Is 4 ohms or 16 ohms "better"?

Matt_Krush

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________

Amen,

And if you have a mystery speaker cab and you don't know the ohms, use a multi-meter and plug-in a speaker cord to the speaker cab and get a reading on the 1/4" mono jack to know for sure. If you run a 4 ohms cab on the 16 ohms Head setting, well it will not be good if you dime the amp for a while. We have jacks with a small 12" pigtail and alligator clips to test speaker cabs. Some cabs have a bad speaker and measure as low as 2 ohms sometimes. If you don't know what's in the cab, find out.

After you lose an OT on a good amp it wakes you up.

This is why most cabs are stenciled or labeled on the rear in a touring band.

not exactly...impedance and resistance are not the same thing, although both are measured in ohms..

A 4 ohm speaker will not read 4 ohms...
You are measuring passive or real resistance with a multi-meter.
The speaker is rated as an impedance which includes reactive (capacitance/inductive) as well as real resistance.

Typically...a speaker measured with a multi-meter will read lower than the rated impedance.
 

Marshall & Moonshine

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As you say, the voltage goes up., often to a point where the insulation fails and the primary shorts. This is why the practice of using shorting speaker jacks as on most fenders is better than the open jacks Marshall always used. Vox use a compromise in the old days with a 470 ohm resistor across the output jacks, so there was never a no load situation.
J
I don't mean that the primary voltage goes up as a spike, rather that it goes up and down, following the signal voltage in a normal way. As long as the valve output sees the primary impedance, why should it matter if there was an open circuit in the secondary? It still seems to me that having a speaker fail open wouldn't hurt anything, and similarly, if the speaker jack didn't switch closed with nothing plugged in, no damage should occur when operated with no load.
I can't imagine that the primary side wouldn't be rated for all the voltage that the B+ could throw at it. I mean, is it an inductive spike or something?
 

KenG

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There are about a million combo owners who better run out and grab another cab then. Definitely shouldn't run a tube amp without a load but if sound stops coming out I would assume most people would shut off the amp. Ot s don't blow quite that easily. I'd be replacing one a year if that were the case.

I find 16 sounds slightly better on a Marshall but in a band context it's lost.
Output transformers rarely blow period, I don't know where people get these myths from, usually a total lack of understanding of electronics. However I've repaired a few amps where the reflected power smoked the output tubes and components in the power amp stage, required considerable work.:slash:
 

KenG

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I don't mean that the primary voltage goes up as a spike, rather that it goes up and down, following the signal voltage in a normal way. As long as the valve output sees the primary impedance, why should it matter if there was an open circuit in the secondary? It still seems to me that having a speaker fail open wouldn't hurt anything, and similarly, if the speaker jack didn't switch closed with nothing plugged in, no damage should occur when operated with no load.
I can't imagine that the primary side wouldn't be rated for all the voltage that the B+ could throw at it. I mean, is it an inductive spike or something?
A transformer in itself is not a load unless both sides have circuit paths for current flow. Thats why it's called a transformer, it only changes the voltage or current, it does not dissipate it (the wattage rating is for the amount of current that will flow through the coils) It's a magnetic device the energy that travels through the primary has to couple over to the secondary and get absorbed, if it doesn't (because there's no signal path (open load) on the secondary) the primary acts as an inductor that is pulsed with a voltage. Once the voltage (signal) is removed the inductor's magnetic field collapses and it sends out an equal or higher reverse polarity voltage spike. Modern relays which use magnetic coils (subject to inductance) often contain diodes wired in reverse polarity across the coils to absorb this reverse voltage spike or the designer has to put them in the circuit his/her self.
 

Marshall & Moonshine

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But if the tube sees a coil of wire wrapped around the iron core, with the proper primary impedance, why should it matter if the secondary is open? With no load on the secondary, the only load is the inductive load on the primary, right? It really shouldn't draw that much by itself. I still don't see how this burns out on an open secondary.
 

KenG

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But if the tube sees a coil of wire wrapped around the iron core, with the proper primary impedance, why should it matter if the secondary is open? With no load on the secondary, the only load is the inductive load on the primary, right? It really shouldn't draw that much by itself. I still don't see how this burns out on an open secondary.

I never said it burns out the secondary, as a matter of fact I never said it would even hurt the transformer at all! The reverse voltage comes from the primary side and can ruin the output stage (tubes, plate resistors etc), you can't couple current into the secondary windings if there's an open circuit on that side, there is no path for current flow. Its a basic electrical law. The transformer is not meant to act as purely an inductor, the current going through it must be transferred to the other side.
For example you will never see an amp where the power supply transformer is given power without a minimal load on the secondary side. Whether it's filaments or even just the rectifier and filter there will always be a circuit across the secondary (or portion there of) when the AC switch is on.
 

