Speaker ohms dropping?

[EOT]

Hey all, I recently bought one of the unloaded Diezel cabs from Brad. I pulled a set of speakers(EVM12L's @ 16 ohms) out of my Marshall cab to put in the Diezel. When I first got the EV's they measured about 13 ohms, now they are at about 10.5 ohms. Both individually, and as a set. This has me not only confused, but worried I might damage my amp. I'm probably fine running my amp at 8 ohms but...

Does any one know what's going on? Or if this is bad?

Thanks

 

[glpg80]

resistance does not equal reactance. there is a very large difference. reactance is a vector quantity and cannot be measured with a voltage meter in any way shape or form.

secondly, you will get build tolerance differences with speakers. and no, do not run them on 8 ohms. run them at the designed specs listed on the magnet.

 

[Anonymous]

as long as your amp load is lower than your speaker(s), it will function fine. not optimum sound or function, but it will function properly without damaging. but, for instance, if you have an 8 ohm amp load going into a 4 ohm speaker or cab, this is trouble.

i've multimetered all my cabs and speakers, and they rarely, if ever, have matched the stamped rating on them.

 

[JTyson]

i've multimetered all my cabs and speakers, and they rarely, if ever, have matched the stamped rating on them.

This ^^
Although I will say I've never seen the resistance change once I had established a reading on them
anyone ever seen that??

 

[EOT]

i've multimetered all my cabs and speakers, and they rarely, if ever, have matched the stamped rating on them.

This ^^
Although I will say I've never seen the resistance change once I had established a reading on them
anyone ever seen that??

My main concern is the change in readings. What could possibly cause this?

And glpg80 what is reactance/vector quantity. I did a quick google search, but was quickly confused
Are you sure it's safe to run my amp at 16 ohms? I'm just really nervous about causing any damage to my amp.

 

[glpg80]

i've multimetered all my cabs and speakers, and they rarely, if ever, have matched the stamped rating on them.

This ^^
Although I will say I've never seen the resistance change once I had established a reading on them
anyone ever seen that??

My main concern is the change in readings. What could possibly cause this?

And glpg80 what is reactance/vector quantity. I did a quick google search, but was quickly confused
Are you sure it's safe to run my amp at 16 ohms? I'm just really nervous about causing any damage to my amp.

reactance is what you are trying to measure with a voltmeter which is impossible.

you could get measurement differences if there was a DC blocking capacitor discharging into the coil - meaning your meter is a POS.

you cannot measure a potential-energy potential in the imaginary plane. what you are trying to read is not readable - your values will be skewed because electromagnetic properties of speakers are not physically measurable. ever.

i have said it once and ill say it again - run the speaker on the designed specifications listed on the magnet and nothing else. are they 16 ohms or 8 ohm coils? if there are no markings then we will go from there depending on how you are wiring the cabinet.

 

[sled]

My main concern is the change in readings. What could possibly cause this?

Are you using the same Meter?

Maybe your initial reading was in error.

If all of the speakers are now reading 10.5 and all of the speakers previously read ~13, the common element here is the Meter. Maybe put new batteries in the Meter.

Try a different Meter.

 

[JTyson]

I would look for consistancy between the readings, maybe your battery is down (as was said)
RX1 scale on ohms, if they are close to the rating, they should be fine if the readings are all similar
I've never seen them ohm out to exactly what they were rated. I dont know why that is, seems like it would be easy to get the rating you were after using rate of resistance/length to achieve the desired number, but in the real world, my guess would be tolerance variances in the copper wire itself that were sourced by the manufacturer
But I'm an idiot

 

[roadifier]

Hey all, I recently bought one of the unloaded Diezel cabs from Brad. I pulled a set of speakers(EVM12L's @ 16 ohms) out of my Marshall cab to put in the Diezel. When I first got the EV's they measured about 13 ohms, now they are at about 10.5 ohms. Both individually, and as a set. This has me not only confused, but worried I might damage my amp. I'm probably fine running my amp at 8 ohms but...

Does any one know what's going on? Or if this is bad?

Thanks

Are the speaker coil leads soldered or crimped at the terminals. I have had speakers from Eminence appear to have failed but later come to find that the crimped terminals are not making good contact so I soldered them up and everything was back to normal.

When I had my EV's they had little posts and they went through the post, and had little screw down knobs to hold the wires in place.

