#1
So needing a new guitar amp soon, haven't played guitar primarily in a band in a while. Been looking forward to getting a new amp setup!

Since I want it somewhat big, but somewhat small and portable, i'm going for a 2x12 combo and going to get a 4x12 to throw under it!

6x12 Marshall JCM 900 setup, 2 x G12T-75's in the combo and 4 V30s in the cab..

Here's the Question for the techies! Is there an external output on the back of the JCM 900 that will allow me to run the extension 4x12 cab and the combo at the same time without doing any damage?


Thanks, this is vital to my buying any combo so want to know before I make any offers!

Thanks, and this is my first post but glad to have found a good musician community for any future adventures!
Last edited by DEV_L at Jun 27, 2014,
#2
Yes. That is how I ran my 4102. Impedances is what you have to be careful with. Here's the issue. The combo speakers are in paralleled 16 ohm speakers - so 8 ohm. The 4x12 (assuming also 16 ohm speakers) can be run at 4 or 16 ohm.
So you can't run the 412 at 4 ohm because that would be 2.6 ohm total. You could run the amp at 4 ohm and probably get away with it, but it's not wise. It's way too close to KABOOOM!.
If you run the 412 at 16 ohm the resultant resistance is 5.3. If you run the amp at 4 ohm into that, it will work fine. But there is a problem; the power isn't equally distributed across the speakers. Twice the current goes into the combo speakers as does into the 412. So you're looking at the combo overpowering the 412 and largely defeating the purpose of running both.
But, there is a solution. Rewire the combo speakers in series. That will give you 32 ohms. That in parallel with a 16 ohm 412 results in 10 ohm. Run the amp at 8 ohm and you're cooking. With that configuration every speaker receives the same amount of current. You've effectively made a 3 leg series/parallel system.
Just one warning - you can't run the combo on its own unless you rewire it back to stock. Well, in reality you can. I accidentally played a whole 3 hour gig once with just the combo at 32 ohm and it handled it ok. I was running the volume pretty damn hard too. BUT you shouldn't. The power amp may be a bit of a tank but I wouldn't do it again on purpose. That would be tempting Murphy way too much.

Hope that answers your questions. I still have my 4102, it's now my backup amp.

As I used to run it:



Edit: I've since replaced it with a Randall RM100 212 on an angled stand and quite frankly, it kicks its arse.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
Last edited by Cathbard at Jun 27, 2014,
#3
^ Assuming that he haven't bought anything yet, I guess the easiest and safest way might be getting an 8ohm 4x12" like a Mesa, then he can just run the amp at 4ohm with 2x8ohm loads.
EGC Baritone | ESP M-I | Gibson LP Jr
BC Rich USA ST3 | Fender Telecaster
VHT/Fryette GP3 | Verellen Meatsmoke pre | Mesa 2:90
Soldano HR50+ | Marshall 2203KK | Krank RevJr 50 | Holden 50
Orange PPC412 | Laney GS412 | Mesa Thiele 112 | Alron 115
Last edited by godgrinder at Jun 28, 2014,
#4
Still have a similar issue. Same current into both sections but the 412 has four speakers - so the top speakers are running twice as hard as the bottom ones. My method is better.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
#5
yeah I haven't bought anything yet, still debating on what to do. That rewiring issue seems great, but it would defeat the purpose of the setup if I had to rewire it back to use the combo seperatly. That was sort of what I was looking for, the option to use it all or just the combo easily!

The cab i was planning to get was an 8ohm cab, quite possibly a Mesa. If there's still possible damage being done, i'd rather not chance it and find a setup that is properly matched.

Maybe a head is the better option after all, that was my first choice i just wanted to explore options and have a multi use amp as a stack or combo.
#6
You could add a switch to go between parallel and series. I never got around to it because I usually used all six speakers but it wouldn't be hard. You could mount it to the inside of the combo in a little box.


If you were to use an 8 ohm cab it wouldn't be too bad. My setup is better but it would work reasonably well.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
#7
Rewiring of the cabinet might also be another option depending on how it is wired.
#8
Quote by DEV_L
Maybe a head is the better option after all, that was my first choice i just wanted to explore options and have a multi use amp as a stack or combo.


I would say get a head. A 212 tube combo is by no means "portable", they are usually super heavy and awkward to lug around. A head + 212 cab would be easier since you can carry them in 2 loads. You can get another 212 cab (of the same ohm value), for smaller gigs you can bring one 212, for a bigger one you can bring both.