Marshall & Moonshine

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Ok!!.... I get it now. I was thinking this whole time that it was the OT that takes the hit from blown speakers, but just because I read it a billion times on the interwebz. It never really made sense. I'll have to read that a few more times, but I think you've gotten me past the part that was holding me up a little. :)
So it's actually the output tube(s) that get damaged. What's the "reverse voltage" from? Is it like a CEMF or something from the collapsing field?
 

Wrench66

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Ok, quick question. I run my 18 watt clone with the speaker selector set to 16 ohms and the speaker is an 8 ohm Greenback. It sounds a lot better and research I've done on the web says you can mismatch up or down one step (8 ohms) with no problem. I've had it this way for about 6 months and have seen no ill effects yet (amp is about 9 months old). What are your guys thoughts here?

The amp is a 18 watt clone I built myself from a kit. I figure if I damage something, I should be able to fix it easily, sense I built it myself anyways.
 

Ginger Beer

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As sometimes happens, as soon as I posted my question I came across this post: http://www.mylespaul.com/forums/squawk-box/311342-4-ohm-vs-16-ohm-observation.html

I guess I'll just experiment and see what I like best.

Thanks for the responses!!!
That was my thread. I don't think there is a wrong or right answer, just what sounds best to your ears. Since the original posting, I've gone back and did the A/B test once again. Just like the first time, 4 ohms yielded a "slightly" more defined signal while 16 ohms yielded a "slightly" more smeared sound. The differences were very, very small.

I have been generally using the 4 ohm tap but that's more due to not worrying about the cab impedance that I'm plugging into (various ones at different practice / jam locations) than anything else.

Avoiding a detrimental mismatch is my only concern.
 

ScottMarlowe

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Just FYI, what the power tubes see from the primary of the OT is a reflection of what's on the other side at a given ratio. So if the other side is 4ohms and the windings are such that that looks like say 4k to the power tubes, when the speaker disappears, the power tubes see an impedence of infinity * 1000. What happens in this instance is that the amplitude across the primary coil of the transformer now swings wildly high, and can measure much higher than the B+ voltage applied through it to the plates, into the thousands and more volts. This high voltage can result in arcing within the power tubes, across the wiring in the amp, and through the insulation of the windings of the output transformer itself, blowing up just about anything from the plates to the transformer, and possibly the rectifier of the B+ section as well.

It's not about heat, it's about high voltage and arcing.

Note that an amp that's on, with no output won't usually blow up. It's when you keep strumming power chords with it turned up, trying to get noise out of it that it blows the OT/tubes/wiring etc. As soon as an amp goes quiet STOP PLAYING and turn it off.
 

Tele Jr.

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Bruce Zinky was kind enough to teach me all about this stuff some time ago, so I mayswell share it and pass it on. I hate to see threads like this with people groping for the truth and so much mis-information to filter through...



When you have a 2x12 v30 cab, for example, with two 8 ohm speakers, wiring to 4 ohms will give you a more classic so called vintage sound, more rounded with less highs and lows. This is how most vintage amps were wired.

If you wire the 2 v30's to 16 ohms, you will have the so called modern sound, yielding more highs and lows.

Also for perspective note that a 16 ohm v30 will sound a lot like 2 8 ohm v30's wired to 16 ohms, when compared to an 8 ohm v30 which will have a more vintage sound as per this discussion.

Also when you use a 16 ohm cab and have 4,8, and 16 ohm taps on your amp you can get the so called power reduction effects 1/3 and 2/3, when using the 8 and 4 ohm taps which are way cool and worth having available imho. Note this is how Bruce designed his Fender heads to work. See the Tone Master owners manual for more info on how they explain it there.

But doing the 4 ohm wiring on the cab and using the 4 ohm tap on your amp would give you a classic more vintage style sound that might be more what you are looking for.

I have one 2x12 v30 cab that I can change over easy, but I have to remove one of the speakers to do it. I mostly am favoring using the 16 ohm wiring because I find the so called power reduction effects to provide very versatile tone shaping options with my high powered amps.

I was talking with Bruce one day and told him I had 2 v30 cabs, one wired each way, could hear the difference, and really liked the sound of each, but didn't really understand what I was hearing or how to interpret it. So he hauled off and went ahead and told me how all this stuff works. And made it pretty simple to understand I believe. So I hope some of this can be passed on and become conventional wisdom for those who wish to know how someone like Bruce would explain it.

You need a little experience with listening to the cab wiring choices and using the power reduction effects to learn how versatile the tone shaping options can be with a simple 4/16 ohm cab and amp with multi tap 4/8/16 tranny.
 