 

[EOT]

Ok, I changed batteries on my pos multimeter(GB industries from Walmart) and tried again. And I used two different speaker cables, a cheaper one I don't know the name/brand of, and a Monster Studio Pro
The results:
The original reading of 10.5 ohms is still there with the cheapo cable but...
with the Monster speaker cable the reading started at 13 and slowly climbed to 24 ohms,
then I checked my 8 ohm cab(Vader) with both cables and they both measured about 7 ohms
I then checked my other 16 ohm cab(Marshall) with both cables producing 14 ohm readings.

Bottom line is I still don't know what's going on with the EV's. I'm starting to think glpg80 is right. I can't properly measure them. I may as well just keep playing through them like I always have and not worry about it. Sometimes ignorance IS bliss.

And baron55, the speaker terminals are spring loaded like these
http://www.parts-express.com/images/ite ... 1144_s.jpg

 

[Anonymous]

This is incorrect. This would apply to a solid state power amp only. A lower speaker load on a tube amp is actually better than a higher speaker load. In either case the correct load of course is optimal.

i beg to differ my friend! let me clarify because i think you thought i had it backasswards!

if you have an 8 ohm speaker, and your amp is set on 4 ohms, being a tube amp, you can most CERTAINLY put this into the speaker. its not optimal, but it WILL work without risk to hurting anything.
not the case if it's reversed: if you have a 4 ohm speaker, and the amp is set to 8 ohms, you CANNOT put the amp into this, or you'll most likely damage the amp.

or, if you have a 16 ohm speaker, you'd want to put the amp on 16 ohms for optimal sound, BUT you can run the head on 4 or 8ohms SAFELY.


with 2 16 ohm cabs, (16x16 = 256) then 256/32 = 8 ohm setting on your head.

or, if you're running an 8 ohm cab and 16 ohm cab, (8x16 = 128) 128/24 = 5.333 meaning a 4 ohm setting on your head. it runs safely.

either way, go by what is stamped on the speakers and you should be fine!

 

[glpg80]

This is incorrect. This would apply to a solid state power amp only. A lower speaker load on a tube amp is actually better than a higher speaker load. In either case the correct load of course is optimal.

i beg to differ my friend! let me clarify because i think you thought i had it backasswards!

if you have an 8 ohm speaker, and your amp is set on 4 ohms, being a tube amp, you can most CERTAINLY put this into the speaker. its not optimal, but it WILL work without risk to hurting anything.
not the case if it's reversed: if you have a 4 ohm speaker, and the amp is set to 8 ohms, you CANNOT put the amp into this, or you'll most likely damage the amp.

or, if you have a 16 ohm speaker, you'd want to put the amp on 16 ohms for optimal sound, BUT you can run the head on 4 or 8ohms SAFELY.


with 2 16 ohm cabs, (16x16 = 256) then 256/32 = 8 ohm setting on your head.

or, if you're running an 8 ohm cab and 16 ohm cab, (8x16 = 128) 128/24 = 5.333 meaning a 4 ohm setting on your head. it runs safely.

either way, go by what is stamped on the speakers and you should be fine!

yeah yeti is correct, a lower reactance load than what the secondary of the output transformer is selected for will cause permanent damage to the OT due to eddy currents causing heat because of the resistance of the internal wire used to wind the secondary.

the rule of thumb is assuring the amplifier see's a higher load than what is expected, meaning an amplifier set to 4 ohms running into a 16 ohm cabinet will indeed work and work fine. you will not have max power transfer therefore the higher reactance load could or could not cause an OT to heat up depending on build quality and wire diameter/DC/AC current ratings and other properties of the transformer, but it will work indeed without any other problems.

as has been said before, match impedances if you dont understand for safe measure. transformers are expensive to replace

 

[JamesPeters]

Baron is trying to help. I'd recommend anyone wondering "which way is ok to mismatch" a tube amp, to read this:

http://aga.rru.com/FAQs/technical.html#imp-1 (Read that and the point below it.)

