Marshall JCM900 Mk III and SLX - In-Depth Review 

Thanks! That's what I'm going for

He's the Johan Segeborn but for 80's rock

@@1thess523 haha I appreciate that but I'm not quite as cool as he is I think!

@@totallyradguitars959 you need a baseball hat and crocs and bingo, oh and come up with a "let's go" saying 🤣. Try Autobots roll out 🤔

I would recommend getting a legendary tones lynch mod or hot mod for your 2204. It doesn't require any modding just slides into V2, it increases to the gain by up to double and the amp sounds better at low volume. Before I got my Lynch mod my 2203 was never played and I play it a lot.

@@andrewspade7969 Great suggestion. I have always wanted to try one of those. I use a distortion pedal in front of everything. I can use the pedal volume to keep the amp vol. where I want it.

@@andrewspade7969 That’s what I did and to be honest my 2204 sounds better now at 7 o’clock than it at 11 o’clock.

Thanks! The Mk III is really a beast, I couldn't believe how good it sounded recorded just from these clips - I actually taped my mic stand to the floor after recording this so I wouldn't lose the spot! Now this is an amp they should reissue.

Absolutely, and people praise amps like Cameron's, Ceriatone Chupa/Yeti, etc. and those all have clipping too. It's just a flavor thing, some prefer no diodes and that's fine but it doesn't make it a bad amp to have them - I wish more amps did it like the Mk III where you can basically turn the diodes completely off if you want. My Ceriatone KK also has a switch that turns off the diodes as well so you have both options - very nice to have.

Tube Screamer or Maxon 808 in front of pure tube Marshalls is a classic sound.

Thanks dude!

It's a hell of a guitar! I actually used it for the Vintage Modern video then realized in editing that I clipped the mic the whole time. Doh! So I re-recorded that one and had already put this one back in the case. It deserved to be in a video, super well taken care of too.

Thanks! Still need to do a video on that one - great amp. Just need a little more free time!

Agree! I found one and I can’t stop playing it now. It’s a keeper no matter what!

I was curious about that too - I’ve been learning a lot lately about gain stage dragon and this circuit always confused me whether it was 4 or 5 stages and it seems like there’s one stage that isn’t adding any additiona distortion characteristics so I’m glad I’m not crazy in thinking that. I have a 6100LM I need to do a video on soon. The lead mode on that amp with the gain boost is what I thought the SLX would be - it’s extremely high gain especially for 1992. The SLX is a little more classic and less “modded” sounding, if that makes sense, to my ears at least. Love them both

@@totallyradguitars959 Played a 6100 nearly 30 years ago. It was killer, back then I couldn't afford one. I like the Marshalls from that era. At least back then they were trying different things circuit-wise. But us guitar players are fickle, and terribly un-accepting of things different or new.

Thank you! It's one of my favorites for sure

Hi! 1986 G12-T75’s, the ones with the vented back, in the Marshall 4x12 underneath is what I used for both amps. The internal speaker of the Mk3 wasn’t used in this demo but it is a G12-H100 which I also love

They are killer amps. I lucked out and got a JCM800 2203 before the prices went nuts on them and while I'm glad I have it, I'd be just as happy with a 2500/2100 (either SLX or Mk III) especially at current prices!

Man. So true. I also used to have 2x12 jcm800 i forget the model but basically 2203 but combo. Nice amp, absolute nighmare to carry around. Hey bwt, in your 2500, if you set master a and b the same, b is quite a bit louder? In mine master at 9 o’clock A=barely on while master B takes your head clean off. I have stared the schematic but I dont fully understand how the 2 masters operate and if that is supposed to be like that. Anyway, thank you for the informative video content!!

@@vl292 I don't have this combo anymore, but I have a 2100 Mk III Head instead. On that amp, the volumes are incredibly close when set the same - nearly impossible to tell. I *think* I hear Master B (footswitch on) being the tiniest, slightest bit louder, but only by a little bit. There is a little tonal shift, it feels punchier on Master B to me. I'm also not an expert once the opamps get involved, and both masters run through opamps in this circuit. It looks to me like the footswitch jack is grounding Master B out when not engaged from the + voltage line of the opamp there, but then when it's disengaged it looks like both volumes are active... But I don't find that to be the case on the amp itself (I.e. if I have Master B on via footswitch, and I turn Master A up or down, I don't notice any volume/tone change). Looking at the schematic, if I didn't have the amp in my hands right now, I'd expect to hear a slight tone/volume change if I were on Master B and started turning up Master A, but that's not what I'm hearing. It's possible I'm just using my ears too much and not my eyes, or it's also possible there is a schematic revision/update I don't have or it's just missing a part on there. If there's a drastic difference as you describe though, I would take a look at the area around Master A: R16, C19, VR3 pot, and see if there are any loose solder connections or any sign of damage. If those components are good, then I'd really start to suspect the opamp on that side. Of course, the usual safety speech goes here, don't touch anything with two hands, don't hurt yourself, it's not worth it, take it to a tech if you're not comfortable! Cheers!

It’s got a pretty great dirt sound that’s for sure. Rock on

Thanks man! I'm a little partial to the Mk III but neither are particularly low volume friendly, well depending on what you mean. I play about 90-100 db at home but I have very understanding neighbors, if you need to go quieter, and attenuator or even just going down to a 1x12 can make even the loudest Marshall feel a little more manageable. There are 50w versions of both of these amps, including the SLX (model 2500). Cheers!