Also the JCM900 dual reverb have solid state clipping diodes in them, unless you want to mod it, you would be better off with a JCM2000.
EGC Baritone | ESP M-I | Gibson LP Jr
BC Rich USA ST3 | Fender Telecaster
VHT/Fryette GP3 | Verellen Meatsmoke pre | Mesa 2:90
Soldano HR50+ | Marshall 2203KK | Krank RevJr 50 | Holden 50
Orange PPC412 | Laney GS412 | Mesa Thiele 112 | Alron 115
#9
I'd tend to agree. The best way to go really is a head, a 412 and a 212. Then you have options. Depending on your needs you can go 2x12, 4x12 or 6x12.
And yes, unless you mod the amp like I have, a JCM2000 DSL is far better. But as I said, I have now gone to a 2x12 RM100 combo on an angled stand. My JCM900 is now just my backup amp. The last time I used my 1960A was when I loaned it to my rhythm guitarist because his Laney cab sounded like crap.


Btw. The only reason I have the 4102 rather than the 4100 is that I got it REALLY cheap from a junky, too cheap to turn down.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
Last edited by Cathbard at Jun 28, 2014,
#10
yeah i've pretty much decided that i'll go for a head and 4x12.

I had a JCM 2000 for a few years at the same time as i had a JCM 800 2203, and whenever i played the 2000 it sounded much more a fuzzy type gain.

In the past year i've tried two JCM 900's, not sure the models was just what the jam space gave me.. and usually get a great sound out of it.

I have a few options on heads too.. will post some pics when i get the new rig!
#11
There are three different types of JCM900 and they are all very different amps - sure it's the Dual Reverb series you were looking at? The JCM900 SL/X is a beast, not fizzy at all - but it is a single channel amp (with two master volumes). Completely different animal to the Dual Reverb.

Edit:
An unmodded JCM900 4100 is far, far, far fizzier on the lead channel than a JCM2000 DSL. The SL/X on the other hand is in no way a fizzy amp. Im thinking that's the one you've heard before, not the Dual Reverb.

Here's the go with JCM900's.
The 2100 series, called the Master Volume MkIII. They are essentially a JCM800 2203 with some op-amp stages in front of it, with the clipping circuit from the 2210. At the end of the preamp is another op-amp network, this one providing two switchable master volumes. The didn't provide two gains. So it's a single channel amp with two MV's.
These can be turned into a great litle amp with a pair of sidecutters. All you need to do is literally cut the clipper out. You can do it without removing the board. Just hack it out of there. Now you have a clean boosted 2203. You don't lose any distortion at all, it's just better, lots better.

Then there's the 4100. The much maligned Dual Reverb. It is a friggin nightmare. A gazillion op-amps before V1, two channels. Channel 1 has what amounts to a tubescreamer with a different EQ. Same type of clipping (ie. diodes in the NFB). It's not too bad. If you keep the gain down, beautiful cleans. An 800 with an overdrive pedal in front of it. Not too bad.
Channel 2 is ridicuous beyond credulity. At the output of one of the op-amps there is that god awful clipper out of the 2210 again. It limits how much signal is available to V1. If you get rid of it you can get more distortion - and it's all coming from V! instead of that shitful fuzz box piece of crap that is in there. That's the fizz everybody complains about.
It's a bit harder to remove, you have to replace the clipper network with a resistor. Bit trickier, but not too hard.

Last but certainly not least is the mighty SL/X. (also called a 2100 IIRC) With that, they took all the op-amp/clipper nonsense at the start of the Mk3 away and replaced it with a 12AX7. Everything else is the same. ie. the op-amp driving two switchable MV's. Identical amp to the Mk3 functionally - but it kicks it's arse.
Mk3's are a good thing though if you cut out the clipper and they sell quite cheaply at times.

With the JCM2000 DSL's, they dropped all the op-amp shit and went back to proper tube amps. They're not fizzy if you use the right tubes and speakers. It's what Gary Moore used live, his tone is hardly fizzy.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
Last edited by Cathbard at Jun 29, 2014,
#12
yeah someone close to me is selling a 4100 head for 550$ canadian, but from what you and other reviews have said, i'll hold off on that offer unless he gets real desperate to sell
#13
If you do buy it, give me a yell (ie. resurrect this thread) and I'll explain how to mod channel 2. Try to find an SL/X if you want a brutal Marshall though. They are by far the best 900 unless you want cleans. I'd go as far as saying that they were the best high gain amp Marshall has ever made.
Not my favourite Marshall of all time (that would be out of the JTM45, the 1974X or the 6100) but I don't really go in for high gain stuff. If I was playing metal the SL/X would be my "go to" Marshall.


Edit: I mentioned it in passing but take a look around for a Randall RM100 too. Load it up over time with whatever modules float your boat. You're in Canada so maybe check US ebay, they come up really cheap at times over there. They're a good thing.
http://salvationmods.com/index.php?page=modification
http://jadedfaithmods.com/mts-mods.html
If you can't find what you need from one of those guys, you didn't look.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
Last edited by Cathbard at Jun 29, 2014,