 

[Black Cat]

This is incorrect. This would apply to a solid state power amp only. A lower speaker load on a tube amp is actually better than a higher speaker load. In either case the correct load of course is optimal.

i beg to differ my friend! let me clarify because i think you thought i had it backasswards!

if you have an 8 ohm speaker, and your amp is set on 4 ohms, being a tube amp, you can most CERTAINLY put this into the speaker. its not optimal, but it WILL work without risk to hurting anything.
not the case if it's reversed: if you have a 4 ohm speaker, and the amp is set to 8 ohms, you CANNOT put the amp into this, or you'll most likely damage the amp.

or, if you have a 16 ohm speaker, you'd want to put the amp on 16 ohms for optimal sound, BUT you can run the head on 4 or 8ohms SAFELY.


with 2 16 ohm cabs, (16x16 = 256) then 256/32 = 8 ohm setting on your head.

or, if you're running an 8 ohm cab and 16 ohm cab, (8x16 = 128) 128/24 = 5.333 meaning a 4 ohm setting on your head. it runs safely.

either way, go by what is stamped on the speakers and you should be fine!

yeah yeti is correct, a lower reactance load than what the secondary of the output transformer is selected for will cause permanent damage to the OT due to eddy currents causing heat because of the resistance of the internal wire used to wind the secondary.

the rule of thumb is assuring the amplifier see's a higher load than what is expected, meaning an amplifier set to 4 ohms running into a 16 ohm cabinet will indeed work and work fine. you will not have max power transfer therefore the higher reactance load could or could not cause an OT to heat up depending on build quality and wire diameter/DC/AC current ratings and other properties of the transformer, but it will work indeed without any other problems.

as has been said before, match impedances if you dont understand for safe measure. transformers are expensive to replace

I will keep this brief, because the last thing I want to do is get involved in any potential flame war, and this is my first post, but.......

Gruppe, This is incorrect, and there are many determinates that need to be accounted for, and becomes obvious when you consider both closed load (grounded load), and open load (infinite load) Scenarios when discussing class ab1 push pull valve output stages, and then plotting some theoretical loadlines both extremes of the spectrum, on the curves of the valves you are using,and also taking into consideration both the plate, and screen voltages that are used in guitar amps. (most valve datasheets do not show curves for screen voltages near what are used in typical guitar amps, you will have to use the transfer characteristics listed to redraw them).

A higher reflected primary impedance is potentially more dangerous (ie running a 16ohm load into the 4ohm tap) than a lower load into a higher tap, for the reasons that article James posted states, and will be the scenario to do potential harm to the OT

a lower than desirable reflected a-a load is only dangerous because the odds of the load line passing above the maximum dissipation curve over much of the tubes operation is increased, which will lead to "Red Plating" under signal conditions. Depending on how bad the situation is, This can be remedied by biasing colder, lowering the screen voltage, and also depends on how efficient the power supply is. this stress is mostly hard on the output valves however. The article posted sort of says this.

Now how much in either direction does it take to have these adverse effects?, and how much leeway do we have? the general rule of thumb is that there is quite a bit of leeway, because there is no such thing as an ideal perfect a-a load, theres an ideal one for power transfer, and an ideal one for linearity, (the datasheets often provide one for power transfer) and a mismatch by one step in either direction is generally acceptable, but again this will depend on how close to edge you already are in either scenario as is, and the only way that I know of at least to figure that out is draw loadlines.

Cool place guys!

-Black Cat

 

[JamesPeters]

Now how much in either direction does it take to have these adverse effects?, and how much leeway do we have? the general rule of thumb is that there is quite a bit of leeway, because there is no such thing as an ideal perfect a-a load, theres an ideal one for power transfer, and an ideal one for linearity, (the datasheets often provide one for power transfer) and a mismatch by one step in either direction is generally acceptable, but again this will depend on how close to edge you already are in either scenario as is, and the only way that I know of at least to figure that out is draw loadlines.

Something else people should keep in mind is: are their amps already running mismatched impedance to a degree, before they decide to further mismatch the impedance (which can be the "push over the cliff")?

Some amps which can run on EL34 or 6L6 don't bother having mention in their manuals that the OT's impedance doesn't change to compensate (if there's a switch for the bias range, that's all it is, not something to change the impedance). EL34 and 6L6 (among other tubes too) "expect" different impedance to operate as expected, all else being equal. So depending on the amp and tubes used, you could already be running a mismatch factor of approximately 1:2 or 2:1.

Also, some tubes these days aren't manufactured to the specs they once were, including the primary impedance. Getting proper specs from tube manufacturers is generally not possible since they tend to reprint data from old datasheets instead of testing their new tubes.

