Ohm rating questions... Is this dangerous?

[StevieD_FenderP2009]

Hi guys

I've got (figure of speech, picking it up within the next week or 2) a Trace Elliot RAH250 SMX head with 2 speaker outputs on the back that are rated at 8 ohms each but at home I've got 1 Marshall JCM 800 Bass Series 4x12 fitted with 4 Celestion G12-65 15 ohm inpedance speakers as well as a Marshall JCM 800 Bass Series 2x15 fitted with 2 Celestion G15-200 speakers (Not sure on the impedance, they will either be 8 or 15 ohm)

I'm just wondering if it will in any way damage my head or cabinets plugging one into each of the outputs on the head? If you guys could help it would be great.

Cheers
Stevie

[flyfisher]

It really depends on how the multiple speakers in each cabinet are wired up - in other words what is the overall impedance of each cab?

The bottom line is that the amp can drive two 8 ohm cabs, which really means it can drive an overall 4 ohm load.

As long as you don't connect a load that is less than 4 ohms then you won't do any harm.

My guess (and it really is a guess - you should check the manufacturer's specs) is that the 4x12 marshall cab is a 4-ohm overall cab, made by connecting the 4 x 15ohm speakers in parallel and the 2x10 is an 8-ohm overall cab, also made by connecting 2 x 15ohm speakers in parallel.

If my guess is correct, then you can safely connect [u]either[/u] cab to the amp.

BUT, if you connect them both then you'll present the amp with a 2.6 ohm load and you'll be getting into 'magic smoke' territory.

[icastle]

[quote name='StevieD_FenderP2009' post='1035945' date='Nov 25 2010, 09:46 AM']Hi guys

I've got (figure of speech, picking it up within the next week or 2) a Trace Elliot RAH250 SMX head with 2 speaker outputs on the back that are rated at 8 ohms each but at home I've got 1 Marshall JCM 800 Bass Series 4x12 fitted with 4 Celestion G12-65 15 ohm inpedance speakers as well as a Marshall JCM 800 Bass Series 2x15 fitted with 2 Celestion G15-200 speakers (Not sure on the impedance, they will either be 8 or 15 ohm)

I'm just wondering if it will in any way damage my head or cabinets plugging one into each of the outputs on the head? If you guys could help it would be great.[/quote]

The Marshall JCM 800 Bass Series 2x15 is rated at 8Ω
Can't find any data on the Marshall JCM 800 Bass Series 4x12 but you can be 99% sure that it will be 8Ω as well.

HTH

[obbm]

[quote name='icastle' post='1036011' date='Nov 25 2010, 10:31 AM']Can't find any data on the Marshall JCM 800 Bass Series 4x12 but you can be 99% sure that it will be 8Ω as well.

HTH[/quote]

Don't see how you can say that when it has 4 x 15-ohm speakers. The 4x12 cab is probably 15-ohm.

[icastle]

[quote name='obbm' post='1036020' date='Nov 25 2010, 10:37 AM']Don't see how you can say that when it has 4 x 15-ohm speakers. The 4x12 cab is probably 15-ohm.[/quote]

Sorry, my mistake.
From the description given it's a 260W 16Ω cabinet model 1935A

[Ghost_Bass]

See here:
[url="http://www.drtube.com/marshall.htm"]http://www.drtube.com/marshall.htm[/url]

go to the bottom of the JCM800 Bass series and you'll find what i think may be your cab:
[quote]1935A JCM800, Bass 4x12" 260W/280W 16Ω angled front cabinet.
1935B JCM800, Bass 4x12" 260W/280W 16Ω straight front cabinet.
These cabinets were in production form 1980 up to 1986. They were fitted at first with 4 Celestion G12-65 T3101 16Ω speakers and later (1983 onwards) with 4 "Marshall-Celestion" 12" 16Ω speakers.[/quote]

So it would be a 16ohm cab and not 15ohm.
If the 2x15 is realy 8ohm and this is confirmed to be a 16ohm cab then you're safe to connect both cabs.

[StevieD_FenderP2009]

Yeah, I'm in agreeance with OBBM over this, I don't know how the cabinet can be 8ohm when the 4 speakers are 15ohms each. I know there's a website that says that the cabinet is 8ohm but I don't understand how it could be.
From what I have kinda understood, 2 8ohm speakers make the impedance of a cabinet 4ohms, am I correct in thinking that?

