#1
Okay, quick question, and this probably belongs in the Gear thread but I'll ask here.

I have some 2x12s Im going to run through my Bugera 333 head. The outside of the cabinets says its peak wattage is 70 watts @ 16 ohms. I was told the cabinets have been modded with better speakers, but Im unsure of the wattage. If the peak wattage is still 70w @ 16 ohms, and considering my amp is 120w, with a switchable impedance, would running that amp into those cabinets blow up the cabinet or the amp? And if possible, how might I avoid this situation short of getting new cabs? (If the 212 doesnt work Ill just run it through some PA speakers.)
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#2
by not pack them with DYnamite....
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#3
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by not pack them with DYnamite....


lol... This makes me laugh but yet somehow Im greatly disappointed. Congratulations. :\
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#4
If you crank the amp you'll def. blow the cabinet.
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#5
Lovecannon - where did you start this thread out in?

you and your threads


well anyway, you need to measure the resistance (impedance) of your 2 cabs with a multimeter like I suggested I think in one of your other threads. You can't guess at this stuff.

If you run 2 - 2x12 that are both 70 watts and 16 ohms (for example) the you will have 140 watts and 8 ohms total. Set amp to 8 ohms. Play. Profit. Hopefully.

You can't just plug a tube amp into PA speakers. It doesn't work that way. Your amp head NEEDs a cabinet and it NEEDs a speaker load. Guitar speakers. Speakers designed to work with guitar frequencies and guitar amp heads.

The only exception would be things like headphone taps and testing equipment which I'm not even going to get into.


you're welcome
#6
My friend runs a 100 watt solid state into the cabs at 8 ohms cranked, doesnt hurt anything.

Would it be possible for me to run the tube amp at 8 ohms? Would that lower the wattage at 16 ohms?

EDIT: just saw the post above me. I only brought one 212 with me from the practice space. How would I go about running that cabinet without blowing it up? And also, the PA speakers should work shouldnt it? They have a driver and a horn, and they are 8 ohms, i think like 300 watts. Are you sure they wouldnt work? Ive seen people run PA speakers with guitar heads, and nothing went wrong
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Last edited by Lovecannon at Oct 6, 2009,
#7
Lol I'd be pretty upset if my brand new amp gets fried from doing something wrong, or the cabs for that matter
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#8
Will running the amp at low volume work? Im not going to be cranking it
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#9
Can anyone at all help me?

Since noone has an answer, could a mod please close this thread/delete it?
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Last edited by Lovecannon at Oct 7, 2009,
#10
Yes you can crank it. If one speaker is 70 watts, then the cab is 140 watts altogether if it's a 2x12. If the the 2 speakers together are equaling 70 watts, then you can still use it, I'd just suggest not cranking it. But if you have 2 cabs, then you're fine no matter what.

And do not hook it to a PA...

Edit: I missed where you said the whole cabinet equals 70w. But why worry about that anyway if you have more than one cab?
Last edited by Ignite at Oct 7, 2009,
#11
What are the speakers? Open up the cab, and they should say how many watts they are. If the total wattage of both speakers is higher than your amp wattage, you'll be fine to crank it. Also, for I've amps you MUST match the resistance of the head to the cab. You can also use a multimeter to find out.

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#12
I have 2 cabs, but only one with me. That's why I wanted to know if it would hurt anything, I wasnt going to crank it, I only wanted to play at low volumes and test out the amp.
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#13
Quote by 311ZOSOVHJH
well anyway, you need to measure the resistance (impedance) of your 2 cabs with a multimeter like I suggested I think in one of your other threads. You can't guess at this stuff.
Are you pulling his leg? Maybe I should explain because a misunderstanding like this could result in the voice coils being embedded in the wall opposite the speaker.
Resistance != impedance with AC. A multimeter is measuring DC resistance, not AC impedance. The DC resistance of the wire in a speaker has no direct relationship to the speaker's impedance which is due to it's inductance. You may be able to work it out if you ran a constant sine wave (of a fequency in the middle of the speaker's frequency response) into the speaker and measured the current and voltage and then did the math but it would be faster and simpler to pull the speaker out and look at what was written on it.

http://en.wikipedia.org/wiki/Electrical_impedance
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#14
^OK, maybe I should NOT have used the word impedance. I stand corrected.

However, if you put the leads of a multimeter on your speaker cable and set the meter to 20 ohms it will show you the resistance in ohms.

No, I'm not an EE nor an amp tech. Never claimed to be one either.