Lastly, all it takes is a dirty cable end or jack to add another variable to the equation.

So generally speaking, I'd say to match the impedance all the time. If you're in a bind temporarily, it's not so bad to mismatch if your amp is set up well (including biasing) but even so I always recommend not running the amp very loud in those cases.

 

[Rogue]

Baron is trying to help. I'd recommend anyone wondering "which way is ok to mismatch" a tube amp, to read this:

http://aga.rru.com/FAQs/technical.html#imp-1 (Read that and the point below it.)

Thanks for that link. Looks like some cool info in there.

 

[Black Cat]

Something else people should keep in mind is: are their amps already running mismatched impedance to a degree, before they decide to further mismatch the impedance (which can be the "push over the cliff")?

Some amps which can run on EL34 or 6L6 don't bother having mention in their manuals that the OT's impedance doesn't change to compensate (if there's a switch for the bias range, that's all it is, not something to change the impedance). EL34 and 6L6 (among other tubes too) "expect" different impedance to operate as expected, all else being equal. So depending on the amp and tubes used, you could already be running a mismatch factor of approximately 1:2 or 2:1.

that kinda pertains to the point I was trying to make, and for all intents and purposes there is no such thing as a set in stone "perfect Impedance" for any given valve to be honest, and you really do have quite a bit of wiggle room, or a wide pallet of "acceptable" reflected plate-plate loads before you get into trouble, depending on where the design already sits, and it's intended use. what most datasheets will quote (if you can find it, most newer manufacture sheets wont quote a-a load) is really the figure for maximum power transfer for the design figures THEY list, which most of the time do not pertain to, or reflect the design values for common guitar amps. This is made worse because a lot of people that designed these guitar amps initially didn't know any better, or if they did, economics and availability was the principal determining factor, and the general abuse guitar amps take wasn't a consideration. People still don't know any better. case in point, you see a lot of off the shelf 50W/100W Marshall Ot's that list a reflected impedance of 3.4K (50W) 1.7K (100W) because thats what some of the original used. Indeed this is a figure you will see quoted on the Mullard/phillips datasheet for EL-34's, but with the design values listed around 375V on the plates and screens, not the typical 470VDC-480VDC or higher most higher power amps use, especially ones with very stiff power supplies. Not as big of a deal for very high gain designs, where you are not really trying to pound this piss out of the output section.

Whether or not there is, or the degree of "mismatch" rather in any given design cannot be accounted for unless you graph it though, or to the extreme, own very expensive equipment to test the actual physical device. It starts to become kinda of an impractical endeavor at that point though

So generally speaking, I'd say to match the impedance all the time. If you're in a bind temporarily, it's not so bad to mismatch if your amp is set up well (including biasing) but even so I always recommend not running the amp very loud in those cases.

I'd say this is pretty damn good advice, especially if you are dealing with unknowns, which most of the time we are unless we built the thing ourself, and are mindful of such things

-Black Cat

 

[Anonymous]

another reason i love this site..SO much information from so many sources!!!!

riddle me this, just for shits and giggles cause i always wondered why...:
how come mesa boogie states in the manuals of a lot of their amps " is not overly sensitive to mismatches of impedance - and in some cases a mismatch may actually be preferred for tonal reasons, as different loads have different characteristics. However, it is important to always keep a load...."

 

[wolfeman28]

another reason i love this site..SO much information from so many sources!!!!

riddle me this, just for shits and giggles cause i always wondered why...:
how come mesa boogie states in the manuals of a lot of their amps " is not overly sensitive to mismatches of impedance - and in some cases a mismatch may actually be preferred for tonal reasons, as different loads have different characteristics. However, it is important to always keep a load...."

Because when certain conditions are met, it 'is safe' to mismatch impedance with no adverse effects. As long as the builder did their homework, and as long as you don't change the variables, if they say it's ok then it's ok.

Some amps, but certainly not all, can handle a mismatch up and down one setting. But go further than that and you'll have problems. But there are those other amps that wouldn't like a mismatch either way. Add in all the factors that were already talked about in this thread, and it just makes more sense to just match the amp with what it says on the cab.

There is a lot more that goes into it, and maybe someone else can get into the nitty gritty about it if they like typing.

 

[Anonymous]

im all ears, as i like reading and absorbing this type of shite!

thanks for your info on this, as well as everyone else...truly insightful.