that if you must use a non matched hookup, you can use the cabinet with higher impedance value with the amp at a lower setting. So an 8 Ohm cabinet with 4 Ohm amp setting is okay. This type of hookup will also influence the overall instrument timbre due to the fact that the output transformers will have to deal with different load (this is true for tube power amp section, not the solid state one). A hookup to avoid is where you have a lower impedance value cabinet connected to the higher impedance value amp. For example: 4 Ohm cabinet connected to the 8 Ohm amp. This type of hookup will stress output transformers and may cause damage. Don't use a tube amp without being connected to the cabinet (or some dummy load). Also, never use an instrument cable for connecting the amp and the cabinet. 

...copied and pasted most of that but that is basically my understanding in a nutshell.


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## keto (May 23, 2006)

doriangrey said:


> hmmm...not sure what you are referring to when you say 'loading"...we are probably saying the same thing...it's my understanding that if you must use a non matched hookup, you can use the cabinet with higher impedance value with the amp at a lower setting. So an 8 Ohm cabinet with 4 Ohm amp setting is okay. This type of hookup will also influence the overall instrument timbre due to the fact that the output transformers will have to deal with different load (this is true for tube power amp section, not the solid state one). A hookup to avoid is where you have a lower impedance value cabinet connected to the higher impedance value amp. For example: 4 Ohm cabinet connected to the 8 Ohm amp. This type of hookup will stress output transformers and may cause damage. Don't use a tube amp without being connected to the cabinet (or some dummy load). Also, never use an instrument cable for connecting the amp and the cabinet.
> 
> ...copied and pasted most of that but that is basically my understanding in a nutshell.


That was also my understanding, ie:

4 ohm head into 8 or 16 ohm cab = fine

16 ohm head (like, say, an old Marshall) into an 8/4/2 ohm cab was not smart or prudent.


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## greco (Jul 15, 2007)

Impedance mismatch/matching threads ALWAYS amaze me. 

I think of electronics as being mainly science-based with possibly a bit of "art form" added for flavouring. However, these threads can go on forever. 

How many ways can (assumedly comparatively basic) electronics theory be interpreted?

Rant over...sorry. 
Too many impedance mismatch/matching threads for one lifetime.

Cheers

Dave


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## WCGill (Mar 27, 2009)

Sorry guys but you have it bassackwards! Now for a decently built transformer of robust makeup a 100% mismatch either way won't matter but if anything is sketchy at all, better to put a cab having a lower input impedance rating than your amp's output rating. Got it now?


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## GTmaker (Apr 24, 2006)

greco said:


> Impedance mismatch/matching threads ALWAYS amaze me.
> I think of electronics as being mainly science-based with possibly a bit of "art form" added for flavouring. However, these threads can go on forever.
> How many ways can (assumedly comparatively basic) electronics theory be interpreted?
> Rant over...sorry.
> ...


My rant is just starting....
This whole thread is very confusing so I'll bring it down to my level and ask some questions.

I have a Fender Super reberb RI which has 4 tens wired up in a 2 ohm load configuration.
I'll assume that this is OK as this is stock.

For the sake of aguments and my next few questions, lets assume that I run my amp volume at 4.

If I add a 8 ohm speaker in the Ext. Jack.....is this OK or not OK ?

If I unplug the regular speaker jack and run an 8 ohm speaker there (no other speakers attached.) Is the OK or not OK?

If I unplug the regular speaker jack and run (2) 8 ohm speakers ( one in the regular and the other in the EXT. jack )
Is that OK or not OK ?

Thanks in advance for any input or advice.
G.


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## nonreverb (Sep 19, 2006)

With all due respect...wrong! This theory is correct for solid state amps but not tube amps. 
In solid state outputs, the transistors or FETs link directly to the load and load tolerance is more critical. Basically the higher the load the less current is drawn from the output. Attach a load lower than the the spec of the output and the transistors try to deliver too much current and BOOM...time for a service call.
In tube amps exactly the opposite happens but for different reasons. When an exceedingly large load or an open condition occurs, the voltage generated in the transformer primary has nowhere to go so it gets reflected back on itself creating huge voltage swings. This may not cause immediate failure in the transformer and some transformers can take it....but I wouldn't press my luck especially with a vintage Marshall or Fender. 
Connecting too small a load isn't great for the tubes but the stress is far less destructive.
Fortunately, tube amps can generally take more abuse regarding load variations when compared to solid state amps.







doriangrey said:


> hmmm...not sure what you are referring to when you say 'loading"...we are probably saying the same thing...it's my understanding that if you must use a non matched hookup, you can use the cabinet with higher impedance value with the amp at a lower setting. So an 8 Ohm cabinet with 4 Ohm amp setting is okay. This type of hookup will also influence the overall instrument timbre due to the fact that the output transformers will have to deal with different load (this is true for tube power amp section, not the solid state one). A hookup to avoid is where you have a lower impedance value cabinet connected to the higher impedance value amp. For example: 4 Ohm cabinet connected to the 8 Ohm amp. This type of hookup will stress output transformers and may cause damage. Don't use a tube amp without being connected to the cabinet (or some dummy load). Also, never use an instrument cable for connecting the amp and the cabinet.
> 
> ...copied and pasted most of that but that is basically my understanding in a nutshell.


