# Marshall 4X12 cab: replacing two G12T-75s w/Greenbacks - what ohms is result?



## TubeStack (Jul 16, 2009)

Hi there.

I'm hoping to replace two of the G12T-75 speakers in my Marshall 1960A 4X12 cab with Greenback G12M-25's (in an 'X' pattern).

Thing is, I've got a Canadian JCM 800 head, so it has no 16ohm out, only 4 and 8 ohms. Right now I use the 4 ohm out into the 4 ohm jack on my cab (in mono setting, the left jack is 4ohm, the right jack is 16).

If I put in the 2 Greenbacks (paired with the 2 stock G12T-75's), can the cab still be 4 ohms (or 8 ohms), rather than 16 ohms?

Also, if I eventually switch all four speakers to Greenbacks, will the cab still be able to be 4 (or 8) ohms?

Thank you for your help. :rockon2:


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## SteveS (Apr 25, 2006)

I did the same thing with my 1960A, only I replaced the two G12T-75's with V30's.

As long as the speakers you are putting in are 16ohm as well you should be fine. Just make sure you wire it up exactly as it was before.


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## forum_crawler (Sep 25, 2008)

SteveS said:


> I did the same thing with my 1960A, only I replaced the two G12T-75's with V30's.
> 
> As long as the speakers you are putting in are 16ohm as well you should be fine. Just make sure you wire it up exactly as it was before.


What he said.


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## TubeStack (Jul 16, 2009)

Great, thanks guys.

Quick question - after swapping in two 16 ohm Greenbacks, will the cab be 4 or 8 ohms?

Reason I ask is, I may be buying a 4 ohm Hot Plate before doing the speaker switch, and it'd be great if it still worked after the swap (ie I don't want to have to sell it and get an 8 ohm Hot Plate).

Thanks!


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## jimihendrix (Jun 27, 2009)

would a bad cat "leash" be a good choice for an attenuator...???...at least it "looks" adjustable for various ohms...

http://www.badcatamps.com/badcat_pedals.html


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## dwagar (Mar 6, 2006)

with 4 16 ohm speakers in there, your choices are still 4 or 16 ohms. You can't wire it to 8 ohms.

But, to make it a little more complicated:

I'm in almost an identical situation right now, same Cdn JCM800 with only 4 or 8ohm outputs. And I've just started using a Dual Showman 2x15 cab - 2 8ohm speakers, so 4 or 16 ohms options. I've been running them at 4ohms. It's really, really loud, so I've had to turn down so far my tone is suffering.

I just bought a Dr Z attenuator from one of the guys on the forum - it will work for 4, 8 or 16 ohms, and I notice there is another one for sale in the classifieds. You might want to check it out. I haven't rec'd mine yet so I haven't tested it. 

In reading the Dr Z literature, they suggest vintage Marshalls would run better at an 8ohm setting on the head into a 16ohm cab. So that's what I'm going to do. I checked this with THD too (I already own an 8ohm Hotplate), they agreed this would be fine.

But, do your research before trying this, make sure you are comfortable with it. There are definitely a couple of different schools of thought.


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## TubeStack (Jul 16, 2009)

dwagar said:


> ... In reading the Dr Z literature, they suggest vintage Marshalls would run better at an 8ohm setting on the head into a 16ohm cab. So that's what I'm going to do. I checked this with THD too (I already own an 8ohm Hotplate), they agreed this would be fine.
> 
> But, do your research before trying this, make sure you are comfortable with it. There are definitely a couple of different schools of thought.


Thanks, will definitely read more.

I actually just sold a Ho (Ultimate) Attenuator, wasn't happy with the sound.

The Ho unit provides a resistance of 27 ohm (or around there), so it's said to run best at 16 ohms, which I wasn't able to do. I had to use the 8 ohm out, into a 16 ohm cab, too. 

This worked okay, except I lost a considerable amount of power and wasn't able to get as loud as I'd like, ironically enough. Yes, it's an attenuator, but I only wanted to knock a few decibels off the top volume level, and wasn't able to reach that level with the 8 ohm-16 ohm situation.

The sound was also hugely coloured, whether using the 8 to 16 ohm setup or the 4 to 4 ohm (which many don't recommend, anyway), even when only attenuating a little. I think the Ho/Ultimate functions best with a 16 ohm out?

