# Running a 16 ohm amp external speaker outlet into a 8 ohm cab.



## marcos

Vox AC10C1 tube amp 16 ohm external speaker outlet into a 8 ohm Jensen speaker cab. Any damage to the amp or speaker in the long run.
Much appreciate any expert info on this.


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## keto

so especially bad if you are running the stock speaker too and it happens to be 8ohms, as 8+8 = 4 ohms total speakers, that's a bad mismatch - outputs 8 stock speaker + 16 external = roughly 12ohms the OT wants to see.

8 into 16, well, if you keep it low volume and not long term, you're not likely to kill the OT, but no guarantees.

General consensus, and I forget all the tech on it offhand, is that it's better to run the opposite mismatch, ie, 16ohm speaker into 8 ohm tap, though still no guarantees of course.

Also, I often get it backwards so I stand to be corrected 

I've run mismatches quite a bit and never hurt anything, but not at even half amp volume.


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## marcos

Thank you Keto. I wasnt certain about any of this and the info on line is not very helpfull


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## jbealsmusic

keto said:


> General consensus, and I forget all the tech on it offhand, is that it's better to run the opposite mismatch, ie, 16ohm speaker into 8 ohm tap, though still no guarantees of course.


That is correct.

16 ohms output connected to a 16 ohms speaker = Ideal
8 ohms output connected to a 16 ohms speaker = OK
16 ohms output connected to an 8 ohms speaker = BAD (for your amp)


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## cboutilier

jbealsmusic said:


> That is correct.
> 
> 16 ohms output connected to a 16 ohms speaker = Ideal
> 8 ohms output connected to a 16 ohms speaker = OK
> 16 ohms output connected to an 8 ohms speaker = BAD (for your amp)


Depends on the amp. Old fenders were designed to have that 100 percent lower mismatch as soon as you plugged in the extension cab.


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## WCGill

jbealsmusic said:


> That is correct.
> 
> 16 ohms output connected to a 16 ohms speaker = Ideal
> 8 ohms output connected to a 16 ohms speaker = OK
> 16 ohms output connected to an 8 ohms speaker = BAD (for your amp)


Actually, it's the other way 'round. And like cb says, 100% isn't a problem but better a lower load on a higher impedance tap. Research!


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## jbealsmusic

WCGill said:


> Actually, it's the other way 'round. And like cb says, 100% isn't a problem but better a lower load on a higher impedance tap. Research!


Interesting. This is the first time I've heard anyone say that.

I'm no electrical engineer, but I do speak with amp techs and manufacturers on a pretty regular basis. They all say what I said when answering this question. You can find the same conclusion in manuals and FAQs by major companies like Mesa Boogie, Marshall, Vox, and numerous other brands.

They all say you should match impedance. If you have to mismatch, going up is safer for the amp. As in, connecting an 8ohm amp output to a 16ohm speaker is a safer mismatch than connecting a 16ohm output to an 8ohm speaker. Less commonly, some have said that a 2:1 mismatch in either direction is quite safe for the amp (though you have to mind the differences in output power).

Can you really blame a guy for trusting such a widespread belief held by those who are generally perceived as experts?

I'm not saying you're wrong. I wouldn't call myself an expert on the subject. Please share more info if possible. Thanks!


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## WCGill

You should talk to better guys-seriously, like R.G. Keen. Seek and ye shall find.


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## sambonee

WCGill said:


> Actually, it's the other way 'round. And like cb says, 100% isn't a problem but better a lower load on a higher impedance tap. Research!


WCGill
Please elaborate somewhat. Thanks


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## marcos

Thank you all for the info. I have decided to sell the empty cab as i dont think i will be using it.
Feel free to continue the conversation as there are a lot of opinions.


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## jbealsmusic

WCGill said:


> You should talk to better guys-seriously, like R.G. Keen. Seek and ye shall find.





sambonee said:


> WCGill
> Please elaborate somewhat. Thanks


I assume he's referring to this article:
http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#mismatch

Summed up in laymen's terms (which techs never seem to be able to do for some reason ):

NEVER operate your amp without connecting to a load. That is a sure fire way to ruin your OT.
Matching impedance is safest, but you can experiment with mismatches because there may be a tonal difference you find preferable.
Going down (8 ohm out to 4 ohm speaker) puts more stress on the tubes, but is safer for the OT.
Going up (8 ohm out to 16 ohm speaker) has no negative impact on the tubes, but puts more stress on the OT.
A 2:1 mismatch in either direction shouldn't be too bad with well designed OTs. However, certain brands (he claims Marshall) are not well built for a mismatch that in the upward direction.

Based on how he writes, it is clear he is more okay with added stress on the tubes as opposed to added stress on the OT.
Conclusion: It is safer to mismatch going down than it is to go up because certain OTs are not well suited for going up.
That is all very interesting. However, it poses a clear problem for non-techs.

Considering he is the outlyer in his opinion vs the majority of amp techs and manufacturers, how do we know whose advice we should take? In most things of this nature, it is generally better to go with the majority consensus. What makes his opinion worth more than the majority?
I'm not trying to be a dick or anything, but it is an important question. MJF$#


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## Kerry Brown

All I can say is that many times I've used a 16 ohm cab with a 8 ohm output with no problems. I've never tried the other way.


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## davetcan

Ohm cooking 101: understanding amps, speakers and impedance

Case 1: running a 16 ohm speaker with an 8 ohm amp output

With this combination, the voltage at the speaker output will rise, while the current will almost halve. The power will drop, although you probably won’t notice it too much, as this combination will likely increase the mids in your tone. So long as you don’t overdo it – make it a rule of thumb not to connect a speaker with more than double the output impedance of that of the amp – this method can be quite useful, as you can effectively boost the mids in a cab that might otherwise be lacking in this department.

Case 2: running a 4 ohm speaker with a 16 ohm amp output

Here, the complete opposite to Case 1 occurs: while the speaker output voltage significantly falls, the flowing current only marginally increases. This will alter your tone too, but in the exact opposite way from before – you’ll get a drop in your mids! If you’ve got a cab that is way too mids-heavy, then, this can be the perfect method to better balance your overall tone. Because this combination can help lessen the self-induction effect that might occur in your tubes, it is generally safer to use than the example in Case 1. In terms of the amount of mismatching you can get away with here, we’d go with the 25% rule: so, for instance, don’t connect a speaker with less than a 4 ohm output to a 16 ohm amp output!


