#1
This is really just double checking due to a gig being tomorrow and sharing my cab, but as far as i'm aware the head is 8ohms (and 4 ohms option) would 8 ohms to the cab be okay? Would even the 4 screw up my cab if the guy did it wrong by mistake?

Everywhere i've read says it should be okay but i just wanna keep it safe. Its a 4x12 marshall, four 25w greenbacks i believe. A 16ohms cab
#2
Quote by Zoot Allures
This is really just double checking due to a gig being tomorrow and sharing my cab, but as far as i'm aware the head is 8ohms (and 4 ohms option) would 8 ohms to the cab be okay? Would even the 4 screw up my cab if the guy did it wrong by mistake?

Everywhere i've read says it should be okay but i just wanna keep it safe. Its a 4x12 marshall, four 25w greenbacks i believe. A 16ohms cab

Yes you want to match your amps output to the cabs ohms. So 8 to 8 is fine. 8 to 4 however is really not ok unless you are not playing for very long. Mismatching ohms runs the risk of ruining your output transformer which is not a cheap fix. Just make sure before you start playing.
#3
Quote by Sputnik1
Yes you want to match your amps output to the cabs ohms. So 8 to 8 is fine. 8 to 4 however is really not ok unless you are not playing for very long. Mismatching ohms runs the risk of ruining your output transformer which is not a cheap fix. Just make sure before you start playing.

Not my amp, some guys putting his head on after we get off stage so it will be his head into my cab. I'll tell him to use 8 but ultimately i wanna make sure that if he makes a mistake and puts the 4 ohms from his head into my 16 ohms cab that it wouldn't be my cab being screwed up
#5
if his amp only has 4 and 8 ohm outputs, it's 99% a solid state head, in which case it doesnt have an output transformer and it doesn't matter so much. simple rule is that solid states have a minimum load they have to meet, which is why they usually have 4 and 8 ohm outs. With tube amps it's best to match impedence, though being one setting off is fine. Eric Johnson runs his heads mismatched cause his golden ears tell him too.
#6
The cab wont get damaged. He could damage his amp head though. The danger is where you have the cab impedance less than the matching output from the amp. So you really shouldn't run an 8 ohm output from a head into a four ohms rated cab and if you run cabs in parallel together the actual impedance load is reduced ie two 8ohm cabs together in parallell give you a 4 ohm load and this catches folk out. You would maybe get away with it for a short period of time but you dont want to risk it. Ideally you want amp output and cab input to be the same. From what I understand the amp will run safely if the impedance load of the cab is higher than the amp output setting though ie cab 16 ohms and amp 8 ohms and I have had to do this a few times in rehearsal studios where I was using the in house cabs and they had a 16 ohm rating. My amp was fine with that.
#7
Quote by S McAllister
The cab wont get damaged. He could damage his amp head though. The danger is where you have the cab impedance less than the matching output from the amp. So you really shouldn't run an 8 ohm output from a head into a four ohms rated cab and if you run cabs in parallel together the actual impedance load is reduced ie two 8ohm cabs together in parallell give you a 4 ohm load and this catches folk out. You would maybe get away with it for a short period of time but you dont want to risk it. Ideally you want amp output and cab input to be the same. From what I understand the amp will run safely if the impedance load of the cab is higher than the amp output setting though ie cab 16 ohms and amp 8 ohms and I have had to do this a few times in rehearsal studios where I was using the in house cabs and they had a 16 ohm rating. My amp was fine with that.


Nah, it's actually bad to mismatch either way. IIRC if you mismatch one step down (8 ohms to four) the power valves might get damaged, whereas a mismatch upwards could damage both the power valves and the output transformer.
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#8
This topic always leaves has me confused since different people have had different experiences with it.

If think it went something like this,
Tube head + No Cab connected = Infinite load on the head.
4ohm out on head + 16ohm cab = 4 times the load on the amp head.
8ohm out on head + 16ohm cab = 2 times the load on the amp head.
so the other way around was considered safe or something.

Anyone who makes amps here would like to explain it or correct the replies mentioned in this thread? (including mine as well)
#9
This always tends to be a bit confusing. I'm an engineer so I will try to explain from that standpoint. All amps have output impedance measured in ohms. Speakers (cabs) have an input impedance again measured in ohms. Theoretically and ideally you really want the two to match if possible. The higher the input impedance to your cab over your amp output impedance the more load current your amp has to deliver which can be bad for the amp depending on whether it's solid state or tube. It would be better to have a cab with a "very slightly" lower input impedance than the amp output impedance. The lower you go though then the amp will deliver more current to the speakers and could cause them to blow (the amp driving a low resistance means higher current from the amp naturally). A 16 ohm cab with an 8 ohm amp should be fine. It won't cause the amp to have to overdrive the output and heat up too much. In the future try to match the impedances as closely as possible. It's not necessary to be exact but the closer you can get the better. If you had 2 16 ohm cabs in parallel and drove it with an 8 ohm amp then the impedances would match because parallel impedances reduce and in this case it would be exactly in half. But they would have to be in parallel! If they were in series they would add so suddenly 16 ohms becomes 32 ohms with an amp rated at 8 ohms output. Amp would be toast trying to drive that load if it could drive it at all. Again that depends on the amp, it's output impedance and rated output current to drive external speakers.
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#10
Quote by darkwolf291
No, you have nothing to worry about.
Your cab would be fine.
His head would go boom though

Right, well i'll tell him to use the 8 ohms out, it can be a hassle though, a lot of people don't research these things at all. Although someone else said it's probably SS which wouldn't cause it much trouble, i have a feeling it may very well be SS too.
#11
SS: Any cab is ok.
Tube: 2:1 cab to amp mismatch is usually fine (i.e. 16 ohm cab to 8 ohm amp). 1:2 cab to amp is sometimes fine.

