touji-za-nai
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#1
My 6505+ can be set to 4, 8, and 16 ohms.. my marshall 1960A is either 4 or 16 ohms...

what does that mean for me exactly?
The Ohms on the head can be set to anything as long as the cab is higher correct? ie
Head@4 Cab@4/16
Head@8 Cab@16
Head@16 Cab@16

yes?

Please help me understand, thanks
-tj
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bowen
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#2
Well, when you say that your cab can be at 4 or 16 ohms, does it have 2 amp inputs that state the ohmage of both, or one that gives you the choice between 4 or 16 ohms on your 6505+?
Shred-Hed
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#3
ohms is a mesure of resistance, it is easier for current to travel through a 4 ohm circuit than say a 16 ohm one, and as a result 4 ohm circuits are louder. Just make sure the numbers on your head match the numbers on your cab.
touji-za-nai
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#4
Quote by bowen
Well, when you say that your cab can be at 4 or 16 ohms, does it have 2 amp inputs that state the ohmage of both, or one that gives you the choice between 4 or 16 ohms on your 6505+?


The 6505+ has a movable switch in the back with 4, 8, and 16 options.
The Marshall 1960A has two separate jacks, a 4ohm one and a 16 ohms one.

What if I want to use the 8ohms setting on my amp head?
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JanB
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#5
ok... always match your resistance, yes you can run out 4ohms into higher resistance but in the long run nothing good is coming of it

i looked up the the 1960A and it has seperate jacks for 4 or 16ohm mono

i think if u run it at 4 you won't be as loud, which is good for the 6505+ as it doesn't have a master volume so you can turn the post up to get crunchier distortion
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Nalakram
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#6
Amazingly, I was actually researching a similar question when I saw this post. I've gotten a little confused with some of my results though.

For example, one site that was talking about a 2x12 cab mentioned that it wires two 16 ohm speakers in parallel to make an 8 ohm cab. I'd assume then that you'd plug the speaker wire from the cab into the 8 ohm jack on the head.

The original poster would be asking this question in a slightly different way - asking if he could plug the speaker wire from the cab into either the 8 or 16 ohm jack on the head. I don't know the answer to the 16 ohm part of that question.

My final related question is why would you choose a 4 ohm cab or an 8 or a 16 ohm cab? I get the fact that a 4 ohm circuit is louder according to Shred, but is there a tone difference or signal quality impact to consider as well?

The most important part of this line of questioning is understanding whether plugging into the wrong ohm amp jack will damage the head?
touji-za-nai
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#7
Quote by Nalakram

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The most important part of this line of questioning is understanding whether plugging into the wrong ohm amp jack will damage the head?


Ya, that i do know.. Say you set the head to 8ohm or 16ohm, if you're plugged into the 4ohm you'll blow out the amp head
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al112987
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#8
Quote by touji-za-nai
Ya, that i do know.. Say you set the head to 8ohm or 16ohm, if you're plugged into the 4ohm you'll blow out the amp head


You should not be mismatching higher or lower.

Its technically safer to run a lower impedance load than the impedance set for the head. I've run my amp at 16 ohms and mismatched before at 8 ohms before. It really puts a strain on your power tubes though. However, if the load impedance is higher than that of the head, you'll blow the output transformer. That's very bad.

Most people set the amp to 16 ohms and run a 16 ohm load. That's what I do, higher impedance = more windings used on output transformer = bigger, richer low end.
Last edited by al112987 at Sep 30, 2008,
Nalakram
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#9
The following is from the Avatar FAQ section:

"If your amp has a minimum 4 ohm load, and you want to get one cab now and leave the door open to expand and add another cab later, then get one 8 ohm cab now and another 8 ohm cab later. When you run a single 8 ohm cab on an amp that can go down to 4 ohm, you lose a little power that only translates to about a 3 db drop in volume which is just barely perceivable, but you can expand and add another 8 ohm cab later which will not only give you more speakers to move more air, but your amp will then be at optimum power at the 4 ohm total load."

If I interpret that correctly, it means that if the head is set to 4 ohms and the cab is 8 ohms your volume will be lower on a relative basis. Theoretically then, it sounds like you can influence the volume of where your tube amp begins to break-up.

However, I'm sure there is some engineer out there cringing at how I've oversimplified things and come to potentially dangerous conclusions.
al112987
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#10
Quote by Nalakram
The following is from the Avatar FAQ section:

"If your amp has a minimum 4 ohm load, and you want to get one cab now and leave the door open to expand and add another cab later, then get one 8 ohm cab now and another 8 ohm cab later. When you run a single 8 ohm cab on an amp that can go down to 4 ohm, you lose a little power that only translates to about a 3 db drop in volume which is just barely perceivable, but you can expand and add another 8 ohm cab later which will not only give you more speakers to move more air, but your amp will then be at optimum power at the 4 ohm total load."