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## jb welder

jbealsmusic said:


> Considering he is the outlyer in his opinion vs the majority of amp techs and manufacturers, how do we know whose advice we should take? In most things of this nature, it is generally better to go with the majority consensus. What makes his opinion worth more than the majority?
> I'm not trying to be a dick or anything, but it is an important question. MJF$#


I'm not aware of any manufacturers who recommend going lower impedance at the load (as opposed to too high)?
I've seen some who don't care, and some who say never ever mismatch.
I am interested in your "majority consensus" statement and wonder where it comes from. This is a sincere question.
I'm wondering if tech's that recommend going high at the load are looking at tube life and not considering the OT. Running low at the load does work the tubes harder which will reduce tube life. Otherwise, no problem as was nicely pointed out by cboutilier and his Fender example above.
Also, with solid-state amps, too low of a load impedance is a death sentence, so perhaps there is some confusion added in by people more well versed is solid-state power amps?
Anyway, I agree with RG that up to a 2:1 mismatch should be fine in most instances, and also agree with his hesitation about going above rated load impedance with Marshall's and other OT's that are marginally spec'd, at least when used in high power/heavy clipping applications.


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## jbealsmusic

jb welder said:


> I'm not aware of any manufacturers who recommend going lower impedance at the load (as opposed to too high)?


I didn't say recommend. I said they consider it safer to go up.



> I am interested in your "majority consensus" statement and wonder where it comes from. This is a sincere question.


It comes from a few places actually.

Personal experience: Early 2000s when I was more into tube/analog gear, I had a cab that didn't match my amp and I contacted the amp manufacturer to ask about it (Mesa Boogie at the time). They said that matching impedance is ideal. If a mismatch is unavoidable, going up is safer (8 ohm output to an 16 ohm cab). But avoid going more than double. I had the same conversation with Carvin.

Work experience: I admit up front that I am no amp tech, but I've had a lot of conversations with techs and small builders. This subject has come up on occasion and the same conclusion is usually drawn.

Forum crawling: I mostly post on this forum, but I crawl/creep several forums and read posts related to amps/cabs/etc. This exact question is asked on every major forum at least once a year and the conversation usually goes the same way. "Matching impedance is ideal. Going up is safer. Some say 2:1 either way is fine." You can't always trust forum posts, but people have shared similar stories to mine where they contacted the maker of their amp and were told the same thing. I don't often see anyone suggest otherwise with any measure of support, which is why this conversation interests me so much.

Searching: A quick google search for "Amp Speaker Impedance Mismatch" comes up with quite a few posts regurgitating more or less the same info.

http://www.mesaboogie.com/media/Amplitudes/2013/June/Speaker Impedance Matching and Hookup.pdf
Multiple instances of amp output impedance being lower than speaker impedance, but they are labelled as "Safe Mismatch".​
OHMS & IMPEDANCE from the OUTPUT (speaker) perspective - FAQ courtesy of GollihurMusic.com
"*If the speaker cabinet impedance load is higher than the amplifier rating, e.g., an eight ohm cabinet plugged into an amplifier with a minimum impedance load of four ohms, you're cool.* You're not getting the amp's maximum output, but don't panic over it. (Again, this applies only to solid-state amps - if you have a tube power amp, the impedance _needs_ to match!)
*
If the speaker cabinet impedance load is lower than the amplifier it is now time to panic! Don't try it!*"​
Legendary Tones - All About Ohms
"The answer to the first question regarding whether a 16 ohm cabinet can be run safely with an amp that has settings for 8 or 4 ohms is yes. However, when running the head at a lower ohm rating then the cabinet, the result will be a significant degree of power loss. In the second case of using a 4 ohm cabinet with an amp that must be run at 8 ohms, this will stress an amp and cause it to overheat. Technically, you’ll get more power output (not efficient or stable power output mind you!) to some degree, but again, at the expense of burning out a transformer and/or other components. Not a good idea!"​
Cabinet Connection and Ohm F.A.Q. - General FAQ - Knowledge Base - Knowledge Base
"*Unacceptable Ohm Mismatches*: When connecting any amplifier to an external cabinet it is important to keep in mind that the ohm load of your cabinet should not be lower in number than the ohm rating of your amplifier (which can cause the amplifier to fail). I.e. - If the amp output has an overall rating of 8 ohms *DO NOT* run with an external cab that has an overall rating of 4 ohms.

*Acceptable Ohm Mismatches*: It is acceptable to run the overall ohm load output of an amplifier to a cabinet that is higher in number. I.e. If your amp output has an overall output rating of 8 ohms you *can* run into an external cabinet that has an overall rating of 16 ohms. Be aware that you will lose ~15-20 percent of perceived volume with solid state power amplifiers when running this type of ohm mismatch, but you will suffer no failures"​
It's possible that most of the things I've read and discussed were related to solid state amps rather than tube amps. I admit that I do have a propensity to lean towards information relating to pro audio (which is more often solid state). However, that begs the question, why do some tube amp manufacturers follow the same suggestion? Are their tube amp designs somehow different in such a way that an upward mismatch is preferable to a downward one?


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## greco

I wish I had $0.10 for every thread ever written about impedance mismatch.

Better yet $0.01 for every post in the threads about this topic ...that would be a better financial choice by far...LOL

Carry on...Don't mind my warped sense of humour.


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## BMW-KTM

Here ya go, fellas.

Consider the argument over forever.


Z-Matcher-100 100w Impedance Matcher

There's a 50 watt version as well.


