#1
I accidentally came across the fact my cab was only 8ohm the other day, as when i bought it i was assured that it was 16ohm.

So what damage has this caused?

Thanks
#2
EDIT: disregard this first part. I was wrong.
If it is a tube amp, the possible result is fire and brimstone... well, I am kidding about the brimstone. But you are at risk for drawing too much current through the amp and suffering a meltdown.


But first, Why do you now think that the cab is an 8Ω cab? Because if it has multiple speakers, each speaker could be 8Ω individually, but the total impedance is not necessarily 8Ω. How many speakers are in the cab, and what is the impedance listed on the individual speakers?
For example: http://jensentone.com/wiredia.php
Last edited by cedricsmods at Aug 21, 2009,
#3
If it's still working, there's probably no damage, but it's being really rough on your output transformer. (I'm assuming you have a tube amp) running a tube amp with low/no load ranges from at best blowing all your tubes to frying your output transformer as well to at worst frying EVERYTHING.
#4
yeah its a DSL50 (tube amp) well stupidly i didnt think to even check the back of the cab, which i now know says "8 0hm" but i looked at the speakers and each speaker is 8 ohm...

its a marshall AVT412 cab with celestion g12avt speakers i think.

but i mean i've had it a few months now and its been running fine, sounding alright, just the high gain is REALLY muddy... but i dont use it anyway
#6
Running a speaker of lower impedance on tube amp is actually SAFER than running with a higher impedance than rated, or no load at all. You'll get less than maximum power transfer, but expect no damage if the load is half the rated impedance.


Running a speaker of lower impedance on solid state amp is MORE DANGEROUS than running with a higher impedance or no load at all. You'll get less than maximum power transfer, and will generate excessive heat in your output transistors. This can cause them to fail. IF this happens, your amp will stop working completely. Major repairs will be necessary.
Meadows
Quote by Jackal58
I release my inner liberal every morning when I take a shit.
Quote by SK8RDUDE411
I wont be like those jerks who dedicate their beliefs to logic and reaosn.
Last edited by SomeoneYouKnew at Aug 21, 2009,
#8
Quote by Torbjorn 66
if that's true you have made me very happy indeed
What do you mean IF? I said it, didn't I? Take that to the bank.
Meadows
Quote by Jackal58
I release my inner liberal every morning when I take a shit.
Quote by SK8RDUDE411
I wont be like those jerks who dedicate their beliefs to logic and reaosn.
#9
hahaha sorry i ever doubted you sir, you are a gentleman and a scholar.

but i was just sayin, can't trust everything you hear.
#10
Quote by Torbjorn 66
hahaha sorry i ever doubted you sir, you are a gentleman and a scholar.

but i was just sayin, can't trust everything you hear.
Fair enough, bro.
Meadows
Quote by Jackal58
I release my inner liberal every morning when I take a shit.
Quote by SK8RDUDE411
I wont be like those jerks who dedicate their beliefs to logic and reaosn.
#11
Quote by SomeoneYouKnew
Running a speaker of lower impedance on tube amp is actually SAFER than running with a higher impedance than rated, or no load at all. You'll get less than maximum power transfer, but expect no damage if the load is half the rated impedance.



Are you sure you're not stating that backwards?

I'd feel safe connecting an 8 ohm cabinet to a 4 ohm output, but not the other way around.
#12
SomeonYouKnew, I think you've got that backwards dude. No load is zero Ohms, not infinite. Also, you get less than maximum power from using a cab with too much impedance, but it's still safe.

You know I've heard that some old marshalls had deliberately mismatched speakers (mismatched too low). Added to the character, but they had to be built to take the punishment.
#13
My Mesa Boogie manual actually recommends connecting your cabinet to a lower rated output jack on the amp to get a smoother sound. I routinely run my 16 ohm cab off the 8 ohm output.
#14
Less for tube amps is correct. If you use a higher load, then you cause the tubes to see a higher primary impedance. With the way that a tube works, this may cause the tube to go above its maximum dissipation and burn out or can be much worse and destroy the output transformer.
#15
ok so you're not directly answering my question:

what damage will have been caused by me running the AMP at 16ohm through an 8ohm cab? Bearing in mind the amp still works with no noticeable change from when I first got it...
#16
Quote by Torbjorn 66
ok so you're not directly answering my question:

what damage will have been caused by me running the AMP at 16ohm through an 8ohm cab? Bearing in mind the amp still works with no noticeable change from when I first got it...


