# Stupid Question



## Vox71 (Mar 25, 2008)

Please forgive me if this sounds like a really stupid question, and the answer is really obvious. Is it possible to run a 16 ohm head into an 8 ohm cabinet?


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## greco (Jul 15, 2007)

Vox71...your question isn't stupid at all...but it has been asked and discussed here many, many times in the past (search "impedance matching").

My response would be that it would be OK to run a 16 ohm head into an 8 ohm cab* if *you were going to do this for short durations at low volumes. It will likely result in some shortening of the life of the power tubes. BTW...is the head SS or tube? 

Is it preferred to match the amp to the speaker impedance?....YES. 
Would I do that if I was you ?...YES 
Am I anal about impedance matching? ...YES
Do I sleep well at night because my amp(s) and speaker(s) are impedance matched? YES

Hope this helps. 

Others might respond with a variety of answers.

cheers

Dave


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## parkhead (Aug 14, 2009)

unlike everyone else 

I try all impedances with an amp and choose the one that sounds best 

sometimes its a mismatch 

am I risking a blown output transformer ? according to the interweb I am 

and everything is true on the interweb 

have I blown an output transformer yet? ... nope 

I sleep fine, when I sleep 

mostly I hang upside down 

P


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## greco (Jul 15, 2007)

In fairness to *parkhead*, I should indicate that I have been told that tube amps are fairly forgiving to impedance mismatch.

cheers

Dave


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## Vox71 (Mar 25, 2008)

Well, that's good to know because it is in fact a tube amp I am inquiring about. Thanks, guys


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## WCGill (Mar 27, 2009)

Speaker impedance is an incredibly variable variable, changing with different frequencies. Tube amps are indeed forgiving of mismatches so there shouldn't be any problem. Tubes *may* wear out faster and you won't have full rated power, distortion may increase (probably not noticeably). Rule of thumb here is it's always preferable to go down with your load than up. Have at it.


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## washburned (Oct 13, 2006)

WCGill said:


> Speaker impedance is an incredibly variable variable, changing with different frequencies. Tube amps are indeed forgiving of mismatches so there shouldn't be any problem. Tubes *may* wear out faster and you won't have full rated power, distortion may increase (probably not noticeably). Rule of thumb here is it's always preferable to go down with your load than up. Have at it.


So I should be able to run my VibroChamp XD (5watt into 4ohm) with my single 12, 8ohm cab @ 2-2/3 ohm total, right?


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## greco (Jul 15, 2007)

What is the 2-2/3 ohm total? How did you arrive at that figure?

Aren't you just running a 4 ohm amp into an 8 ohm speaker cab?

Am I missing something here?

Cheers

Dave


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## JHarasym (Mar 27, 2007)

Sounds like he wants to run his amp into a 4 ohm and 8 ohm speaker in parallel:
1/R1 + 1/R2 = 1/Rt, 1/4 + 1/8 = 3/8, ergo Rt = 8/3 = 2.66
see
Series Parallel Speaker Impedance


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## greco (Jul 15, 2007)

JHarasym said:


> Sounds like he wants to run his amp into a 4 ohm and 8 ohm speaker in parallel:
> 1/R1 + 1/R2 = 1/Rt, 1/4 + 1/8 = 3/8, ergo Rt = 8/3 = 2.66
> see
> Series Parallel Speaker Impedance


Thanks JHarasym...I missed that, obviously. 

Didn't have enough morning coffee before reading the post.

Cheers

Dave


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## GuitarG. (Apr 1, 2010)

WCGill said:


> Rule of thumb here is it's always preferable to go down with your load than up. Have at it.


I was always under the impression that it's better to go higher. 8 ohm tap running a 4 ohm speaker load makes the output transformer and tubes work harder. 
Would'nt running a 8 ohm tap into a 16 ohm speaker load preserve tube life?


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## washburned (Oct 13, 2006)

GuitarG. said:


> I was always under the impression that it's better to go higher. 8 ohm tap running a 4 ohm speaker load makes the output transformer and tubes work harder.
> Would'nt running a 8 ohm tap into a 16 ohm speaker load preserve tube life?


I thought the same....lower load = more amps = more heat =transformer burn up. Apparently it's not true in this case.


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## Wild Bill (May 3, 2006)

washburned said:


> I thought the same....lower load = more amps = more heat =transformer burn up. Apparently it's not true in this case.


The reason is, the OT works to transform impedance by a fixed ratio. So if you plug in a higher speaker load you get a higher plate load to the output tubes.