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## dtsaudio

This is more or less correct.
Ohm cooking 101: understanding amps, speakers and impedance


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## WCGill

jbealsmusic said:


> I didn't say recommend. I said they consider it safer to go up.
> 
> It comes from a few places actually.
> 
> Personal experience: Early 2000s when I was more into tube/analog gear, I had a cab that didn't match my amp and I contacted the amp manufacturer to ask about it (Mesa Boogie at the time). They said that matching impedance is ideal. If a mismatch is unavoidable, going up is safer (8 ohm output to an 16 ohm cab). But avoid going more than double. I had the same conversation with Carvin.
> 
> Work experience: I admit up front that I am no amp tech, but I've had a lot of conversations with techs and small builders. This subject has come up on occasion and the same conclusion is usually drawn.
> 
> Forum crawling: I mostly post on this forum, but I crawl/creep several forums and read posts related to amps/cabs/etc. This exact question is asked on every major forum at least once a year and the conversation usually goes the same way. "Matching impedance is ideal. Going up is safer. Some say 2:1 either way is fine." You can't always trust forum posts, but people have shared similar stories to mine where they contacted the maker of their amp and were told the same thing. I don't often see anyone suggest otherwise with any measure of support, which is why this conversation interests me so much.
> 
> Searching: A quick google search for "Amp Speaker Impedance Mismatch" comes up with quite a few posts regurgitating more or less the same info.
> 
> http://www.mesaboogie.com/media/Amplitudes/2013/June/Speaker Impedance Matching and Hookup.pdf
> Multiple instances of amp output impedance being lower than speaker impedance, but they are labelled as "Safe Mismatch".​
> OHMS & IMPEDANCE from the OUTPUT (speaker) perspective - FAQ courtesy of GollihurMusic.com
> "*If the speaker cabinet impedance load is higher than the amplifier rating, e.g., an eight ohm cabinet plugged into an amplifier with a minimum impedance load of four ohms, you're cool.* You're not getting the amp's maximum output, but don't panic over it. (Again, this applies only to solid-state amps - if you have a tube power amp, the impedance _needs_ to match!)
> *
> If the speaker cabinet impedance load is lower than the amplifier it is now time to panic! Don't try it!*"​
> Legendary Tones - All About Ohms
> "The answer to the first question regarding whether a 16 ohm cabinet can be run safely with an amp that has settings for 8 or 4 ohms is yes. However, when running the head at a lower ohm rating then the cabinet, the result will be a significant degree of power loss. In the second case of using a 4 ohm cabinet with an amp that must be run at 8 ohms, this will stress an amp and cause it to overheat. Technically, you’ll get more power output (not efficient or stable power output mind you!) to some degree, but again, at the expense of burning out a transformer and/or other components. Not a good idea!"​
> Cabinet Connection and Ohm F.A.Q. - General FAQ - Knowledge Base - Knowledge Base
> "*Unacceptable Ohm Mismatches*: When connecting any amplifier to an external cabinet it is important to keep in mind that the ohm load of your cabinet should not be lower in number than the ohm rating of your amplifier (which can cause the amplifier to fail). I.e. - If the amp output has an overall rating of 8 ohms *DO NOT* run with an external cab that has an overall rating of 4 ohms.
> 
> *Acceptable Ohm Mismatches*: It is acceptable to run the overall ohm load output of an amplifier to a cabinet that is higher in number. I.e. If your amp output has an overall output rating of 8 ohms you *can* run into an external cabinet that has an overall rating of 16 ohms. Be aware that you will lose ~15-20 percent of perceived volume with solid state power amplifiers when running this type of ohm mismatch, but you will suffer no failures"​
> It's possible that most of the things I've read and discussed were related to solid state amps rather than tube amps. I admit that I do have a propensity to lean towards information relating to pro audio (which is more often solid state). However, that begs the question, why do some tube amp manufacturers follow the same suggestion? Are their tube amp designs somehow different in such a way that an upward mismatch is preferable to a downward one?


Mesa's info seems to only deal with 100% mismatches, which is generally regarded to be safe all ways. The other sources pertain to solid state amps near as I can tell, sorry didn't read it all. R. G. Keen has probably forgotten more about tube amps than most people will ever learn in a lifetime. I consider this to be a very credible source, perhaps the definitive one. The "higher load is safer than a lower load" is one of the most pervasive internet tube amplifier myths. Again, know your sources. 

Consider for a moment that for years Fender amps came equipped with a shorting output jack, a failsafe mechanism to prevent tube and amplifier damage should the user forget to plug in a speaker. Obviously a zero ohm load (lowest possible) is safer than an ordinary jack that would present an infinite (highest possible) load to the amplifier. 

This is just part of the knowledge base as far as I'm concerned. I've yet to see evidence to the contrary.


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## davetcan

dtsaudio said:


> This is more or less correct.
> Ohm cooking 101: understanding amps, speakers and impedance


Yep, that's the one I posted from Hughes and Kettner. I personally always match unless the amp manufacturer tells me it's safe to deviate.


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## marcos

greco said:


> I wish I had $0.10 for every thread ever written about impedance mismatch.
> 
> Better yet $0.01 for every post in the threads about this topic ...that would be a better financial choice by far...LOL
> 
> Carry on...Don't mind my warped sense of humour.



When i started this thread i sort of figured i would get a lot of different opinions. Its really confusing and the Web was full of conflicting opinions. Right now, i am selling the empty cab or if i get my hands on a cheap 16 ohm speaker i will just replace my 8 ohm Jensen.


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## davetcan

My Stiletto Ace has a very comprehensive section on mismatching in the manual. This is the preface. Again this is particular to this amp.

SPEAKER IMPEDANCE MATCHING & HOOK-UP GUIDE: 

IMPEDANCE: Wiring up speakers to provide the most effective load and making sure that all of them are in phase will help in creating the best sound possible. This is not too difficult, as long as you understand a few things about loading and how to connect your speakers to provide an optimal resistive load. MESA/Boogie amplifiers can handle 4 and 8 ohms effectively. Never run below 4 ohms in a tube amplifier unless you are absolutely certain that the system can handle it properly; this can cause damage to the Output transformer. A few amplifiers can handle 2 ohms effectively without damaging them (for example the MESA’s Bass 400+). You can always have a higher resistance (16 ohms, for example) without damaging results, but too low of a resistance will likely cause problems. 