Most likely nothing has been damaged by this. I use an 8 ohm speaker with a 16ohm amp and have never had any problems.
#17
goooood

but from what other people are saying i find it hard to believe that nothing has happened. im a bit paranoid now haha
#18
Quote by Metalhead_28
Are you sure you're not stating that backwards?

I'd feel safe connecting an 8 ohm cabinet to a 4 ohm output, but not the other way around.
Quote by Metalhead_28
My Mesa Boogie manual actually recommends connecting your cabinet to a lower rated output jack on the amp to get a smoother sound. I routinely run my 16 ohm cab off the 8 ohm output.
Randall Smith makes some very nice amps indeed. He's also an idiot when it comes to technical issues.
Feel free to run your Mesa at higher impedances on his recommendation, then sue him for the repairs if the insulation in the primary windings of the output transformer punches through.

But definitely DO NOT recommend this to others.


Quote by Blaster Bob
SomeonYouKnew, I think you've got that backwards dude. No load is zero Ohms, not infinite. Also, you get less than maximum power from using a cab with too much impedance, but it's still safe.

You know I've heard that some old marshalls had deliberately mismatched speakers (mismatched too low). Added to the character, but they had to be built to take the punishment.
It is absolutely NOT SAFE.

Look at the schematic for any Fender tube amp that has its speakers connected via a plug. You'll see a shorting jack. When no speaker is connect to the output, the transformer secondary is shorted DIRECTLY TO GROUND.

Yes folks, that's right.
A DEAD SHORT is safer for the output of a tube amp than open circuit.

I'm not making this shit up, people.
This is real.
Meadows
Quote by Jackal58
I release my inner liberal every morning when I take a shit.
Quote by SK8RDUDE411
I wont be like those jerks who dedicate their beliefs to logic and reaosn.
#19
hahaha fair enough...

thanks SomeoneYouKnew


... so just to clarify, i'm good right? nothing should be harmed?
#20
Should be just fine, man.
Meadows
Quote by Jackal58
I release my inner liberal every morning when I take a shit.
Quote by SK8RDUDE411
I wont be like those jerks who dedicate their beliefs to logic and reaosn.
#21
Quote by Blaster Bob
SomeonYouKnew, I think you've got that backwards dude. No load is zero Ohms, not infinite. Also, you get less than maximum power from using a cab with too much impedance, but it's still safe.

Wouldn't a short to ground be zero resistance? An open circuit (one with no connection to ground) would have infinite resistance. You have it backwards Bob.
After this discussion I realize that I was incorrect in my first post. Going to edit a disclaimer in there.
#22
Quote by SomeoneYouKnew
Randall Smith makes some very nice amps indeed. He's also an idiot when it comes to technical issues.
Feel free to run your Mesa at higher impedances on his recommendation, then sue him for the repairs if the insulation in the primary windings of the output transformer punches through.

But definitely DO NOT recommend this to others.



Okay, you've inspired me to a little reading and I have to apologize. You're right.

I think the recommendation in the Boogie manual is perhaps safe for that particular amp, but certainly not a universal recommendation to make.
#23
Quote by SomeoneYouKnew
Randall Smith makes some very nice amps indeed. He's also an idiot when it comes to technical issues.
Feel free to run your Mesa at higher impedances on his recommendation, then sue him for the repairs if the insulation in the primary windings of the output transformer punches through.

But definitely DO NOT recommend this to others.


It is absolutely NOT SAFE.

Look at the schematic for any Fender tube amp that has its speakers connected via a plug. You'll see a shorting jack. When no speaker is connect to the output, the transformer secondary is shorted DIRECTLY TO GROUND.

Yes folks, that's right.
A DEAD SHORT is safer for the output of a tube amp than open circuit.

I'm not making this shit up, people.
This is real.

Allright, I believe you, but I specifically remember reading my backwards rule in on one of the stickies on UG, which I now can't find. Anyone know where that would be? Because it needs to be fixed.

What I don't understand is why there's a "sweet spot". How is it that going with a lower impedance is fine, until you remove the load entirely and have zero ohms, then it's a recipe for disaster?

Quote by cedricsmods
Wouldn't a short to ground be zero resistance? An open circuit (one with no connection to ground) would have infinite resistance. You have it backwards Bob.
After this discussion I realize that I was incorrect in my first post. Going to edit a disclaimer in there.