A higher load means higher voltage swings in the output transformer. With a severely high mismatch these swings can be high enough to actually break down the insulation in the transformer windings. The OT can end up with internal shorts and burn up.

WB


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## greco (Jul 15, 2007)

Vox71 said:


> Please forgive me if this sounds like a really stupid question, and the answer is really obvious. Is it possible to run a 16 ohm head into an 8 ohm cabinet?


*Vox71...*stiil think it is a was (originally) a "stupid question" with an obvious answer...me neither....LOL

Cheers

Dave


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## GuitarG. (Apr 1, 2010)

Wild Bill said:


> The reason is, the OT works to transform impedance by a fixed ratio. So if you plug in a higher speaker load you get a higher plate load to the output tubes.
> 
> A higher load means higher voltage swings in the output transformer. With a severely high mismatch these swings can be high enough to actually break down the insulation in the transformer windings. The OT can end up with internal shorts and burn up.
> 
> WB


So either way you take the risk of damage whether the speaker load is lower or higher.


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## greco (Jul 15, 2007)

GuitarG. said:


> So either way you take the risk of damage whether the speaker load is lower or higher.


And to add to this comment... Is the risk still somewhat dependant on the volume you play at and the duration you play through the amp at these (high) volumes?

This has been (as previuosly stated) my understanding, from "old" GC threads on this topic. 

Cheers

Dave


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## Wild Bill (May 3, 2006)

GuitarG. said:


> So either way you take the risk of damage whether the speaker load is lower or higher.


With a 2:1 mismatch, the risk is "mice nuts"! And with a lower load, the current goes up a bit but the voltage swing goes down. Copper wire in a winding can usually handle a bit more current better than enamel insulation can handle excessively high voltage, so that means lower loads are a bit safer. 

Still, everybody's nitpicking at something they really don't understand anyway! It would help a lot to look at the data sheets for an output tube. Most will show you typical curves of operation, with a range of plate load impedances, power out in watts and distortion figures. You will quickly see that there is no one specific "best" load. Things change over a range and the tube isn't really hurt within that range.

WB


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## parkhead (Aug 14, 2009)

I'll add that if the transformer blows up 

REPLACE it !! it was CRAP !

a buddy just took a brand new reissue deluxe reverb in for service, 

he's a working pro and gigs for a living 

he was not mismatching, just playing the amp 6 nights a week 

a 6v6 failed and took out his output transformer 

IMHO since those amps use a relativly inexpensive output transformer its a no brainer 

get a better one and be done with it ....

OBVIOUSLY you want to be careful with a 50's tweed old marshall or vox 

but if you are dealing with a modern amp ... the MFG just outsourced the OT to china anyway 

so just blow the junk up and order a nice 1st world made replacement 

thats my .02c and I'm sticking to it 

p


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## nonreverb (Sep 19, 2006)

parkhead said:


> I'll add that if the transformer blows up
> 
> REPLACE it !! it was CRAP !
> 
> ...


Funny, but original trannies in"vintage" amps were by 'n large pretty crappy too. Too many Fender OT were barely rated for the amps they were put in. Not surprising that many have long since been replaced. Problem is they sound great.... These days there are many making trannies that sound great and are more reliable. But hey, if you like the sound of an OEM one, just order another. They can be had pretty cheap. Personally, I've never seen a Reissue Deluxe with a blown OT in my shop. It's usually a screen resistor that's grenaded
...if I had any complaint it would be that they stock them with pretty lame output tubes. Drop a set of JJ's in it and it'll far more reliable.


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## Gizmo (Aug 7, 2008)

I think everyone has addressed the question w.r.t. tube amps (better to have the load lower than higher...within limits)

For a tranny amp though, its the opposite..pretty safe to use a higher load, but not a lower load...in fact don't do it.



Vox71 said:


> Please forgive me if this sounds like a really stupid question, and the answer is really obvious. Is it possible to run a 16 ohm head into an 8 ohm cabinet?


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## nonreverb (Sep 19, 2006)

Correct, a solid state amplifier with a lower load than specified will just try and deliver more current with the result being either a shutdown circuit will kick in or it'll grenade the output transistors and usually a few other things as well


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## WCGill (Mar 27, 2009)

nonreverb said:


> it'll grenade the output transistors and usually a few other things as well


Which wouldn't be a bad thing ;-)


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## nonreverb (Sep 19, 2006)

WCGill said:


> Which wouldn't be a bad thing ;-)


lol...spoken like a true purist ;D


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