MIS-MATCHING: When running a higher resistance (for example 8 ohm output into 16 ohm cabinet), a slightly different feel and response will be eminent. A slight mismatch can provide a darker smoother tone with a little less output and attack. This response is a result of the amplifier running a bit cooler. Sometimes when using more than one cabinet a mismatch will be the only option.

Full section, with diagrams, can be found here, pages 21 thru 26.

http://www.mesaboogie.com/media/User Manuals/stl_ACE.pdf


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## Chitmo

BMW-KTM said:


> Here ya go, fellas.
> 
> Consider the argument over forever.
> 
> 
> Z-Matcher-100 100w Impedance Matcher
> 
> There's a 50 watt version as well.


Haha, you beat me to it


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## Guitar101

BMW-KTM said:


> Here ya go, fellas. Consider the argument over forever.
> 
> Z-Matcher-100 100w Impedance Matcher


_"R. G. Keen has probably forgotten more about tube amps than most people will ever learn in a lifetime."_

If R.G. Keen is so smart, why didn't he invent this?^)@#


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## LexxM3

Knowledge is overrated, when compared against understanding. The myths and confusion will continue until an actual understanding of underlying operation occurs.

Can anyone post an ego-free electronics-based explanation, with basic circuit calcs and circuit diagrams to support that understanding? It's a lot of work, but if done right, would be an immense service to humanity.


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## dtsaudio

LexxM3 said:


> Can anyone post an ego-free electronics-based explanation


Here you go Lexx. They are primarily for Hifi amps, but ALL the theory is the same. There is no voodoo for guitar amps.
In neither of these explanations, you will notice, do they warn about mismatching impedance. They just do the math and show what happens.

https://www.google.ca/url?sa=t&rct=...VnpRhh4mExK5pw&bvm=bv.128617741,d.amc&cad=rja

https://www.google.ca/url?sa=t&rct=...deqv8UWRQ&sig2=-Kw8GFG40Qn4c1TryonzXw&cad=rja


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## jbealsmusic

WCGill said:


> Consider for a moment that for years Fender amps came equipped with a shorting output jack, a failsafe mechanism to prevent tube and amplifier damage should the user forget to plug in a speaker. Obviously a zero ohm load (lowest possible) is safer than an ordinary jack that would present an infinite (highest possible) load to the amplifier.


That's interesting and makes perfect sense. Of course, it begs the question. Why did they stop doing it? If it really is safer, why wouldn't all amp manufacturers do this?



> The "higher load is safer than a lower load" is one of the most pervasive internet tube amplifier myths. Again, know your sources.


"You can always have a higher resistance (16 ohms, for example) without damaging results, but too low of a resistance will likely cause problems." - Mesa Boogie (not a credible source?)



dtsaudio said:


> https://www.google.ca/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&ved=0ahUKEwiAl_TZoqDOAhXBpYMKHf3UDFsQFgg2MAQ&url=http://www.vac-amps.com/TechMonographs/VAC%20Output%20Impedance%20Matching-Technical%20Monograph%2090-9.pdf&usg=AFQjCNEuqJfOKe21KeKEXSqpodeqv8UWRQ&sig2=-Kw8GFG40Qn4c1TryonzXw&cad=rja


Now that was fascinating!

It seems the subject matter is too convoluted to create universally applicable general rules as I instinctively regurgitated in my initial post. Based on the relationship between the tubes and the OT, I could imagine how some amps might be designed to operate with no tolerable mismatch, or a preferable mismatch in either direction (should a mismatch occur).

So, new conclusion: *Always try to match impedance. If you can't, consult your amp's manual or contact the manufacturer to confirm what type of mismatch is tolerable with your particular amp.*

This has been a most interesting discussion.


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## davetcan

I've been doing just that for years with no problems to report to date 




jbealsmusic said:


> So, new conclusion: *Always try to match impedance. If you can't, consult your amp's manual or contact the manufacturer to confirm what type of mismatch is tolerable with your particular amp.*


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## Guitar101

_WCGill said: ↑
Consider for a moment that for years Fender amps came equipped with a shorting output jack, a failsafe mechanism to prevent tube and amplifier damage should the user forget to plug in a speaker. Obviously a zero ohm load (lowest possible) is safer than an ordinary jack that would present an infinite (highest possible) load to the amplifier._


jbealsmusic said:


> That's interesting and makes perfect sense. Of course, it begs the question. Why did they stop doing it? If it really is safer, why wouldn't all amp manufacturers do this?


That's a good question. Here's another.

This is the back of my Valve Junior. Why aren't they all doing this also.


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## greco

Guitar101 said:


> This is the back of my Valve Junior. Why aren't they all doing this also.
> View attachment 22394


Because they have to put in an output transformer that is tapped for 3 impedances and a 3 jacks or a selector switch. 

My ongoing rant is that *all amp heads* should have this feature so that more speaker cab impedances are possible.


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## WCGill

I don't put shorting jacks in anymore either. The main reason is I mostly use multi-tap outputs like the VJ in the picture in your post and a shorting jack isn't compatible. Why doesn't everyone use 4-8-16 multi-tap outputs?-probably cost and not really used in a combo amp. There may be an extension jack a la Fender but not many use extensions with combos.

I have used 680 ohm resistors across the output jack in multi-tap setups so that there will be a load on the transformer at all times. When a speaker is plugged in, the amp doesn't "see" the much larger resistor.


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## WCGill

For a real-world test, take an amplifier, perhaps one of those Mesas, and short the output, dime the amp and pound on your hottest guitar for a spell and see what happens. If the amp is still operable after this, leave the output open and do the same thing. Let us know how it goes. 
You can do us all a favour and short a solid-state guitar amp output and see what happens. I've done this and can tell you that the output transistors were toast.


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## jbealsmusic

greco said:


> My ongoing rant is that *all amp heads* should have this feature so that more speaker cab impedances are possible.


You can actually get upgraded transformers for many amps to make that possible. But yeah, it really should come standard.