That's just it, when there's no load connected, there would be a short to ground at the shorting jack and no resistance.
#24
Quote by Blaster Bob
That's just it, when there's no load connected, there would be a short to ground at the shorting jack and no resistance.



Not all amps have a shorting jack though.
#25
Quote by Blaster Bob
Allright, I believe you, but I specifically remember reading my backwards rule in on one of the stickies on UG, which I now can't find. Anyone know where that would be? Because it needs to be fixed.

What I don't understand is why there's a "sweet spot". How is it that going with a lower impedance is fine, until you remove the load entirely and have zero ohms, then it's a recipe for disaster?


That's just it, when there's no load connected, there would be a short to ground at the shorting jack and no resistance.
Look dude, I don't know where this rule is you're talking about in the stickies, but you have a SERIOUS confusion in the use of the term load. You post is a complete mess because of that.


Heavy load = High conductance = Low resistance.
Light load = Low conductance = High resistance.
No load = Zero conductance = Infinite resistance.
Meadows
Quote by Jackal58
I release my inner liberal every morning when I take a shit.
Quote by SK8RDUDE411
I wont be like those jerks who dedicate their beliefs to logic and reaosn.
#26
SomeoneYouKnew  
this is probly gonna make me sound stupid, and thats cool becuase i feel it, but 1stly, thanks for finally explaining this.. ive read so many threads where they say " incorrect impedence will damage your amp" then they never say how.. now i know, cheers.. 
 now to the part that makes me sound stupid...and  very lucky i`d assume..  i built my own jcm800 clone 4 years ago.. i have no experience in that stuff im just a guitar player that gets bored and has a little becker (cheap ass) in him and isnt afraid of a challenge.. besides i had the little road map that ceriatone and triode give you and if you can read a road map you should be ale to follow that.. schematics is a different story, for me anyway.. like i said i can follow the map and build it but if something doesnt worked im stuffed, it takes days or weeks of reading and tinkering to sort it.. anyway, i have been using the amp at home in the studio and live since i built it.. sounds awesome but every 6 months i have been replacing power valves as they or actually its always V5, burns out.. ive always thought i did something wrong in the build.. but it would appear im just a dickhead... it is the only issue i have had other than that its a great amp and loud as f#*k... next to my original 800 the only difference is that its louder and a has a touch more clarity which i attribute to the kiwame resistors over the old allen and bradley carbon comp resistors.. but im probly way wrong there too...
now heres where i sound like a complete dick.. for 4 years i have been running 2, thats 2 marshall vs412 quad boxs from the amp with the impedence slector set at 8ohms.. both boxs are rated at 8ohms but after reading this and a few other things i clearly shoudve had the impedence selector at 4 ohms and i would now assume this is the reason i burn out the power valves so often yeah???..  thankfully i dont think i have damaged the OT as it works fine and as soon i put new valves in it sounds perfect.. so im guessing ive been very very lucky not to have burnt out the OT as well.. i know this makes me sound like a dumb mother f#*ker but im actually not...lol.. but i think i should have known this.. so, in future, i should be running both 8 ohm cabs with the impedence selector set to 4 ohms yeah?? and then i shouldnt be burning out valves so often??  my heads are being retired in a few weeks anyway, getting too old to lug them now and they get bashed about too much so im getting either a kemper profiler powered  head or a positive grid powered bias head for live stuff and just keeping the marshalls for home duties..
#27
*Headscratch*

The higher the number, the greater the impedance. 16 ohms is more impedance than 4 ohms. Most amps are fine pushing more impedance, but can become unstable when the impedance is too low. The other issue is that with some amps the transformer can overheat if it's running into an impedance that is less than what it's rated for. My rule of thumb (other than avoiding any speaker combination of 2 ohms) is that you can usually run one level of speaker mismatch on either side without the likelihood of damage (though you'd certainly want it properly matched). You can probably get away with running 8 ohms on the 16 ohm tap or 16 ohms off the 8 ohm tap without letting smoke out of the amp.

In any case, it's unlikely that you've done any damage to your amp. Most 4x12s are wired in series (two speakers) and then in parallel (the two sides), so that you end up with the same overall impedance as the individual speakers. IOW, if you have 8 ohm speakers in a 4x12, you'll usually have an 8 ohm cabinet if you're using all four speakers.