# Ohm mismatch?



## davetcan (Feb 27, 2006)

What would be the possible consequences of plugging an 8ohm speaker into a 16ohm output jack :confusion:


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## greco (Jul 15, 2007)

According to what I remember from Wild Bill's comments is that this only really becomes an issue (i.e. being detrimental for your output tubes and OT) if you are playing at reasonably high volumes for extended periods of time. Please don't ask me to define "reasonably high volumes" and "extended periods of time". 

Personally, I prefer not to have mismatch so that I can relax/avoid worrying about it.
However, in many previous threads on the topic, it appears that many play with impedance mismatch all the time and are not concerned.

Cheers

Dave


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## davetcan (Feb 27, 2006)

Well it wasn't me but it is my amp  Fingers are crossed. It "seems" fine although the drive channel does not sound as good as I remember, could be just tubes though, I'll play with them tonight.



greco said:


> According to what I remember from Wild Bill's comments is that this only really becomes an issue (i.e. being detrimental for your output tubes and OT) if you are playing at reasonably high volumes for extended periods of time. Please don't ask me to define "reasonably high volumes" and "extended periods of time".
> 
> Personally, I prefer not to have mismatch so that I can relax/avoid worrying about it.
> However, in many previous threads on the topic, it appears that many play with impedance mismatch all the time and are not concerned.
> ...


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## nonreverb (Sep 19, 2006)

If you're going to mismatch, it's the better of the two scenarios. The reverse (8ohm out to 16ohm speaker) is where the output transformer can get stressed. Usually not a major problem as most amps can handle a 100% mismatch although better to keep 'em matched up.:smile-new:


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## Church-Audio (Sep 27, 2014)

here is the deal. lets say this was a marshall 100 watt amp.. At 8 ohm tap with an 8 ohm load the output transformer sees a impedance of 1.7k to 2k on the primary side of the output transformer. When you bring up the load on the 8 ohm tap you bring up the primary impedance this is what the tubes see. It will go for 1.7k to as high as 4k this is not really a good load for a el34 tube with most transformers its not really going to be a problem as far as decreased current goes where it will not be a problem is the lifespan of your tubes. It's when you use a lower impedance load in a higher impedance tap you then lower the primary impedance and thus load the power tubes. Plain and simple dont do it if you can avoid it if you are doing it for one night you should be ok. But if you have a crappy old pair of output tubes you could cause them to fail. The other side effect of loading the tubes down? you change the tone of the amp. So yeah you can do it for short periods of time longer periods if your not cranking things. But its not good for the tubes and in some cases if you have an output transformer that is already on its last leg it can in some cases finish it off. you can also use this to change the primary impedance if you want to change your amp from a 100 watt amp to a 50 and remove two tubes. Then playing around with this load to primary relationship is beneficial. I edited this so it made sense I guess I should hit be posting deeply technical stuff when I'm half asleep lol.


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## dcole (Oct 8, 2008)

An increase in primary impedance would cause a decrease in primary current. This should stress the tubes less. Is it an increase in primary voltage to maintain the same output that stresses the tubes?


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## jb welder (Sep 14, 2010)

dcole said:


> An increase in primary impedance would cause a decrease in primary current. This should stress the tubes less.


 You are correct in terms of current. Running into a lower impedance load (than rated) will give a decrease in primary Z which will mean tubes conduct more current. This decreases tube life as you are basically putting on more "current hours" with use.
But running into a higher impedance load (than rated) has other hazards, such as high voltage spikes on the OT which can stress the OT or cause insulation breakdown between windings.
Amps that have problems red-plating at high power can sometimes be tamed by increasing primary Z (or running into a higher than rated load) due to the decrease in tube current.
Most OT's have enough leeway (over-rated) that I don't worry about the mismatch either way. Some manufacturers even encourage experimenting with mis-match to get different tone colorations. I have an old Ampeg V4 manual somewhere that says that, and I believe some boogies also suggested this. Of course, any mismatch will lower the power output.
But amps that have less robust OT's, especially amps that are pushed hard, are more prone to problems when mis-matched. So I wouldn't do it with a Marshall, and this may be part of the reason they seem to have more OT failures than other brands.

And all the posts in this discussion have been about tube amps.
For solid-state amps you should _never_ run into a lower impedance load than the amp is rated for.