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## jbealsmusic

WCGill said:


> I don't put shorting jacks in anymore either. The main reason is I mostly use multi-tap outputs like the VJ in the picture in your post and a shorting jack isn't compatible. Why doesn't everyone use 4-8-16 multi-tap outputs?-probably cost and not really used in a combo amp. There may be an extension jack a la Fender but not many use extensions with combos.


Maybe I should have been more specific. Why don't they continue to do it on amps with a single output jack? A shorting jack costs essentially the same as a standard jack, so cost can't be it. Or would some other components need to be wired in as well?



WCGill said:


> For a real-world test, take an amplifier, perhaps one of those Mesas, and short the output, dime the amp and pound on your hottest guitar for a spell and see what happens. If the amp is still operable after this, leave the output open and do the same thing. Let us know how it goes.


Not exactly an analogous test. The effects of the difference between 1:0 and 1:∞ is not at all the same as 2:1 and 1:2, which are the types of mismatches we're discussing. When an amp manufacturer says it is safer to go up, they are not in any way saying that it is safer to go up to infinity.


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## marcos

Could i put two 8 ohm speakers in the cab to make it 16 ohms? If someone has a diagram that would help.


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## WCGill

jbealsmusic said:


> Maybe I should have been more specific. Why don't they continue to do it on amps with a single output jack? A shorting jack costs essentially the same as a standard jack, so cost can't be it. Or would some other components need to be wired in as well?
> 
> Not exactly an analogous test. The effects of the difference between 1:0 and 1:∞ is not at all the same as 2:1 and 1:2, which are the types of mismatches we're discussing. When an amp manufacturer says it is safer to go up, they are not in any way saying that it is safer to go up to infinity.


If we're discussing 2:1 or 1:2, then there's nothing to discuss. Totally safe mismatch.


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## WCGill

marcos said:


> Could i put two 8 ohm speakers in the cab to make it 16 ohms? If someone has a diagram that would help.


Absolutely. Wire them in series, + to - between the speakers and hook the remaining + and - terminals to your amp or input jack.


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## marcos

WCGill said:


> Absolutely. Wire them in series, + to - between the speakers and hook the remaining + and - terminals to your amp or input jack.


Much appreciated.


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## davetcan




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## jbealsmusic

WCGill said:


> If we're discussing 2:1 or 1:2, then there's nothing to discuss. Totally safe mismatch.


Sorry to nitpick, but now you're going against what R.G. Keen suggests, who you referenced as a highly regarded expert on the subject. You're saying that a 2:1 or 1:2 mismatch is "totally safe". But in Mr.Keen's work referenced earlier in this thread, he mentioned that he wouldn't do a mismatch like that with certain amps because "they are not all that sturdy under that load" (under a 2:1 upward mismatch).

Based on a second review of the info in R.G. Keen's article and the white paper posted earlier by *dtsaudio*, it is clear that the amp's individual design has an effect on the results of mismatching impedance. For that reason, I'll stick to my earlier revised conclusion that we should go with whatever the amp manufacturer says. Full disclosure, I did not read the entirety of each article. I only read the sections on how the output transformer and tubes handle loads.

I have learned a lot from the contributions to this thread. Thanks for introducing me to R.G.Keen's work. I look forward to continuing through the rest of the articles at a later date.

For now, it is back to what this is all about. Jam time! Maybe I'll do some impedance mismatching just to see what happens. 

PS - I did notice R.G.Keen thanked our very own Mark Hammer in the linked article. Very cool!


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## marcos

davetcan said:


>


Thanks Dave


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## WCGill

jbealsmusic said:


> Sorry to nitpick, but now you're going against what R.G. Keen suggests, who you referenced as a highly regarded expert on the subject. You're saying that a 2:1 or 1:2 mismatch is "totally safe". But in Mr.Keen's work referenced earlier in this thread, he mentioned that he wouldn't do a mismatch like that with certain amps because "they are not all that sturdy under that load" (under a 2:1 upward mismatch).
> 
> Based on a second review of the info in R.G. Keen's article and the white paper posted earlier by *dtsaudio*, it is clear that the amp's individual design has an effect on the results of mismatching impedance. For that reason, I'll stick to my earlier revised conclusion that we should go with whatever the amp manufacturer says. Full disclosure, I did not read the entirety of each article. I only read the sections on how the output transformer and tubes handle loads.
> 
> I have learned a lot from the contributions to this thread. Thanks for introducing me to R.G.Keen's work. I look forward to continuing through the rest of the articles at a later date.
> 
> For now, it is back to what this is all about. Jam time! Maybe I'll do some impedance mismatching just to see what happens.
> 
> PS - I did notice R.G.Keen thanked our very own Mark Hammer in the linked article. Very cool!


Are you saying he's not an outlier anymore? ;-)


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## cboutilier

BMW-KTM said:


> Here ya go, fellas.
> 
> Consider the argument over forever.
> 
> 
> Z-Matcher-100 100w Impedance Matcher
> 
> There's a 50 watt version as well.


That's what I need! My 2 ohm output tranny is a bit of a PITA when it come to cabinet options.


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## jbealsmusic

WCGill said:


> Are you saying he's not an outlier anymore? ;-)


Not quite.  An outlier is someone who stands apart from his/her group for some reason (behaviour, beliefs, practices, etc.) If he claims that a mismatch going down is *always* better than a mismatch going up, then he would still be an outlier considering that is not the more widely held belief. Whether he is right or wrong has no effect on that.

I'd be interested to know how he would respond to the many questions and comments in this thread. Or, how he would respond to amp designers/manufacturers who recommend an upward mismatch over a downward one.


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## R.G.

Hi. I got a request to come over here and try to sort out some things. I'll try, but be sure to ask questions where I muddy it up, or where I get it flat wrong. 

There are two "kills your amp" scenarios, which are different for solid state and tube amps. 

Solid state amps have trouble coping with too-low impedances or short circuits. Most of the better ones have internal protection circuits to try to catch them before they vaporize the output devices, but some of the older, vintage ones don't. And there are some special cases where funny loads can drive them to oscillate their brains out, but this gets rarer as engineers accumulate more info on how to keep the wolves at bay. For solid state amps, follow the maker's recommendation on the minimum load impedance. But you can generally run them with an open output, no speaker or resistor load at all, and they're fine. 

