# Ohm mismatch



## guitarman2 (Aug 25, 2006)

I have a combo amp and a 2X12 cabinet that has 2 8 ohm speakers. the cabinet is wired 4 ohms. The combo I have is has 3 10" 8 ohm speakers. The transformer handles a 2 ohm load. Now I am not at all an amp techie so bear with me if this is a dumb question. 
I would like to test the combo amp with the 4 0hm cabinet plugged in to it. Probably have it plugged in and test it for about 30 minutes. Is this a bad idea? I just want to see what the amp would sound like with 2 12" speakers.


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## hollowbody (Jan 15, 2008)

Hard to say, Terry. For a short period, the transformer should be ok, but you never know. Usually a well designed amp can handle a slight mismatch and 2 to 4 ohms is a slight one. Also, going up in impedance is less stress on the amp than going down, so that's in your favour too.

The bigger problem is this. At 4 ohms, your amp's power will be roughly halved, so you won't actually be hearing your amp the way you're used to hearing it because it won't be putting out the same amount of current.


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## guitarman2 (Aug 25, 2006)

hollowbody said:


> Hard to say, Terry. For a short period, the transformer should be ok, but you never know. Usually a well designed amp can handle a slight mismatch and 2 to 4 ohms is a slight one. Also, going up in impedance is less stress on the amp than going down, so that's in your favour too.
> 
> The bigger problem is this. At 4 ohms, your amp's power will be roughly halved, so you won't actually be hearing your amp the way you're used to hearing it because it won't be putting out the same amount of current.



Thanks for the response. Its a 35 watt amp. So you're saying it will actually be like about 18 watts?


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## hollowbody (Jan 15, 2008)

guitarman2 said:


> Thanks for the response. Its a 35 watt amp. So you're saying it will actually be like about 18 watts?


It depends on a lot of things, but yes, as impedance increases, power output tends to go down. Best bet is to contact the manufacturer and find out from them.


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## Wild Bill (May 3, 2006)

guitarman2 said:


> Thanks for the response. Its a 35 watt amp. So you're saying it will actually be like about 18 watts?


Is it a solid state amp, Terry? The 'half power' thing is only true with solid state amps. Tube amps are different.

If it is a solid state amp, it won't care about a higher load. As said, it will just put out a bit less power.

If it's a tube amp, the mismatch won't bother it. Tube amps are tough! You will still get approximately the same amount of power. There might be a slight tone change due to the power tubes seeing a bit different load on the primary side of the output transformer from the different speaker load but it would likely be 'mice nuts'.

Go ahead and wail!:smile:

:food-smiley-004:


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## hollowbody (Jan 15, 2008)

Wild Bill said:


> Is it a solid state amp, Terry? The 'half power' thing is only true with solid state amps. Tube amps are different.


Really??? I didn't know that! Any idea why, Bill?


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## guitarman2 (Aug 25, 2006)

Wild Bill said:


> Is it a solid state amp, Terry? The 'half power' thing is only true with solid state amps. Tube amps are different.
> 
> If it is a solid state amp, it won't care about a higher load. As said, it will just put out a bit less power.
> 
> ...


You were right Bill. When I plugged my Celestion Blue Cabinet in to the Victorilux it was actually louder. I attribut it being louder because the Celestion Blues are a much more efficient speaker then the Webers. 
The Celestion Blues actually sounded very nice with the vikky. 
Bill do you think that long term use of this ohm mismatch would cause damage. I wouldn't mind trying this combo for a night at a gig.


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## Wild Bill (May 3, 2006)

guitarman2 said:


> You were right Bill. When I plugged my Celestion Blue Cabinet in to the Victorilux it was actually louder. I attribut it being louder because the Celestion Blues are a much more efficient speaker then the Webers.
> The Celestion Blues actually sounded very nice with the vikky.
> Bill do you think that long term use of this ohm mismatch would cause damage. I wouldn't mind trying this combo for a night at a gig.


I still don't know if its a solid state or tube amp but I really think it doesn't matter, Terry. Either way, the amp should be fine.

:food-smiley-004:


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## Wild Bill (May 3, 2006)

hollowbody said:


> Really??? I didn't know that! Any idea why, Bill?


Solid state amps normally have no output transformer. They just drive current directly into a load. If the load is lowered then more current flows. The designer has to put a lower limit on the load or the amp will eventually be pumping so much current into too low a load that the transistors will overheat and burn out.

Any higher load just means less current flow. Techs actually can work safely on a solid state load with no speaker connected at all!

Tube amps have an output transformer. They do NOT just push current into the load! If you look at the data sheets for an output tube you will find curves of plate load resistance vs. power out and also distortion. There is a range of safe power loads for the tube, just as there's a range for distortion. Actually, rarely do maximum power and least distortion track at exactly the same load resistance. So designers would pick one according to what the amp was supposed to do, such as low distortion hifi or screaming Marshall rock and roll machine!