Does the Dr. Z really switch ohm levels, or is it a "one size fits all" situation like the Ho/Ultimate?

Will look more into using a 4 ohm Hot Plate. Thanks.


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## TubeStack (Jul 16, 2009)

Are you buying the Dr Z because of dissatisfaction with the Hot Plate?


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## dwagar (Mar 6, 2006)

no, I'm happy with the Hotplate. I actually bought the Dr Z because I thought I was going to be stuck with 4 ohms. I thought having a 'one size fits all' attenuator might be a good idea.

I'm going to compare the 2, may sell one or just keep them both. 

I've never tried a Ho, I'd heard there were the best though.

From what I've read on the Dr Z, it's real good for taking the edge off, they don't recommend it for bedroom use (even though it has a bedroom setting). It steps down by 1.8dB, less than others do. Their idea was you could be turned down at the start of a gig, and easily bump up as the night gets louder. It's designed by Trainwreck btw.

I've been happy with the HP down to -12, I think you start losing tone beyond that, well, maybe some at -12. But still further ahead than setting the crazy Marshall at about 1 on the MV.


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## TubeStack (Jul 16, 2009)

dwagar said:


> ... same Cdn JCM800 with only 4 or 8ohm outputs...


How are you liking the JCM 800! I love mine, best rock amp I've ever owned. :rockon2:

Have you had some chances to crank yours up? Mine goes through a _major_ tone transformation when the Master gets up to 8 (preamp around 3-4). It's a remarkable difference from when it's on 7, or even 7.5. At 8 it's like a different amp, thickens right up and takes on these beautiful mids and phat-ass low-end.

I'm also happy with my low-volume tones. I can get a decent tone for practicing/jamming/writing, with the Master around 1, the preamp at 8, and using some overdrive/boost pedals (Expandora and a Keeley Java).

I mainly want an attenuator to have some options for band practice, gigs, etc.

Is yours a 2204 or 2203? (Mine's a 2204.)


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## TubeStack (Jul 16, 2009)

I also really dig the look and feel of the Canadian heads, with the nice, heavy metal toggle switches for power & standby and the cool, plexi-like power light. Love it.


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## dwagar (Mar 6, 2006)

mine's a 2204 too (actually it's the head out of my 4010 combo - I pulled it and build a head box for it). I love this amp, best amp I've ever had. 

yeah, with the master cranked almost to the pin and the gain down, through an attenuator so you don't take the paint off the walls, these amps are monsters.


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## SteveS (Apr 25, 2006)

I've got a 76 2203, and an 83 2204. They're both great amps.

I've also got a Dr Z Airbrake and as previously mentioned it is great for up to 2 or 3 notches of attenuation. After that it loses a lot.


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## dwagar (Mar 6, 2006)

I'm a bit concerned the Dr Z won't give me enough attenuation (I haven't received mine yet), I agree with Tubestack that when you crank the master to about 8 (on the 2204) there is a significant change.
But I'm also running though 2 D130s, and they really push.

I run my Hotplate at -12dB, 3 notches on a Dr Z is only about -6dB, correct?

How does it work for you Steve?


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## Caribou_Chris (Feb 15, 2009)

Hey I've got a Canadian '83 2204 too, love it to death. 

I am actually in the same ohm situation (isn't that a Motley Crue song?)...I sold my 1960A cab's G12T75 speakers, and am planning on screwing in 2 8ohm Eminence Red White & Blues speakers. Down the road I may add 2 Emi Governor speakers. So my only choice right now, with 2 8 ohm speakers, is to wire them in parallel, for a 4 ohm load, right?


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## Archer (Aug 29, 2006)

SteveS said:


> I did the same thing with my 1960A, only I replaced the two G12T-75's with V30's.
> 
> As long as the speakers you are putting in are 16ohm as well you should be fine. Just make sure you wire it up exactly as it was before.


That is exactly it. Use speakers with the same rating and you are golden.


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## SteveS (Apr 25, 2006)

dwagar said:


> I'm a bit concerned the Dr Z won't give me enough attenuation (I haven't received mine yet), I agree with Tubestack that when you crank the master to about 8 (on the 2204) there is a significant change.
> But I'm also running though 2 D130s, and they really push.
> 
> I run my Hotplate at -12dB, 3 notches on a Dr Z is only about -6dB, correct?
> ...