Tube amps are fundamentally different. Tubes can provide lots of voltage, not much current, so we need output transformers to change that to the lower voltages and higher current that efficient speakers need. The transformers bring in a lot of side effects. One of the side effects is a tendency to oscillate if any feedback is used in the amp. Remember that "oscillate its brains out" from the solid state amps? Tube amps can do the same thing, it just happens in a different manner. Drive them hard enough at a high enough frequency and the output tubes can't dispose of enough of the energy they're handling into the output transformer, and it stays in the tube as heat. Eventually everything dies when it gets too hot.

But there's another set of gremlins lurking. The output transformer itself can cause oscillation, which is literally the amplifier driving itself HARD, and if the conditions are right, not only do the tubes melt, but the transformer, being inductive, can cause voltage spikes that will puncture the transformer insulation or arc over the output tubes - other forms of death. There are other causes, but they're rarer. 

The OT is why no-load is dangerous for a tube amp. It can contribute to the oscillation, or make the voltage spikes. This happens worst when it is open-circuited on the output. It, and the power tubes, can cope just fine with being short circuited. The output tubes will self-limit current ***in most cases*** when the OT is shorted. There are special, unusual cases, but in most otherwise normal cases, shorting won't cause problems. Long term shorting might contribute to OT overheating, but "long term" means "several thermal time constants of the OT", which is on the order of hours. Transistors don't self limit current, which is why shorts on them are a problem.

The question then becomes - how "open" is too much? There were extensive evaluations made back in the Golden Age of tubes, the 50s and early 60s. They ran pairs of tubes at all kinds of loads from shorted to opens to find what gave the best power, least distortion, etc. I've read a lot of these (where I could find them) and there is no warning "don't go above X". The available power simply drooped as load resistance went up. I have personally run tube amps on open circuits. I did this first accidentally, then tinkered to see what happened. 

Nothing happened. No death, no disaster. Then I realized that the amp I was working on did not use feedback from the speaker output. No feedback, no oscillation. I hooked up some feedback around the OT, it oscillated, and I QUICK!! turned it off. 

The question of how much load is needed is tricky, because it's probably different for every amp, OT, and amount of feedback. I suspect that every manufacturer reaches their own favorite answer and none of them wants to put themselves out in public as defining a limit. Best I can tell, 2:1 or even 4:1 mismatch is non-fatal. I dimly remember reading that Fender used to use a 270 ohm resistor on speaker jacks that might be unplugged. That's a lot bigger than 4:1.

I personally view different speaker mismatches as clumsy tone controls. The speakers will sound different when driven off the optimum match. Some better, some worse. But running a 4 ohm speaker on an 8 or even 16 ohm OT setting should not be fatal. If it is - and there are some edge cases of poorly designed or otherwise frail amplifiers that might make it that way - it had other problems. Don't be afraid of a 2:1 mis-match. 

If your manufacturer is screaming about don't do it, they may be doing that because they absolutely know mismatches are fatal (YIKES! What does that say?). They could also be more calmly saying that the output power is lower on mismatches, because it is, and they don't want their amps to get a reputation for not being loud enough, or that they just want fewer and less complicated warranty returns. It's tempting but not logically sound to generalize from one manufacturer's advice to what tube amps will or won't do.


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## Guitar101

If this really is R.G. Keen, please ignore post #24 %h(*&


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## jbealsmusic

R.G. said:


> Hi. I got a request to come over here and try to sort out some things. I'll try, but be sure to ask questions where I muddy it up, or where I get it flat wrong.


Cool! Welcome to the forum! Shame it is under these strange conditions, but awesome nonetheless.



> The question of how much load is needed is tricky, because it's probably different for every amp, OT, and amount of feedback. I suspect that every manufacturer reaches their own favorite answer and none of them wants to put themselves out in public as defining a limit. Best I can tell, 2:1 or even 4:1 mismatch is non-fatal. I dimly remember reading that Fender used to use a 270 ohm resistor on speaker jacks that might be unplugged. That's a lot bigger than 4:1.


So, despite the widespread fear mongering on mismatching tube amps, we're actually quite safe in either direction provided the mismatch isn't too great.

However, that leads to a question:


> If your manufacturer is screaming about don't do it, they may be doing that because they absolutely know mismatches are fatal (YIKES! What does that say?). They could also be more calmly saying that the output power is lower on mismatches, because it is, and they don't want their amps to get a reputation for not being loud enough, or that they just want fewer and less complicated warranty returns.


...which answers that question. 

Considering the contrast of your comments on certain Marshall amps not being stable using an upward mismatch vs a company like Mesa who says upward mismatches are safer on their amps. Do you think it is conceivable that an amp manufacturer could design an amp that might favour a mismatch in one direction over the other?


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## R.G.

Guitar101 said:


> If this really is R.G. Keen, please ignore post #24


No problem.

My best guess about what's in the matcher is that it's an audio autotransformer, one coil on a core with a zillion taps, since no isolation is needed. I had no chance to invent it, as the audio autotransformer predated my birth. It's from the early part of the 1900s.

It's not nearly as fun as some of my "inventions"  . For instance, I really did personally invent the flying lawn mower, using the rotation of the blades as a fan to float the housing of a rotary lawn mower by ground effect. That was while I was in grade school. 

I bring this up because how dumb is a flying lawn mower?  The cut is uneven every time you move the handle, it slides sideways so you can't mow a straight line, and don't even try it on a slope in ANY direction. Sigh. Another great invention in the tradition of Wallace, and Bart Simpson.

Years later I saw it had been independently invented and commercialized as the "Fly-Mo", which is still sold in the UK, I think. AAAACK!!


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## R.G.

jbealsmusic said:


> Considering the contrast of your comments on certain Marshall amps not being stable using an upward mismatch vs a company like Mesa who says upward mismatches are safer on their amps. Do you think it is conceivable that an amp manufacturer could design an amp that might favour a mismatch in one direction over the other?


I spent a long time in a major computer manufacturer's employ, and have some insight into how things get designed. It is entirely conceivable that an amp manufacturer would design their amps to be better at mismatches in one way or the other - if they had any clue how to do that. 