Ps, OBBM, I'm going to be contacting you soon over having some custom cables made

And double ps, the speakers all say 15ohms each on the back of them
[url="http://www.wikizic.org/1-029-029919-Celestion-G12-65-Rola.jpg"]http://www.wikizic.org/1-029-029919-Celest...G12-65-Rola.jpg[/url]

Edited November 25, 2010 by StevieD_FenderP2009

[Noisyjon]

[quote name='StevieD_FenderP2009' post='1036069' date='Nov 25 2010, 11:10 AM']...From what I have kinda understood, 2 8ohm speakers make the impedance of a cabinet 4ohms, am I correct in thinking that?...[/quote]

Speaker wiring can be a bit more complex than that. See below:




Regards,
Jon

[Noisyjon]

And for 2 speakers:

[StevieD_FenderP2009]

Ok, I was wrong, it's not how I thought it was at all haha
Well, I give up trying to understand impedance. Cheers for your diagrams though Jon! They do look very helpful.

Right, so back to the question at hand, AM I GOING TO KILL MYSELF/MYAMP/MY CABS? haha

Cheers
Stevie

[Ghost_Bass]

[quote name='StevieD_FenderP2009' post='1036239' date='Nov 25 2010, 12:44 PM']Ok, I was wrong, it's not how I thought it was at all haha
Well, I give up trying to understand impedance. Cheers for your diagrams though Jon! They do look very helpful.

Right, so back to the question at hand, AM I GOING TO KILL MYSELF/MYAMP/MY CABS? haha

Cheers
Stevie[/quote]

Based on John's diagrams try to find out your cabs exact impedance. Can't you see how they're linked the same way you saw their model and impedance?

[icastle]

[quote name='StevieD_FenderP2009' post='1036239' date='Nov 25 2010, 12:44 PM']Ok, I was wrong, it's not how I thought it was at all haha
Well, I give up trying to understand impedance. Cheers for your diagrams though Jon! They do look very helpful.

Right, so back to the question at hand, AM I GOING TO KILL MYSELF/MYAMP/MY CABS? haha

Cheers
Stevie[/quote]

Nope - your neighbours might kill you, but this setup wont

The 2x15" is an 8Ω cab (2x 16Ω wired in parallel)
The 4x12" is a 16Ω cab (2x 16Ω wired in series + 2x 16Ω wired in series = 2x 32Ω and then those 2x 32Ω outputs wired in parallel to give 16Ω)

Your amp will handle anything [b]above[/b] a 4Ω load - using these two cabs together will give you a 12Ω (ish) load - so well inside the spec of the amp.

Edited November 25, 2010 by icastle

[flyfisher]

Yep, no magic smoke with those impedances (5.3 ohms for both cabs in parallel), but realise the amp's power won't be evenly split across the two cabs.

Edited November 25, 2010 by flyfisher

[Prime_BASS]

Might worth checking of the amp actually outputs at 4ohm.
I've had guitar amps that have output 2x8ohm lol the ashdown superfly head.

[Ghost_Bass]

Hey! We're talking about a trace head! I'm sure there won't be any lack of power to the cabs coming from it!

Trace gear puts out 4 ohm and not 2x8ohm. It's true that with 2 cabs with different ohmage the output isn't going to be the same for both, the 8 ohm cab will be a tad louder - can't do the math to tell how much louder, maybe someone can help on this?

[xgsjx]

You could rewire the 4x12 to run at 8 ohms to get the same going to both, but the volume difference wouldn't be worth the hassle.

[obbm]

[quote name='xgsjx' post='1037328' date='Nov 26 2010, 07:30 AM']You could rewire the 4x12 to run at 8 ohms to get the same going to both, but the volume difference wouldn't be worth the hassle.[/quote]

I'd be really interested to see how you would do that with 4 x15-ohm speakers. You could have solved an age-old problem.

[obbm]

[quote name='Ghost_Bass' post='1037231' date='Nov 26 2010, 12:50 AM']Hey! We're talking about a trace head! I'm sure there won't be any lack of power to the cabs coming from it!

Trace gear puts out 4 ohm and not 2x8ohm. It's true that with 2 cabs with different ohmage the output isn't going to be the same for both, the 8 ohm cab will be a tad louder - can't do the math to tell how much louder, maybe someone can help on this?[/quote]

Amplifiers don't put out ohms. They put out Watts into a load which is specified in Ohms. An amplifier has a minimum load into which it will deliver its maximum specified power.