I've only ever used the Dr Z and I find it works very well. Up to 2 or 3 clicks anyway. 

I bought it mainly to use with my 71 JMP 50, AC30TBX, and Germino Lead55LV as they don't have master volumes. To get them sounding good the volume has to be over 5 at least and by the time you get there it's really loud. 
1 or 2 clicks down is usually enough to get good tone but not lose too much of the dynamics. It's still fairly loud though, just not unbearable.

I never used it much with the 2204, or 2203 as the master volume on those works pretty good, but I'll have to give it a try with the gain set lower and the master high just for fun. 

These things really don't have a bad sound in them anywhere. If you like that Marshall crunch, they can't be beat! :food-smiley-004:


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## dwagar (Mar 6, 2006)

thanks Steve, I'm looking forward to testing the Dr Z.

Chris, the other option is to wire to 16 ohms (2 8s in series). I think I mentioned above, and maybe it only applies when you're using an attenuator, but Dr Z suggests a vintage Marshall head is safer (the OT will run cooler they said) to run at 8 ohms into a 16 ohm cab. 

That's what I'm running right now, but I wouldn't suggest anyone else do it unless they've researched and are comfortable with it.


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## Caribou_Chris (Feb 15, 2009)

dwagar said:


> thanks Steve, I'm looking forward to testing the Dr Z.
> 
> Chris, the other option is to wire to 16 ohms (2 8s in series). I think I mentioned above, and maybe it only applies when you're using an attenuator, but Dr Z suggests a vintage Marshall head is safer (the OT will run cooler they said) to run at 8 ohms into a 16 ohm cab.
> 
> That's what I'm running right now, but I wouldn't suggest anyone else do it unless they've researched and are comfortable with it.



Better than running the head at 4 ohms into a 4 ohm cab? I'll have to do some research on that one.


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## TubeStack (Jul 16, 2009)

SteveS said:


> I did the same thing with my 1960A, only I replaced the two G12T-75's with V30's.
> 
> As long as the speakers you are putting in are 16ohm as well you should be fine. Just make sure you wire it up exactly as it was before.


Thank again, guys.

Seems obvious now, but I had just started to worry that my Canadian-version-ohms situation was going to limit my options re. speakers. I also didn't know if Greenbacks came in different ohm versions, or only one, etc.


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## TubeStack (Jul 16, 2009)

Caribou_Chris said:


> ... I am actually in the *same ohm situation (isn't that a Motley Crue song?)...*


Ha! Nice one - LOL - thanks for the laugh.


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## Caribou_Chris (Feb 15, 2009)

OK so I did some research and have decided to wire the 2x12 in series for a 16 ohm load, and run the 800 at 8 ohms into it, as was suggested in this thread. This sums it up nicely:

(from http://www.jumbosunshade.com/swd01.htm)

_4-ohm, 8-ohm or 16-ohm cabinet(s): which is best?
Resistance is measured in Ohms. *A 4-ohm cabinet offers less resistance than a 16-ohm cabinet (allowing more wattage to the speakers) and is therefore a GREATER load on your amp than the 16-ohm cabinet*.
Up to an amplifier's limit, using a lower ohm cabinet will deliver more power to your speakers (although not necessarily more volume), but the trade-off in working your amp harder and hotter may not be worth it._

Since I do not want to fry my nice new RWB's, I'll go the 16 ohm route.


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## Caribou_Chris (Feb 15, 2009)

Ok, so I got around to wiring up the RWB's in the 1960A cab, in series for a 16 ohm load (JCM 800 at 8 ohm output). It sounds fantastic! If you've never tried Eminence speakers, do so. I'll get another youtube clip up soon.

Next up is replacing a blown 15" speaker I have in a nice old Traynor cab (not the one I use in this video: http://www.youtube.com/watch?v=NrhqMqxYlXA ; a different Traynor 1x15). I will be replacing it with the Red Coat series Big Ben model. Can't wait!

http://www.guitarplayer.com/article/the-patriots/Jan-05/422

http://www.guitarplayer.com/article/the-red-coats/Jan-05/2346


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