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## davetcan (Feb 27, 2006)

So just to clarify, and thanks for all the posts so far, It is an 8 ohm speaker. The amp has taps on the back for 4, 8, and 16 ohm. It is a 4x EL84 amp. The speaker was plugged into the 16 ohm tap. I was always under the impression that this was "bad". No problem running it the other way, 16 ohm speaker into an 8 ohm tap. I've also read that "most" modern tube amps can stand a mismatch of 2:1 in either direction with perhaps nothing more than reducing tube life.

Similar to this.


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## greco (Jul 15, 2007)

davetcan said:


> ... It is an 8 ohm speaker. The amp has taps on the back for 4, 8, and 16 ohm.


Why not just use the 8 ohm tap for the 8 ohm speaker?...or is that tap not functioning? 
Must be some reason for not choosing it...correct? ...or not...LOL

Cheers

Dave


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## dtsaudio (Apr 15, 2009)

> I was always under the impression that this was "bad". No problem running it the other way, 16 ohm speaker into an 8 ohm tap.


 No other way around. It is better to have a lower load (speaker) into the higher tap.
Think of it this way. You always hear "never a run a tube amp without a speaker connected" So putting a 16 ohm on a lower tap starts to make things worse. Solid state amps are the exact opposite. 
Interesting thing, I used to own two Traynor Bassmaster amps. The speaker jack was actually a shorting jack that shorted the output to ground if the speaker wasn't plugged in. Safe enough in short term on a tube amp, but do that to a solid state amp and smoke will be the result.


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## davetcan (Feb 27, 2006)

No reason at all, and that's the way the amp was received by the other party. When I got it back it was plugged into the 16. I'm trying to determine if any damage could have been done.



greco said:


> Why not just use the 8 ohm tap for the 8 ohm speaker?...or is that tap not functioning?
> Must be some reason for not choosing it...correct? ...or not...LOL
> 
> Cheers
> ...


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## greco (Jul 15, 2007)

davetcan said:


> No reason at all, and that's the way the amp was received by the other party. When I got it back it was plugged into the 16. I'm trying to determine *if any damage could have been done.*


Gotcha!...I knew there must be a simple explanation for this. 

Cheers

Dave


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## keto (May 23, 2006)

If it's the Memphis 30, and if it's the same as the manual for the SigX, they ENCOURAGE mismatches to explore different sounds. Hardy iron in Fryettes, should be no damage.


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## davetcan (Feb 27, 2006)

Found this great link. I seem to remember seeing something like this on the Weber forums a long time ago.

http://www.geofex.com/tubeampfaq/tafaqndx.htm

the pertinent section:

Q:Will it hurt my amp/output transformer/tubes to use a mismatched speaker load? 
Simple A: Within reason, no. 
Say for example you have two eight ohm speakers, and you want to hook them up to an amp with 4, 8, and 16 ohm taps. How do you hook them up? For most power out, put them in series and tie them to the 16 ohm tap, or parallel them and tie the pair to the 4 ohm load. 
For tone? Try it several different ways and see which you like best. "Tone" is not a single valued quantity, either, and in fact depends hugely on the person listening. That variation in impedance versus frequency and the variation in output power versus impedance and the variation in impedance with loading conspire to make the audio response curves a broad hump with ragged, humped ends, and those humps and dips are what makes for the "tone" you hear and interpret. Will you hurt the transformer if you parallel them to four ohms and hook them to the 8 ohm tap? Almost certainly not. If you parallel them and hook them to the 16 ohm tap? Extremely unlikely. In fact, you probably won't hurt the transformer if you short the outputs. If you series them and hook them to the 8 ohm or 4 ohm tap? Unlikely - however... the thing you CAN do to hurt a tube output transformer is to put too high an ohmage load on it. If you open the outputs, the energy that gets stored in the magnetic core has nowhere to go if there is a sudden discontinuity in the drive, and acts like a discharging inductor. This can generate voltage spikes that can punch through the insulation inside the transformer and short the windings. I would not go above double the rated load on any tap. And NEVER open circuit the output of a tube amp - it can fry the transformer in a couple of ways. 
Extended A: It's almost never low impedance that kills an OT, it's too high an impedance. 
The power tubes simply refuse to put out all that much more current with a lower-impedance load, so death by overheating with a too-low load is all but impossible - not totally out of the question but extremely unlikely. The power tubes simply get into a loading range where their output power goes down from the mismatched load. At 2:1 lower-than-matched load is not unreasonable at all. 
If you do too high a load, the power tubes still limit what they put out, but a second order effect becomes important. 
There is magnetic leakage from primary to secondary and between both half-primaries to each other. When the current in the primary is driven to be discontinuous, you get inductive kickback from the leakage inductances in the form of a voltage spike. 
This voltage spike can punch through insulation or flash over sockets, and the spike is sitting on top of B+, so it's got a head start for a flashover to ground. If the punchthrough was one time, it wouldn't be a problem, but the burning residues inside the transformer make punchthrough easier at the same point on the next cycle, and eventually erode the insulation to make a conductive path between layers. The sound goes south, and with an intermittent short you can get a permanent short, or the wire can burn though to give you an open there, and now you have a dead transformer. 
So how much loading is too high? For a well designed (equals interleaved, tightly coupled, low leakage inductances, like a fine, high quality hifi) OT, you can easily withstand a 2:1 mismatch high. 
For a poorly designed (high leakage, poor coupling, not well insulated or potted) transformer, 2:1 may well be marginal. Worse, if you have an intermittent contact in the path to the speaker, you will introduce transients that are sharper and hence cause higher voltages. In that light, the speaker impedance selector switch could kill OT's if two ways - if it's a break befor make, the transients cause punch through; if it's a make before break, the OT is intermittently shorted and the higher currents cause burns on the switch that eventually make it into a break before make. Turning the speaker impedance selector with an amp running is something I would not chance, not once. 
For why Marshalls are extra sensitive, could be the transformer design, could be that selector switch. I personally would not worry too much about a 2:1 mismatch too low, but I might not do a mismatch high on Marshalls with the observed data that they are not all that sturdy under that load. In that light, pulling two tubes and leaving the impedance switch alone might not be too bad, as the remaining tubes are running into a too-low rather than too-high load.