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## WCGill (Mar 27, 2009)

GTmaker said:


> My rant is just starting....
> This whole thread is very confusing so I'll bring it down to my level and ask some questions.
> 
> I have a Fender Super reberb RI which has 4 tens wired up in a 2 ohm load configuration.
> ...


Running an 8 ohm speaker from the ext. jack is no problem because it's in parallel with your 2 ohm load so it's less than 2 ohms, probably about 1.6. Running a pair of 8 ohm speakers yields 4 ohms total load, a 100% mismatch, not a problem either. A single 8 ohm speaker might pose problems.

As nonreverb says, transistors are more prone to damage from lower than specified loads, unlike tubes. Ever notice that the output jack in your Fender shorts to ground when the speakers aren't plugged in?


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## Wild Bill (May 3, 2006)

Output transformers are fixed ratio devices. The usual preferred load to the plates of a pair of 6L6s is about 6600 ohms. The OT will drop this down to the speaker load, which is usually 8 ohms but doesn't have to be. Look at the Super Reverb example, where there are 4 speakers all in parallel. Paralleling speakers is like paralleling resistors. Ohm's Law says the total keeps going down.

Since the step down ratio (or step up, if you are looking at it from the speaker side first) is fixed, If you put a 16 ohm speaker into the jack the OT secondary now sees double the expected 8 ohm value - 16 ohms. So on the primary side, those 6L6s will also see twice the expected load - 6600 x 2 = 13,200 ohms!

Now the output power of the amplifier is a big voltage swing across the OT primary. Lower loads will keep the voltage peaks down. Higher loads or worse yet, no speaker plugged in at all, which is an infinitely HIGH load will allow those peaks to go SUPER high!

Transformers are made of copper wire with a thin enamel insulation. If the voltage swings have peaks that are higher than the insulation rating of that thin enamel layer, the voltage will arc through and cause shorts in the windings.

I once had a lovely vintage JCM800 come to me from a local rehearsal hall. Someone with just enough knowledge to be dangerous had put a line out from the preamp stage, just before it fed into the phase inverter driver tube that drove the EL-34s.

This works, of course. At least, as far as feeding the mixing board. However, what the bozo didn't know was that you still needed a speaker load on the amp! He set it up for his customers to use, with the feed to the board but no cabinet for the amp itself.

Of course, he didn't tell me this when he brought me the amp. He simply said the amp had stopped working. As far as he knew, that's what had happened, I guess.

So I test all the tubes and find that the EL-34s are snackered. I figured they were just old (they were!) and likely had a lot of hours from being used in the rehearsal hall. I replaced the tubes, powered up the amp and set the bias. Everything SEEMED hunky dory!

Then came the final test. I plugged in a guitar and cranked up the amp to take a listen. LIGHTNING shot out of the back! Literally! Big sparks!

The OT was toast! It seemed ok but as soon as you asked it to accept big powerful voltage swings the internal insulation arced over.

So I phoned up the customer and asked him some guarded questions and eventually it came out what he had been doing. He had no idea that he still needed a speaker load. Now the amp needed a new OT. That meant an additional couple of hundred dollars for a quality reverse-engineered OT, plus installation.

So WC is absolutely right when he warns about too high a speaker load. A 2:1 mismatch rarely causes problems but in a worst case scenario of a 4 ohm load into an amp expecting to see a 16 ohm load the primary impedance to the tube plates will be 4 times the design value, or 6600 x 4 = 26,400 ohms! This is so high that if you are running the amp load there is a real chance the voltage peaks could damage the insulation of the windings.

In the real world, if you make a mistake the amp will not instantly blow up. Even if you play the amp as long as its not for a prolonged period and running LOUD you still will probably be ok.

However, if you do it for too long at too high a power level, you are just asking for it!

Wild Bill/Busen Amps


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## greco (Jul 15, 2007)

Wild Bill said:


> ......In the real world, if you make a mistake the amp will not instantly blow up. Even if you play the amp as long as its not for a prolonged period and running LOUD you still will probably be ok.
> 
> However, if you do it for too long at too high a power level, you are just asking for it!
> 
> Wild Bill/Busen Amps


This is what I stated in my original post (#2 in this thread). 

I memorized this from a thread in the past in which Wild Bill had written the exact same comment.

Cheers

Dave


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## nonreverb (Sep 19, 2006)

I think this thread has now officially been beaten to death...


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## GTmaker (Apr 24, 2006)

nonreverb said:


> I think this thread has now officially been beaten to death...


probably true but answer me this.

a tube amp rated min. 4 ohm load...
which will do more damage ( if any).
running a 2 ohm load OR running an 8 ohm load...

I ask this silly question cause after reading all this stuff I still dont know the answer.

G.


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## nonreverb (Sep 19, 2006)

Probably neither if it's a quality built amp. Let's make it a little mor extreme...
Dead short in place of the 2 ohm load? That will burn out the power tubes quickly.
16ohm load or worse, an open condition on the output? That poses a real danger of frying the output transformer. A much more costly repair.




GTmaker said:


> probably true but answer me this.
> 
> a tube amp rated min. 4 ohm load...
> which will do more damage ( if any).
> ...


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