The cab won't take any damage either way.
#12
Quote by darkwolf291
No, you have nothing to worry about.
Your cab would be fine.
His head would go boom though


This pretty much, unless the amp pushed more watts than the cab could handle, the cab doesn't care, it's the amp you have to worry about.
#13
It's usually ok to mismatch when the amp has the lower ohm value. It just won't be efficient and you will not be able to achieve the amps maximum output (setting at 10 will sound like an 8 for example) - but that's moot unless you need the volume dimed.

Missmatching the other way (amp ohm > cab ohm) is very bad.
#14
Zoot, can you ask them what they have?


I have this handy so I'll paste it. Yes, watts on cab can be much much higher than amp but never lower.


For Tube amps:
Amp Head Ohms > Cab Ohms = Possible blown Power Valves and/or Output Transformer.
Amp Head Ohms < Cab Ohms = Strain on the Output Transformer, Power loss.
*Some amps can take a 1 step mismatch but not all.

For Solid State Amps:
Amp Head Ohms > Cab Ohms = Broken amp
Amp Head Ohms < Cab Ohms = Power change, no strain
#15
Quote by 311ZOSOVHJH
Zoot, can you ask them what they have?

Yes, watts on cab can be much much higher than amp but never lower.



Typo. watts=ohms.
#17
Quote by ethan_hanus
Watts do not equal ohms...watts is not even in ohms law. Watts is power produced/used, ohms is resistance, and they do not correlate.

I think he means that he meant ohms when he wrote watts.

Also, watts (power) is in Ohms second law. P=U*I
Quote by Todd Hart
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If only I were the only one at home right now. I don't need my parents asking who Mr. Wiggles is.
#19
Quote by ethan_hanus
Watts do not equal ohms...watts is not even in ohms law. Watts is power produced/used, ohms is resistance, and they do not correlate.



What Ataborman said.

311ZOSOVHJH wrote watts instead of ohm by accident. That's what I was reffering to when i said typod watts=ohms.
#21
Quote by darkwolf291
No, he meant to write Watts.
Your cab HAS to have more watts than the amp outputs or your speakers will blow.
He knew what he wrote and he was correct in writing it.


AH! Now I get it. I missed a couple of replys in between.
Sorry for the confusion.
#22
Quote by ethan_hanus
Watts do not equal ohms...watts is not even in ohms law. Watts is power produced/used, ohms is resistance, and they do not correlate.


Huh?

Ohms law has long been recognized to deal with 4 elements, P (Power, in Watts), I (Current, in Amps), E (Voltage, in Volts) and R (Resistance, in Ohms).
“Ignorance more frequently begets confidence than does knowledge.”
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#23
Quote by ethan_hanus
Watts do not equal ohms...watts is not even in ohms law. Watts is power produced/used, ohms is resistance, and they do not correlate.


There is a definitive correlation between ohms and watts.

Watts = Volts^2 / Ohms = Current^2 x Ohms
#24
Quote by darkwolf291
No, he meant to write Watts.
Your cab HAS to have more watts than the amp outputs or your speakers will blow.
He knew what he wrote and he was correct in writing it.

thanks for watching my back while I was away
#25
Quote by 311ZOSOVHJH
Zoot, can you ask them what they have?


I have this handy so I'll paste it. Yes, watts on cab can be much much higher than amp but never lower.


For Tube amps:
Amp Head Ohms > Cab Ohms = Possible blown Power Valves and/or Output Transformer.
Amp Head Ohms < Cab Ohms = Strain on the Output Transformer, Power loss.
*Some amps can take a 1 step mismatch but not all.

For Solid State Amps:
Amp Head Ohms > Cab Ohms = Broken amp
Amp Head Ohms < Cab Ohms = Power change, no strain

Well the gig is done now, we got there and he had a marshall 100w head of some kind i believe and it turned out that it had options for 8 ohms and 16 ohms so it was fine.
#26
as long as the mismatch is reasonable, the only problem you would have is less clean headroom
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#27
Quote by ethan_hanus
Watts do not equal ohms...watts is not even in ohms law. Watts is power produced/used, ohms is resistance, and they do not correlate.

You should take your act on the road - Patton Oswalt has nothing on this kind of material.
Everyone is entitled to an opinion.

Feel free to express yours so I can make an informed judgement about how stupid you are.
#28
Quote by ethan_hanus
Watts do not equal ohms...watts is not even in ohms law. Watts is power produced/used, ohms is resistance, and they do not correlate.



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