That's a weightier question than it seems. It's not too clear to me how you could design an amp to be safer at mismatching up in output impedance; and yes, that means that it may be possible and I just haven't learned how to do that yet. But at my present state of knowledge, the root cause of killing amps and OTs with upward mismatches seems to be oscillation, and oscillation seems to be related to higher output impedances in the "normal" Class AB push-pull guitar amp.

It may be that they're relying on the internet (or, previously, rumored) wisdom that higher output loads use less current, so there's less heating resulting from the lower currents in the OT. I personally don't think that's accurate. The proof of that would be to do a field test, getting many amp techs to send you dead OTs from amps they repair, with as good a description of the circumstances of the death as possible, then to conduct a statistically significant number of OT autopsies. My comments on Marshall OT death was based on discussions with amp techs, them searching their memories for what died, and why they thought it died, as well as descriptions of the circumstances. 

The autopsies would look for whether the transformer died first from an insulation puncture from overvoltage, or from generalized overheating inside the OT causing insulation failure. Those are the two main ways transformers die. 

Generalized internal heating is a slow death for a transformer, taking place over hours as it heats up in most cases. Voltage puncture can be quick, or can be the instigator of an arc that slowly gets worse. 

Mesa may be putting faith in over voltage clamps, usually done with reversed biased diodes from ground to the plates/ends of the OT. The diodes are there to clamp the off-side winding voltage to no more than 2x or so times B+ by transformer action. I personally don't think this is a great way to protect things, but I suppose it's better than a lucky charm in each box.


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## WCGill

Using diodes is an old Ken Fischer trick and I believe Fender used it as well. I've tried them but in the end clipped them out because I thought I could hear something not quite right but it may well have been psycho-acoustics.
Is this a "poor man's" conjunctive filter? The RCA tube manual suggested using one in the case of a marginal output transformer if I recall correctly.


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## R.G.

The idea behind the diodes is that they turn on when the half of the transformer primary that's closest to ground gets pulled UNDER ground by some transient event. When that happens, the voltage on that half-primary is clamped between B+ and ground by the diodes being turned on. 

The OT center tap sits at B+, and the "on" output tube pulls one end down to between 50 and 75V above ground, driving the secondary. The "off" tube doesn't let current flow, so its half of the OT is pulled up above the CT by the same(ish) voltage that the "on" side is pulled down. So it's maybe 100V to 150V less than twice the B+ in normal operation under full output. 

When some transient pushes either half of the OT primary to a voltage greater than B+, the low side diodes turn on to ground and the two halves are clamped to no more than 2x B+. That's the idea.

It has issues. One of these issues is that there is leakage inductance on each side of the primary, and this inductance is NOT clamped by the diodes. So there's still an unclamped inductor that can provide voltage spikes to puncture insulation. 

Conjunctive filters do a couple of things, primarily messing with the high frequency amplitude and phase response of the OT, but they can provide some conversion of a voltage spike into a damped sine wave ringing. This will be at the frequency of the (unspecified...) leakage inductance and the capacitive part of the conjunctive filter. This could cut voltage transients, but will certainly mess with the high frequency response, for better or worse. Tuning the response of the conjunctive filter for various things can be done, and may become an involved process. 

I personally like voltage clamping across the entire primary, or across each half-primary. I used MOVs in the Workhorse amps, but I understand that TVS parts are lower capacitance. This clamps the voltage on both halves of the primary at once, and can eat the transients from a leakage inductance, where diode stacks can't. I think they're better protection.

Whether any or all of these are noticeable in sound is subject to question. All of them add some capacitance to the OT primary. That may or may not be enough to cause audible differences. It is certain that all of them will cause audible differences if the voltages are big enough to activate the diodes, the MOVs, or cause lots of current in a conjunctive filter. I've always thought that the audibility of a protection circuit was better than no sound at all from a dead amp, but that's just me.


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## jb welder

Thanks for coming over RG! I didn't know you had moved to Van. 
I guess I'm one of those who has contributed to the anecdotal evidence of Marshall having more OT failures than Fender. I've worked on way more tube Fender's than Marshall's, yet have replaced less Fender OT's. In fairness, a couple things could contribute to this. First the fact that Fender's usually have a shorting jack on the output where Marshalls never do. Second, I think the type of music and player is more likely to lead to Marshall output stages being overdriven much harder. Thirdly, if it is true that an under-rated (or less over-rated) OT will sound different than an OT with more cubic inches per watt, then perhaps Marshall has decided not to upgrade their OT size due to sonic considerations.
All speculation, but perhaps some of those factors could be contributors?


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## R.G.

It's a tribute location. 

All of those things have an effect. The deeper/hidden reality in iron core transformers is that the M-field is actually transmitting the collective motions of groups of iron molecules ("magnetic domains") inside the iron. This happens to be very nearly linear, but not quite. It takes a shove to get them to change orientation a little, then it's easier to push them around, and finally they all get lined up and can't magnet-field any harder, so the domains get more "saturated". Along the way, the M-field changes create heating by eddy current conduction in the electrically-conducting iron, as well. 

What this leads to is a lower-than-peak inductance for low magnetic fields, getting bigger, then peaking and flattening out at saturation. If the field happens to be an audio signal, there is some distortion of the signal as a response to the magnetic domains not quite being perfectly in step as they pivot around. That effect happens *under* the transfer of energy to the secondary, which *is* linear - well, a linear transfer of the time derivative of the field changes. The mathematical water gets deeper quickly here. 

Designing an OT is an exercise in manipulating the field intensities so you get as much linear transfer as possible, over a big enough range of signals. Deciding what is "enough" is a big part of that, and the sonic output and music to be played definitely affect that. When Fender and Marshall were making their early stuff, they both used off-the-shelf hifi OTs. Fender probably used them because they were cheap and readily available, Marshall used them because of that and he was copying Fender at the start. Neither expected guitarists to go nutso over how much and what kind of distortion they could get. And the guitarists themselves changed over time.