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## Church-Audio (Sep 27, 2014)

You are correct. I edited my post after I realized it was backwards lol I was tired.

- - - Updated - - -

Mesa boogie is one of these companies that's why they use a higher quality output transformers on the amps they make. They can handle it.


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## nonreverb (Sep 19, 2006)

Absolutely right! I can't stress enough to those reading this thread with a Marshall (insert tube amp model here). DO NOT mismatch impedances on the amp. Particularly a higher speaker impedance than what the output is set at. There is a real possibility that the transformer will fail. Most tech here will know. If like me, they've replaced a few OP transformers in 'em.




jb welder said:


> You are correct in terms of current. Running into a lower impedance load (than rated) will give a decrease in primary Z which will mean tubes conduct more current. This decreases tube life as you are basically putting on more "current hours" with use.
> But running into a higher impedance load (than rated) has other hazards, such as high voltage spikes on the OT which can stress the OT or cause insulation breakdown between windings.
> Amps that have problems red-plating at high power can sometimes be tamed by increasing primary Z (or running into a higher than rated load) due to the decrease in tube current.
> Most OT's have enough leeway (over-rated) that I don't worry about the mismatch either way. Some manufacturers even encourage experimenting with mis-match to get different tone colorations. I have an old Ampeg V4 manual somewhere that says that, and I believe some boogies also suggested this. Of course, any mismatch will lower the power output.
> ...


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## greco (Jul 15, 2007)

nonreverb said:


> *I can't stress enough to those reading this thread with a Marshall (insert tube amp model here). DO NOT mismatch impedances on the amp.*


Repeated to emphasize to all the Marshall owners out there.

I had not heard this before in these impedance mismatch discussions/threads.

Cheers

Dave


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## davetcan (Feb 27, 2006)

Interesting discussion all, thanks for the feedback.

The amp in question is the Fryette Memphis, as guessed by Keto. I played it for about an hour last night with no issues at all, looks like all is good.


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## jb welder (Sep 14, 2010)

nonreverb said:


> Absolutely right! I can't stress enough to those reading this thread with a Marshall (insert tube amp model here). DO NOT mismatch impedances on the amp. Particularly a higher speaker impedance than what the output is set at. There is a real possibility that the transformer will fail. Most tech here will know. If like me, they've replaced a few OP transformers in 'em.


For sure, better safe than sorry. This may have been mentioned already, but another hazard of running into a load that is higher impedance than the amp setting is arcing at the tube sockets.
From Randall Aiken's site (http://www.aikenamps.com/index.php/technical-q-a):
"Running the amp at 16 ohms into an 8 ohm or 4 ohm load at high power will accelerate tube wear and could cause damage, so be sure to match the load whenever possible. Higher than normal impedances can cause output transformer damage or tube arcing."
So there is a hazard to tube life if you run into a load of too low impedance, and a hazard to the output transformer (and sockets) if you run into too high of a load impedance. Tubes are much cheaper to replace than OT's, so if there is any doubt, better to mismatch to the lower impedance load than the higher one.*

**
*


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