Trace amps, and the majority of other makes of amp, have a minimum load of 4-ohms which can be made up of any number of cabs as long as the total impedance does not go lower than 4-ohms. A Trace amp can have 2 x 8-ohm loads which happens to be 4-ohms.

In many cases where there are two speaker output sockets they are labelled 8-ohms each so that dumbos don't stick 2x 4-ohm cabs onto it and see magic smoke.

[Ghost_Bass]

[quote]Amplifiers don't put out ohms. They put out Watts into a load which is specified in Ohms. An amplifier has a minimum load into which it will deliver its maximum specified power.

Trace amps, and the majority of other makes of amp, have a minimum load of 4-ohms which can be made up of any number of cabs as long as the total impedance does not go lower than 4-ohms. A Trace amp can have 2 x 8-ohm loads which happens to be 4-ohms.

In many cases where there are two speaker output sockets they are labelled 8-ohms each so that dumbos don't stick 2x 4-ohm cabs onto it and see magic smoke.[/quote]

You're right about the ohm explanation and i know that but i didn't read properly what i wrote before posting.
As for that 2x8ohm... of course trace amps can take a 2x8ohm load! My reply to Prime_Bass had to do with him saying to look out as some amps have 2 separate outputs each running into a min load of 8ohm!

[xgsjx]

[quote name='obbm' post='1037380' date='Nov 26 2010, 08:53 AM']I'd be really interested to see how you would do that with 4 x15-ohm speakers. You could have solved an age-old problem. [/quote]
Haha, I wish I could have fixed that!

Was his 4x12 not confirmed to be this tho?

[quote]1935B JCM800, Bass 4x12" 260W/280W 16Ω straight front cabinet.[/quote]

[chris_b]

Unless someone has fiddled with this cab the plate on the back should say: Model 1935A, 260 Watts RMS, Bass 4x12, 16ohm, Serial Number xxxx".
I'd be surprised id if the 2x15 was anything other than 16 or 8 ohm, so you shouldn't damage your amp.

That's a lot of very big and heavy bass cab for very little volume!

Just my 2p but I'd sell the lot and get something smaller, lighter and louder.


Edit: I'd take a look at that Mesa Boogie 215 that was in the FS section last month.

Edited November 26, 2010 by chris_b

[obbm]

[quote name='xgsjx' post='1037576' date='Nov 26 2010, 11:33 AM']Haha, I wish I could have fixed that!

Was his 4x12 not confirmed to be this tho?[/quote]

I thought the giveaway was in the very original post. How could he have known this without having the bcak off?

[quote]I've got 1 Marshall JCM 800 Bass Series 4x12 fitted with 4 Celestion G12-65 15 ohm inpedance speakers[/quote]

[xgsjx]

[quote name='obbm' post='1037637' date='Nov 26 2010, 12:08 PM']I thought the giveaway was in the very original post. How could he have known this without having the bcak off?[/quote]
Sorry bud, I've read thru the other posts & the OP hasn't said it's the 16 ohm (it was in a couple of other posts that it could be).
He could still rewire it to draw 7.5 ohms & the Trace should still run them ok (if slightly warmer).

Anyway, whether it's 15 or 16 ohms I wouldn't bother with the hassle of rewiring it.

[StevieD_FenderP2009]

ChrisB - No hope in hell of me selling these cabinets. They work perfectly for the sound I want to achieve and they achieve the level of volume I need as well. The Celestion G12-65's provide a really warm bottom end with really clear highs which is perfect for the tone I want. The 2x15 bottom cabinet is mainly there for the extra bottom end boost but it also provides some highs too.

Everyone else - I know which speakers they are from taking off the back panel of the cabinet and reading what it says on the speaker itself as I needed to replace one speaker which was blown when I bought the cab (the f***ing dipshit i bought it off decided it was a good idea to replace a slightly damaged G12-65 with some £30 heap of junk that blew straight away when he used it, he then never supplied me with the original speaker so i could get it re-coned) so yeah, I know the exact model of the speakers, the exact model of the cabinets and what ohm's the 4 12" speakers run at as it's stated on the speaker itself. "Impedance 15 ohm", not 16 like people keep trying to correct me on.

I think i'll just risk it until I get the power amp i'm after anyways. Cheers for your help. thread finished!

[xgsjx]

If you can be bothered, take the back off again & compare the wiring with one of Jonthebass's diagrams (on page 1) & that should give you an indication as to it's ohmage.

Alternatively, just keep an eye on how hot your amp gets when using them