The anecdotes I've heard are that Marshall stayed with a smaller OT per watt. This is quite possibly because Marshall amps evolved into putting out less bass on average, and saturation effects in transformers are primarily a low frequency effect. Internet wisdom aside, it is nearly impossible to saturate an OT with a treble signal. Remember that "linear transfer of the time derivative" stuff? How hard a transformer's fields work depends on the frequency and size of the signal - the volts times cycle time. Saturation in audio transformers is a bass signal effect. Cut bass by using smaller capacitors in the signal path and less bass gets to the OT, so a smaller OT actually does handle more power, because the power is at a higher frequency. So if you're making "brighter" ( = less bass ) amplifiers, you don't need as big a transformer. Smaller transformers have less mass, and heat faster. They also get cooked more if your amp tech replaces some of those smaller signal caps with bigger ones so the net sound is "warmer".

Finally, since economics is above the law, it is entirely possible that Marshall stayed with smaller transformers because they were cheaper, being a function of the amount of iron and copper, and the amps were selling well with the smaller transformers. My bet is that neither Fender nor Marshall expected or even thought about their 1960s production of amps being around for use 50 years later.


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## Kerry Brown

I want to thank all the contributors to this thread. I have learned a lot.


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## WCGill

Briefly (if possible) R.G., what effect do different types of steel have on these transfer characteristics?


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## R.G.

Briefly if possible??
You know me, I see. 

As a practical matter, it is unlikely that any guitar amp made since the 70s has anything except ~~4% silicon-iron in either grain-oriented or non-grain-oriented varieties. I can go launch off into the subtleties of nickel, nickel-iron, cobalt-iron, and other stuff, but unless you wind it yourself or pay a fortune for it, you probably won't get it. There are exceptions, but they will very much be exceptions. 

Grain oriented ( = "rolled HARD in one direction to mechanically orient the grains in a preferred direction") transformer iron is pretty much the standard. It gives high permeability (translates to high inductance per turn) along the preferred direction, has high saturation flux density ( = you can make smaller transformers with it than otherwise), soft edges to saturation, and low remanence (it doesn't tend to take much of a magnetic set in one or the other direction) and it's cheap. It makes good AC power transformers and good audio transformers.

The biggest variations are in lamination thickness. The thinner the lamination, the less power loss and heating to high frequency signals, and the easier it is to get more highs through it. High end hifi stuff goes for thinner laminations. In a guitar amp, it might or might not be audible. 

There are transformer irons for special purposes that have sharper edges to the saturation region. They used to be used for self-oscillating things and magnetic amplifiers. Not so much any more, I suspect. You can get other irons, but once you get off the main path, expense goes up a lot. 

I'm painting with a yard-wide brush here. To keep the verbiage down, what kind of steels did you have in mind?


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## WCGill

Well, grain-oriented seems to be the one most in use. I was wondering as well what the "M-6 and M-19" designations mean and when nickel and amorphous steel get used. Thanks.


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## R.G.

That helps. 

The M numbers increase in terms of increasing core loss. So M6 has a lower core loss than M19 for a given excitation level. Lower core loss is good, as it means that the steel has a smaller internal B-H curve, the side effect of which is that the curve is more linear. Probably you're squinting at gnats for the difference between M6 and M19 for OT use in guitar amps, but if you can use M6, use it. The premium for that isn't much. 

Nickel alloys have a very high permeability (inductance per turn) and low core loss, which are good for audio transformers, but have less than half the saturation flux density of silicon iron. For audio use, this makes them good for low level transformers, not so much where you're trying to push a lot of power through them because you'll have to use more core material and turns to keep the flux density down. This makes the cost issue worse, of course. I have seen either 50% or 80% nickel used in premium input transformers and interstage transformers. I'm sure that there are tweako and high end hifi OTs from nickel, but they're in the cost-is-no-object range. Same comments but more so go for cobalt alloys, which are even trickier to process into laminations. 

Amorphous steel is ordinary transformer iron alloy, but extruded molten into a water bath at very high speed as a thin ribbon. The water and thin section freezes the iron from molten to solid so quickly that it's an iron "glass" and iron-crystal magnetic domains don't have time to form before the molecules are locked solid. This makes the domain size very small, and produces high permeability, low loss and more linear iron. Good stuff. 

But limited. The cooling demands and very high hardness from the quenching mean that it's only available as long ribbons, not laminations, so it's limited to being wound into toroids.


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## R.G.

Having let this settle a little on my mind, I think that you might be interested in my opinion on OTs in general. Messing with different core materials is fun, but everything inside an OT is so interactive that sorting out the options for what's a lot of return for the effort, and what is not so much.

The biggest bang for your buck (or labor, which amounts to the same thing) is in how you wind the coils, much more so than getting a super premium core. The guys in the Golden Age had their finger on what makes a good OT, and they quoted the figure of merit as being the low frequency primary inductance divided by the leakage inductance. Really good OTs had this goodness factor as 100,000 to 200,000. 

Getting to there required big primary inductance, which generally meant high permeability iron, usually grain oriented silicon iron, and lots of it, along with many turns to run the primary inductance higher than you'd get from just keeping the iron out of saturation on volt-time peaks. The iron was often worked to only half it's saturation flux. This had the side effect of lowering the distortion introduced by the nonlinearity of the B-H curve and also lowering the loss per cycle around that B-H curve.

It also required good interleaving and sometimes good sectionalizing to get the leakage inductance low. Side effects in the pursuit of low distortion came in the flavor of highly interleaved primary halves so that each half-primary was tightly coupled ( = low leakage inductance) to each other as well as to the secondary. This helped minimize the crossover glitch in primary current when the tubes crossed over. 

This was the issue solved by the McIntosh "unity coupled" design of OT, tight coupling of the two halves to keep crossover down by magnetic means as well as biasing. The McIntosh split both half-primaries into a plate side and a cathode side and ran both output tubes with a plate and cathode winding. It kept the coils coupled all the time by distributing these two sections around interspersed with each other. 'Course that ran the voltage gain down on the output tube, so it needed a lot of drive voltage, but everything good costs you something.


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## WCGill

Very cool stuff, many thanks.


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