#15
That reading in ohms you see is only showing the dc resistance of the wire, not the speaker's impedance due to its coil. Those two figures are not related. A coil with low resistance wire can have a high impedance to AC and similarly a coil with high resistance can have a low impedance to AC (well not much more than it's dc resistance anyway). If you have ever measured 8 ohms resistance with a multimeter on an 8 ohm speaker it was a complete coincidence.

PS: I'm a technician by trade (with an associate diploma in EE)

Addendum: actually it's not possible, because the total impedance is it's resistance plus it's reactance (reactance is the term for the ac component of impedance). If it has an 8 ohm resistance it's impedance must be greater than 8 or you are looking at a straight wire
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Last edited by Cathbard at Oct 7, 2009,
#16
^fair enough.


All I know is that what I suggested is a common way to find the resistance in ohms of something if you are not sure.

Lovecannon was not sure what the total resistance of his cabs were (he has 2 of them). He said the speakers were upgraded. Chances are it would read something close to 4, 8, or 16. I was just trying to give him a simple quick way to check, without opening the cab and getting into something he doesn't understand anyway.

For example, my cab has 2 - 16 ohm speakers wired in parallel for 4 ohms total resistance. When I check the speaker cable with a multimeter, it reads 3.9 every time.

Another example, I have humbucker with a 'output' resistance of 14k. When I put a multimeter on it I get:





So if all of this is just hocus pocus, I'll quit suggesting to people to test that way.




I have a degree in Psychology
Last edited by 311ZOSOVHJH at Oct 7, 2009,
#17
Quote by 311ZOSOVHJH
I have a degree in Psychology

Haha, me too.

I'm kind of doubting that that method is incorrect. I've heard that from a number of trustworthy people.
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#19
Your humbucker has a resistance of 14k, that is not it's impedance. Read the addendum in my previous post and the wikipedia link I gave you in my first post. If they aren't clear to you I'll try to find a simpler description of resistance vs reactance for you. This is a common mistake people tend to make, you aren't alone in this misconception.
This link may help you to understand too:
Reactance
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#20
Impedance is dependent on frequency isnt it? As Inductive reactance is 2*pi*freq*inductance, and then impedance is the vectorial sum of the ohmic and reactive resistances in a circuit. If i remember correctly from HS.
So, just for interest sake, where does this impedance value come from?
#21
Your humbucker has a resistance of 14k, that is not it's impedance.
I didn't say my bucker had 14k of impedance.

Honestly, most of this, including your wiki link, are over my head.

I still don't see how measuring a cab with a mulitmeter is going to cause the voice coils of his speaker to blow through the cabinet and into the opposite wall.

Let's just focus on trying to help Lovecannon out in a way that he can understand. K?

If he has 2 cabs that are both rated at 16 ohms then his total resistance/reactance is 8 ohms no?


PS: Welcome to UG
#22
Yes it is dependent on frequency. This is because reactance is proportional to the rate of change of current. Let me explain:
When you apply a voltage to a coil it tries to set up a magnetic field and the coil tries to impeded any change in it's present field by the production of a phenomenon called "back EMF", effectively creating a current in the opposite direction that reduces the overall current flowing.
With ac, as the voltage increases and decreases on successive cycles the coil tries to oppose these changes in it's magnetic field. As the rate of change is the important factor the faster the frequency the greater the rate of change and hence the greater the resistance to the of change, ie a higher reactance.
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#23
Quote by 311ZOSOVHJH
If he has 2 cabs that are both rated at 16 ohms then his total resistance/reactance is 8 ohms no?
Yes, two 16 ohm cabinets connected in parallel results in 8 ohms impedance.
(32 if connected in series but he wouldn't be doing that)
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#24
Quote by Cathbard
Yes it is dependent on frequency. This is because reactance is proportional to the rate of change of current. Let me explain:
When you apply a voltage to a coil it tries to set up a magnetic field and the coil tries to impeded any change in it's present field by the production of a phenomenon called "back EMF", effectively creating a current in the opposite direction that reduces the overall current flowing.
With ac, as the voltage increases and decreases on successive cycles the coil tries to oppose these changes in it's magnetic field. As the rate of change is the important factor the faster the frequency the greater the rate of change and hence the greater the resistance to the of change, ie a higher reactance.



That actually made sense. Thanks.
#25
Oh and why I said his coils might fly out of their boxes was that if he measured the impedance with a multimeter he would have the wrong values. Really, there is no actual danger because he'd be working on the safe side of things (ie he'd think they were lower impedance than they were) but he would be working with false figures. I was just being colourful.
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#26
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by not pack them with DYnamite....


lol, beat me to it. I was going with C4 or semtex, though.
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