Marshall & Moonshine

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Just FYI, what the power tubes see from the primary of the OT is a reflection of what's on the other side at a given ratio. So if the other side is 4ohms and the windings are such that that looks like say 4k to the power tubes, when the speaker disappears, the power tubes see an impedence of infinity * 1000. What happens in this instance is that the amplitude across the primary coil of the transformer now swings wildly high, and can measure much higher than the B+ voltage applied through it to the plates, into the thousands and more volts. This high voltage can result in arcing within the power tubes, across the wiring in the amp, and through the insulation of the windings of the output transformer itself, blowing up just about anything from the plates to the transformer, and possibly the rectifier of the B+ section as well.

It's not about heat, it's about high voltage and arcing.

Note that an amp that's on, with no output won't usually blow up. It's when you keep strumming power chords with it turned up, trying to get noise out of it that it blows the OT/tubes/wiring etc. As soon as an amp goes quiet STOP PLAYING and turn it off.
So if I understand it correctly, it's because the secondary isn't loading down the primary, and it exceeds the rated voltage because of wild swings, like over-revving an engine in neutral, right?? That would make perfect sense to me.
 

tolm

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But doing the 4 ohm wiring on the cab and using the 4 ohm tap on your amp would give you a classic more vintage style sound that might be more what you are looking for.
Aligns with what Mark Bartel told me when I asked about the Royalist 2x12 being wired in parallel for 4ohms - he explained he'd A/B'd both and the 4ohm parallel wiring just sounded "better" for that vintage Marshall-y tone the amp is designed for.
 

ScottMarlowe

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So if I understand it correctly, it's because the secondary isn't loading down the primary, and it exceeds the rated voltage because of wild swings, like over-revving an engine in neutral, right?? That would make perfect sense to me.
Exactly. The reactive, inductive load from the transformer's primary results in massive voltage swings.
 

D-POLAND

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Just FYI, what the power tubes see from the primary of the OT is a reflection of what's on the other side at a given ratio. So if the other side is 4ohms and the windings are such that that looks like say 4k to the power tubes, when the speaker disappears, the power tubes see an impedence of infinity * 1000. What happens in this instance is that the amplitude across the primary coil of the transformer now swings wildly high, and can measure much higher than the B+ voltage applied through it to the plates, into the thousands and more volts. This high voltage can result in arcing within the power tubes, across the wiring in the amp, and through the insulation of the windings of the output transformer itself, blowing up just about anything from the plates to the transformer, and possibly the rectifier of the B+ section as well.

It's not about heat, it's about high voltage and arcing.

Note that an amp that's on, with no output won't usually blow up. It's when you keep strumming power chords with it turned up, trying to get noise out of it that it blows the OT/tubes/wiring etc. As soon as an amp goes quiet STOP PLAYING and turn it off.
I thought I would try to explain it in a more common setting!

Most everyone has an ac transformer/wall wart, that charges their cell phone , the transformer, if left without a load/cell phone being charged, will handle that without a problem when its energized!

In a tube amp the transformer is feeding the power control circuit directly,without any disconnects/it's hard wired in, unlike a cell phone that is disconnected from it's charging/ power control circuit when you unplug it from the phone!

The tube amp with NO LOAD leaves the power control side/ tubes, in a no load state with the very high voltages now being applied to it ! with no place to go or rather the weakest link in the circuit! :hmm:
 

Marshall & Moonshine

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Exactly. The reactive, inductive load from the transformer's primary results in massive voltage swings.
Cool, man! Thanks! I've been coming at this from an electrician's point of view, not from audio electronics. I install 3kVA transformers that take two men and a boy to lift, and they see a steady voltage on the primary side and don't give two poopies what's connected to the secondary, as long as the rated load isn't exceeded for long.
So this thing where voltage swings are bouncing back and forth, with AC and DC mixing up (kinda--superimposed AC on steady DC, right??) is all new to me. I'm getting there, but I ain't there.
You're one of a small group of guys I really trust on this stuff, even when you sometimes debate amongst yourselves. That part is actually pretty great. I feel like I snuck a chair at the grownups' table. :)
 

D-POLAND

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Sounds like your back round is similar to mine! I have some controls experience too, but mostly in a trouble shooting scenario and mosly without any schematics! that makes you think in systems,logic sorta way!

As far as the amp stuff is concerned, another example that is similar , is a 12 volt car battery charger you always hook up the load/ battery before applying the the power to it!

The car charger is a step down transformer where the amp is a step up tranny!Jjust some basics that you are probably aware of already!:wave:
 

LJGriggs

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Last night I rewired my "Tone Tubby" 2x12 for 4ohms and changed the amp selector to 4 ohms as well. It may be my imagination, but it does sound better to me. The overall sound seems to be more "precise". The tone and response is tighter. It also appears to have cleaned up some of the "mush" in the mids. Of course, I could just be imagining things. If it still sounds the same tonight, then I'll say there is a difference between 16 and 4 ohms. (My amps never seem to sound the same two days in a row...)
 


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