# NAD: The Much Maligned Marshall JCM900 Mark III Hi Gain Dual Master



## keto (May 23, 2006)

Man, I don't get the hate! Sounds like a Marshall to me and, at least through a 1x12 with a Greenback, I can get good rockin' tone at house (not bedroom or TV watching) levels.

So, I've long known of the hatred for JCM900's. I know they have SS components in the preamp. But I went and read the 100+ page thread over on the Marshall Amps Forum, and learned that:
-there are 2 gain knobs. The one labelled 'PreAmp Volume' is still an all tube stage, and turning this up to 10 is like diming a JCM800 or very close
-the 2nd labelled 'Gain', is where the solid state circuitry lies and leaving this at 0 (well, 11 really, it goes from 11-20 whereas the one above goes 1-10) essentially takes it out of the amp. But turning it up is just like stepping on a pedal with an op-amp in it - yup, I do that even with high gainers. So, I turned off my all tube prejudice for this one.
-I also learned that the effects loop is its own gain stage and that just running a patch cord through it and fiddling with the fx volume adds substantial ooomph. I did this and yeah, the amp is way louder and gainier with a patch cord in there.

It's just a single channel amp. The Mk III was supposedly only produced for 2 years starting in 1990 and, like all the JCM900s, was available in 50W and 100W versions. The Dual Reverbs were true 2 channel amps and put out at the same time, and for a longer production run - if you see a JCM900 the likelihood is very high that you are looking at a DR (and the reissue now available is a DR, I believe). The last 900's were the SL-X, where they took out the SS pre and added an extra preamp tube gain stage - the SL-X's are supposedly the gainiest of the 900s. 

Anyways, this MkIII has the very typical Marshall tone stack TMB + Presence, and the added bonus of dual master volumes. Combined with the 2 gain controls, there are a lot of tones in it...I actually had it dialed clean and loud, it sounded great! But that's not how I'll be using it. That said, I did run through some knob twiddling combinations that sounded horrid, so if you weren't experienced and got this for your first amp, and dialed everything to max, and no patch or pedals in the loop, I can see where it might get a bad rep. 

I believe all the 900's have the pentode/triode switch, mine certainly does. It makes a difference and I can still get firm tight power chords with it in triode mode, lots of amps I have used with power scaling get mushy fast.

Mine is 1990 production and is and has always been an EL-34 amp. One other reason the 900's are less popular is that a lot of them were made with 5881's, which is more like a 6L6 and not the classic Marshall sound. Allegedly, people used to stick EL34's in without rebiasing (requires resistor changes for the different tube type) and got lousy sounds.....go figger duh.

Anyways, these things go cheap - I got it off TGP, the guy wanted $500 shipped for the head...I did pay him some extra to get it into Canada. There's a DR combo on TGP, or at least was yesterday, for $500 or under. After the reading I'd done, I figured I'd probably love the MkIII but if I didn't it was a relatively cheap experiment. I only messed around with it for half an hour so far but it kills! I also bought a speaker from StevieMac, a G12-65, which is supposed to be a great speaker for an 800 or 900 Marshall, tames a touch of volume by being relatively inefficient plus rounds off the top end a touch. Can't wait for that to land too.....it sounds good through the Greenback, but I won't be cranking it too hard through that 20W speaker.

Pics to follow, but it's just an old road dog Marshall, nothing really special to look at


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## Destropiate (Jan 17, 2007)

Congrats on the Marshall!!

Just like you I have heard so much hate for these amps on various forums but every single one I have played through sounded just like a Marshall should to my ears. Even the uber hated Dual Reverb version sounded pretty good to me. I did like the MkIII a bit better though for a straight up rock sound. The only version of the 900 I haven't had a chance to play is the SL-X. Personally I'm glad they get a lot of trash talked about them because it really seems to keep the prices on this line nice and low on the used market. If I ever get the Marshall bug again I'll be hunting for a 900 for sure.


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## keto (May 23, 2006)

I think you're right and I have half a mind to buy 3 or 4 if I can find 'em in the $500 and under range and just stick em away as an investment (meaning take some risk, of course, that they are never worth anything more), especially the MkIII's, given their rather short production run.

Played it some more, this is totally 'hotrodded 800' tone that we've all heard on a zillion radio tunes etc. Of course, it sounds so much better in the room 

Well, like I said, it's an old road dog Marshall, nothing special here. A few dings to the tolex and some minor staining on the faceplate I'm not sure will come off.


Lucky me, I can run a Marshall 50 watter at this volume at home  It's not blazingly loud through a 1x12, I imagine through a half stack it might annoy the neighbours. At '6' it's really giving up the goods, sounds huge.

It has it's back panel. I put fresh Shuguang pre's in it, I can't identify the power tubes all writing is rubbed off.


Here's that trick I mentioned above, about patching the loop to add a gain stage. My son and I took about 5 minutes dialing the inset pot back and forth, where it's set at 11 o'clock the amp is tightest...it gets just a touch quieter and noticeably looser moving in either direction.


We also messed around with the high/low mode (pentode/triode). It does drop volume appreciably, it also makes the amp sound more compressed/less crisp. I tend to like crisper, so will probably always run it full power. Low mode also makes the bass less full, I much prefer the bass response in high mode even though I have to dial it back a touch.


This ain't your dad's plexi


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## hardasmum (Apr 23, 2008)

It's funny, growing up these were my favourite Marshalls. Congrats!


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## sulphur (Jun 2, 2011)

Congrats Kent, you're on a roll with the amps. 8)


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## nonreverb (Sep 19, 2006)

Ahh...where to begin. There are goods and definitely bads about this model. The goods: Great sound and options that were not available on the JCM800 like the lo-hi switch and it sounds reasonably good in both postions. The bads and this is important...Marshall's new idea of mounting several components to a sub-board on the back of the amp was a disaster waiting to happen. The lo-hi switch as ingenious as it is, places the screen resistors on this board in close proximity to other comonents. I have one in my shop where the part of the board holding said resistors literally burned off. Probably due to a catastrophic tube failure which lit up the screen resistor and ruined the board.
The next problem is bad solder and design. These amps have the same impedance switch found on the later 2000 series stuff. Problem is they were too cheap to use all 6 lugs as a redundancy in case of solder cracks....and there's solder cracks on almost every one. Not good for the output transformer.

As seen in the pics, the good old Marshall loop jacks start getting finicky. Fortunately, an easy fix as demonstrated in the pics.
Last, but certainly not least, the dreaded bias supply cap. Marshall had this brilliant idea that they would derive the bias voltage from the main un rectified B+ using a cap to help reduce the voltage. The cap was not spec'd well and is prone to failure. Guess what happens when it goes? Hint: it doesn't go short. Now you're beloved amp has no bias at all and your power tubes are about to launch themselves into oblivion!

Here's my suggestion and forgive me if it feels like I'm sh*ting on the parade here: When buying one of these amps, please take it to a qualified tech. He'll know exactly where to look. Once serviced/modified, the amp will be far more reliable allowing you to enjoy it without fear of problems.


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## keto (May 23, 2006)

Thanks for that, Richard, I don't mind hearing it. I'm going to pull the chassis later this week and check bias, I'll refer to your notes when I poke around in there. Yes, I know not to touch anything, and how to discharge it.

Jock, I've always been way more an amp head than a guitar guy though I've had plenty of both. I love exploring different tones available.


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