If I interpret that correctly, it means that if the head is set to 4 ohms and the cab is 8 ohms your volume will be lower on a relative basis. Theoretically then, it sounds like you can influence the volume of where your tube amp begins to break-up.

However, I'm sure there is some engineer out there cringing at how I've oversimplified things and come to potentially dangerous conclusions.


This applies only to solid state amps where mismatching doesn't matter as long as you don't go below the minimum in which case gives you the highest power output.

With a tube amp, never mismatch high (ie. cab impedance too high)
Nims
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#11
Quote by Nalakram


However, I'm sure there is some engineer out there cringing at how I've oversimplified things and come to potentially dangerous conclusions.


That is correct sir!

-James (resident engineer)
wasted-years
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#12
so lets say i have a valve king 212 with 2x12" 16ohm speakers. i set the ohm switch on 8. but my question is if i get a cab i just plug the other external speacker jack into the 8 ohm jack on the cab (or set it if it has a switch) right?
vitchb
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#13
you can mismatch if you do it right. For instance i hook my mesa's 8 ohm OP to the 16 ohm in on my marshall 1960.
The mesa manual for the amp has a list of all the safe and unsafe mismatches. So dont say go 4 ohms from the amp to an 8ohm Input on the cab...
ShredGod George
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#15
4 ohm 100 watt SS head to an 8 ohm crate v series cab?
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Ashdownlover
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#16
Quote by ShredGod George
4 ohm 100 watt SS head to an 8 ohm crate v series cab?


is fine would just sound about 70 wattish falt out.

SS can take a lot more abuse than tubes
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`digitaL.braVo
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#17
Quote by Ashdownlover
is fine would just sound about 70 wattish falt out.


Mine sounds just fine, so unless you can provide cited proof of this, you're full of shit.

The 100w solid state head into the Crate V series cab will be a-o-k. That's "technically" the same hook up that I'm running.
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#18
Are the ohm switch, the little switch in the back of my 5150 that doesnt say anything under it? Because, I've switched through the 3different spots you can have them, and it doesnt make my amp sound any different, but I have it a low volume.
al112987
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#19
Quote by `digitaL.braVo
Mine sounds just fine, so unless you can provide cited proof of this, you're full of shit.

The 100w solid state head into the Crate V series cab will be a-o-k. That's "technically" the same hook up that I'm running.



Mismatching high with solid state amps causes a decrease power output. Its science.
311ZOSOVHJH
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#20
Everyone who has taken time to post in this thread is not full of sh*t so I don't know why that was necessary.

Here are a couple of formula's that might help. I'm not an expert here but al112987 usually is spot on with his posts and I do believe that an SS amp can handle mismatches better.

^wasted-years - post in the Valveking thread and we'll help your particular case if you like.

1 = 1 + 1
R R1 R2
(thanks jof for posting this somewhere, i pasted into wordpad


If you want to find the total resistance of resistors (or speakers) in parallel, it's R=(R1*R2)/(R1+R2)

So, for a 8 ohm cab for example the R=8 so: (8*8)/(8+8)=64/16=4 ohms.

TS - I think your situation is easier just match 16 to 16 right.

To the guy with the 5150 - I don't think that is good idea but I don't own a 5150 head. I'm sure you can find the correct settings for your switch and write them on there yourself and then match accordingly.
Last edited by 311ZOSOVHJH at Sep 30, 2008,
TNfootballfan62
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#21
I haven't read through the whole thing, but the safest, most logical thing to do is to set the head and the cab to the same resistance. If you have any other questions about resistance, repost them and maybe me or other EE students can try to answer.
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#22
I was under the impression, that if you were mismatching, that you'd want the cab to have a higher ohm rating than the head.

I do agree with everyone saying 16 -> 16, doesn't make sense to mismatch when you can match.
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touji-za-nai
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#23
Quote by Kevin Saale
I was under the impression, that if you were mismatching, that you'd want the cab to have a higher ohm rating than the head.

I do agree with everyone saying 16 -> 16, doesn't make sense to mismatch when you can match.


the sound is altered a little... 16 -> 16 is a more solid sound.. smoother... where 4 -> 4 is a little less define... not sure why you would want that but still...

So I was wondering if it was safe to play with 8 -> 16 to test the tone
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#24
The reason you want to match the impedances is because it ensures efficient power transfer from head to cabinet. It's built like that to get optimal performance and sound.

Mismatching high doesn't usually sound good, it kills off higher frequencies and it makes your tone sound very dull and muffled, I've heard amps running a high mismatch, and it can be bad for your amp. In some cases, mismatching low is preferred, a lot of people run JTM45s with mismatched loads to compensate for the bass circuit being used for guitar, some transformers handle it, others don't do it so well.

Here is the thing to the best of my knowledge, mismatching high causes your ouput stage to run at higher voltages, it makes tubes and your output transformers susceptible to shorting out. Its the same reason why people tell you not to run the amp without a speaker load attached (when your amp would be seeing infinite impedance).