The typical load for a push-pull pair of 6L6's is 6600 ohms, for guitar amps. You will find other values. As I said, there is a range of values and not one exact point.

Once you've chosen your plate load impedance you DON'T want it to change! You use an output transformer to (among other things) transform that 6600 ohms down to 8 ohms. This works in a fixed ratio. If you put a 4 ohm load on the transformer you will see only 3300 ohms on the plates of the tubes. The ratio is fixed and can't be changed. To accommodate 4 or 16 ohms you have to tap the secondary winding so that you have less or more turns being used on the secondary side, in order to change that ratio to preserve getting 6600 ohms on the primary side.

When you put a mismatch on the speaker side it doesn't change the current being drawn from the tubes, like with a solid state amp. All it does is mismatch the plate load impedance on the other side of the transformer. As I said, tubes don't have an optimum impedance but rather it follows a curve of values. The power out changes along the curve but only a bit and not linearly, like half power for half load.

The real worry with mismatches is when they become extremely high. Low mismatches and even a short won't likely burn anything out, although it is hard on tube life. High mismatches, like a 16 ohm load on a 4 ohm tap, can be scary 'cuz they reflect such a high value load to the tubes that you can develop very high voltage swings as you drive signal into the transformer. These voltages can be high enough to puncture the insulation on the winding wires and short the transformer out! 

Worst case is no speaker plugged in at all. Some amps will put a resistor of a few hundred ohms at 5 or 10 watts across the speaker jack. When running in parallel with the speaker the resistor will only take a tiny bit of power, being so much higher in resistance compared to the speaker. If the speaker cable wasn't there, it still provides some kind of load to the transformer to limit those possible voltage spikes.

Hope this all makes sense!

:food-smiley-004:


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## hollowbody (Jan 15, 2008)

Ok, so what's the deal with OTL amps? I know in the audio world some designers swear that tube amps without output transformers sound better. Any credence to that claim?

btw - i realize this is a major de-rail. Sorry Terry!


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## guitarman2 (Aug 25, 2006)

Wild Bill said:


> I still don't know if its a solid state or tube amp but I really think it doesn't matter, Terry. Either way, the amp should be fine.
> 
> :food-smiley-004:


Its a tube amp


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## sfx70 (Sep 16, 2009)

You could blow your output tranny by overheating it...


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## parkhead (Aug 14, 2009)

guitarman2 said:


> You were right Bill. When I plugged my Celestion Blue Cabinet in to the Victorilux it was actually louder. I attribut it being louder because the Celestion Blues are a much more efficient speaker then the Webers.
> The Celestion Blues actually sounded very nice with the vikky.
> Bill do you think that long term use of this ohm mismatch would cause damage. I wouldn't mind trying this combo for a night at a gig.



relax if its a victoria tube amp everything should be fine

since its a "fender clone" it should handle the impedance mismatch without a problem 

if you run a 2 ohm tube amp at 4 ohms you are doubling the primary impedance on the tube side so a 4k primary becomes 8k 
the sound becomes more like an early 50's super than a late 50's & 60's amp 
since early tweeds had much higher primarys like the small amps they developed from 

Harp players mismatch in this way all the time 

when fender designed the tweed bandmaster 3-10 (3.2 ohms) and the bassman 4-10 (2 ohm) they used the same 2 ohm output transformer
giving the bandmaster a slightly different sound 

Keep in mind when amps are being designed the designer actually has a range of impedances he can run the tubes at, in the old days 
he chose an off the shelf Transformer with a range of taps and used the various taps to get close to the impedance he wanted 

Marshall started with 6k6 as their primary impedance then dropped to 3.4k 
on their jtm 45 then 50 watter 
They ran the real 60's Bluesbreaker pre set to its 16 ohm tap at an 8 ohm load from the factory (no impednace selector) 
The JTM 45/100 had a 4 k primary the in 67 they went to a 1.7 k primary 


Damage to output transformers happens when the have no load Ie no speaker ... the current builds up in the transformer and arcs across the windings burning them
this is why fender has a shorting jack on their outputs and Traynor used a 100ohm resistor across the output, a dead short or 100ohm load is better than a disconnected secondary

the exploding output transformer mismatch myth is as common on the internet as spam for viagra ... or chain e-mails 

there was a story in Guitar player 20+ years ago where Joe Walsh claimed the he used a Pair of super reverbs on chairs, one as the amp 
and the second set of speakers as an "extension cabinet" showing a 1 ohm load to the main amp, he claimed this worked great and he always had a "spare amp" 
perhaps he needed the spare but most likely he could run a super this way for hours ....

If an output transformer goes south it was probably under designed for the amp 
ie a tweed deluxe uses a 10 watt OT and the amp can push 15 -20 at full clip where it sounds best 

has anyone seen a old tweed deluxe without a replacement OT ? 



p


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