Attention JCM 900 owners!

glassman

glassman

Electron Herder
This is intended for informational purposes only and is prompted by the third JCM 900 in my shop in a week for various reasons. Two of the owners ran them into a 4 ohm cab with the impedence switch set to (4)16. One of these guys is buying a new output transformer.

The JCM 900 impedence switch is set up as follows:

The 8 ohm selection is always 8 ohms
The (4)16 selection is wired to the 4 ohm tap on combo amps
The (4)16 selection is wired to the 16 ohm tap on heads

If you are running your 900 (or any other Marshall with the two position switch) through a 4 ohm cab with the selector on (4)16, stop and take it to a tech and have him wire it correctly.
 
Re: Attention JCM 900 owners!

GoDrex said:
why are they using 4 ohm cabs anyway?
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I'm not sure. As thier tech, my job ends at asking what impedence the cab is. I set the amp up to match their rigs. I made too many suggestions years back when I was just getting started and ended up alienating a few players. Now I tend to keep my mouth shut and work with them/thier rig.
 
Re: Attention JCM 900 owners!

sosomething said:
or sell it and buy something that doesn't suck
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That really depends on how they are set up. In the "straight from the factory" setup, they are running wicked cold and don't sound right. I imagine that this was done to extend tube life beyond the warranty period. Bias them up warm-hot and they take on an entirely different character. They sound like a different amp; the Marshall character really comes out in them and the bass really intensifies.
 
Re: Attention JCM 900 owners!

Yup, knew that one about the heads. I run mine in to a 16 ohm load so no worries there. However, I did not know the combos where wired like that. If you don't mind, glassman, I'd like to add that small tip to my JCM 900 MkIII page on my web site.

The MkIII's don't suck. IMO, they're the best ones out of the 900 series.
 
Re: Attention JCM 900 owners!

ErikH said:
Yup, knew that one about the heads. I run mine in to a 16 ohm load so no worries there. However, I did not know the combos where wired like that. If you don't mind, glassman, I'd like to add that small tip to my JCM 900 MkIII page on my web site.

The MkIII's don't suck. IMO, they're the best ones out of the 900 series.
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Please do add that to your page; I think it's important for all 900 owners to know this so that they can avoid unnecessary repairs. Also, add a link to your page, I'd like to check it out!
 
Re: Attention JCM 900 owners!

glassman said:
Please do add that to your page; I think it's important for all 900 owners to know this so that they can avoid unnecessary repairs. Also, add a link to your page, I'd like to check it out!
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I agree, it is necessary knowledge. Added it on there.

Here's the link: http://www.erikhansen.net/?page_id=8

The page is specific to the MkIII Hi Gain Master Volumes. I've never been able to find any other info about them anywhere so I decided to put together my own page. Some of the info I got from other sources, and noted as such.
 
Re: Attention JCM 900 owners!

i can't think of many cabs you can buy that come 4ohms... must be do-it-yourselfers...
 
Re: Attention JCM 900 owners!

Thanks for the info....I was wondering about the 4/16 setting on my Marshall MKIII head......tho I can't say I've ever encountered a 4Ω cabinet...........
 
Re: Attention JCM 900 owners!

For information purposes: the 6100 30th Anniversary has the same selector and works the same way. I run mine with on 16 ohms.

As for 4 Ohm cabs, JCM900 Series or newer Marshall cab with the Mono/Stereo switch can be run in either 4 or 16 ohms in Mono Mode. AND, since the CSA (Canadian) versions of the JCM800 only have 4 or 8 ohm selections, a single cab must be run at 4 ohms when matched with one of these cabs.

I don't know why they went away from the 3 position rotary selector.
 
Re: Attention JCM 900 owners!

GoDrex said:
why are they using 4 ohm cabs anyway?
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The 4 ohm option is for 900 combos. Using the stock 1x12 in a combo gives 8 ohms. Plug in another extension speaker and you get 4 ohms because the plugs are wired in parallel.
People who want more speakers tend to buy heads:
The heads are commonly plugged to a 4x12 box wired at 8 ohms. A further 4x12 box wired in series (as is the case with the heads) gives you 16 ohms. The heads speaker plugs are wired in series.
However The labelling on the back of the amps can be confusing. It really would be simpler if marshall just etched the correct values on the amp. (or at least texta mark over the incorrect one)
Now some general knowledge:
1 x 8 ohm speaker = 8 ohms
2 x 8 ohm speakers in parallell = 4 ohms (quite common in 2x12 combos)
2 x 8 ohm speakers in series = 16 ohms
standard 4 x 12 - 2 parallel pairs of speakers wired in series = 8 ohms
2 quads in series = 16 ohms
 
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Re: Attention JCM 900 owners!

Re 900s. I used a jcm900 dual reverb hi gain for many years back in the day. It never sounded as warm as the 50w 800 master volume or 800 lead series 100w combo that I played thru at different stages. Its clean sound was not very inspiring. But it did do high gain 80s sounds pretty darn well. Funny tho - for an amp that everyine seems to hate, marshall has just reissued it!
However the mark III 900 master volume is a different beast altogether. It sounds just as great as the 800 master volume but can get way dirtier. It will also give you crunch at less volume than an 800. Its a brilliant amp that covers all the classic marshall sounds from ballsy 70s style rock to heavily saturated Van Halen type tones. Thumbs up for the mark III!
 
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