@totallyradguitars959 Thank You :)

Great idea, I never thought of that. I'm bad at doing two things at once (i.e. stepping on footswitch while also rolling down volume) but with some practice it would work really smoothly.

Guess I’ll find out .. maybe it wasn’t such a bonkers idea afterall lol

I have already had it open and checked most of the values in the circuit and nothing looks wrong, and when I listen to other youtube demos mine has about the same amount of gain so I doubt anything is wrong. When I say it doesn't have much gain, I meant that compared to other amps also available in 1993 - like the SLO, Dual Rectifier, Peavy 5150, Steavens Poundcake, etc it does not have nearly the same type of heavy, chug palm mute type gain that those have, yet that's what I expected based on other online reviews before I owned it.

@@totallyradguitars959 I totally agree with your second paragrah. However, what speakers is it running through. I have both the G12t-75's & the Marshall Vintage Speakers. The Vintage speakers sound way better with the SL-X as it tends to be a little darker and the Vintage brightens it up.

@@guitarexpert2245 I’m very lucky and happen to have a huge selection of speakers too - I just really love the ‘86 T75’s so I use those most of the time (and in this video). I have a quad of ‘96 V30’s (same as Marshall Vintage speakers) and they certainly brighten up this amp and others. They have an aggressive sounding boost in the high high mids that really cuts and can make some amps sound piercing, but some amps get more presence and cut with them. I’ve actually been meaning to do a speaker comparison video but what can they are mounted in has a noticeable effect too so it’s going to take me a while to get it all tracked since I’ll have to rotate speakers through the same cab. Also, I’m going to check through the SLX again when I have a little spare time and just check. You’re not the first to mention it seems to have less gain than theirs now so I think that merits a follow up on it.

@@totallyradguitars959 Hey, sounds good. You certainly know your gear!

It's certainly a LOUD amp but I'm not sure I'd describe it as having a lot of clean headroom. It's similar to a single channel 800 - you can get it loud with the preamp gain set low, but the master is already at almost full tilt by "5" and if that's not loud enough for whatever purpose (recording, gigging, etc) then you're out of luck. I've had a HIwatt DR103 and THAT is clean headroom, but it was also so ungodly loud that I never got to find the limit in any reasonable setting I could play it in. So it probably depends on A) where you are going to be using it like that and B) if you need really pristine cleans or can put up with a little bit of sparkle/grit Either way, try to swing the 100w version if that's a concern. There is a 50w on guitar center used listings right now at the time of writing this comment, I don't think the price is terrible but not an incredible deal either - so if that's the one you're looking at and clean headroom is the concern, be patient and wait for a 100w. (or get the 50w from GC and try it out, and if it doesn't work for you, return it - but you'll loose the shipping cost to find out) Cheers.

@@totallyradguitars959 Ah, OK. My main amp at the minute is a Bluesbreaker run clean with a T Rex Mudhoney fir crunch/dirt, but I've always fancied a 900. I used tonown a horizontal input JCM800 bass head, clean for days. Should have kept it! Thanks for the reply!

I will have to check and get back to you on that one after I have a chance to open it up again. I remember some of the TSL's had a similar problem with resistor values on earlier units too.

They kind of fly under the radar, especially since the Dual Reverb's are so poorly regarded I think these got lumped in with those since they are both in the same range and use clipping diodes. The thing is, diodes aren't bad on their own, it's how they are used, but the SLX was clearly much more popular (it ran from 93-99)

I have but I didn’t notice a massive difference. I know on the dual reverbs it pulls another tube into the chain (just a buffer tube though not a gain stage). I’m not saying it doesn’t work but I’ve had 4 Mk III’s now and can’t say I would be able to tell the difference between any of them patch cable vs no patch.

@@totallyradguitars959 yeah, I tried it and it did get slightly louder maybe but I’m not sure if jumping the loop improves the tone. They are great amps, compared to my 2204 though the mk iii is a bit thinner, more brittle sounding. Still really good though, especially after I changed the resistor in the bias circuit so it can be biased more to specs. Cheers

@@vintagesound8112 There are a couple of cap/resistor changes you could do that would make it sound nearly identical to a 2204, but it would make the diodes/sensitivity muddier in the low end. I think that's why the Mk III's are thinner by default, to make them more usable if you max both gains out. I had some thoughts about altering the values to match 2203/2204 and then adding a high pass filter across the diode circuit only to prevent it from getting muddy as you add the diode gain... but that's more in depth modding than I'm willing to do to a vintage amp at this point. Maybe if I built one from scratch though...

Very nice, totally agree. I love to look at schematics, but I'm no snob - if it sounds good, I'm happy. The Mk III and 2210/2205 are my favorite Marshalls and they are both diode clipper circuits. On the other hand I've heard all tube amps I didn't like. It's all up to how it gets used!

I’ve heard a few BS reasons like that, but I suppose it may have been a return that was already damaged. Always a 50/50 chance when ordering anything from GC. I’m sure you’ll find another one, it’s well worth having in addition to the 4100 since they do sound a lot different!

@@totallyradguitars959 I decided to try a JCM 2000 DSL since I’ve seen some pretty interesting videos comparing them to JCM 800s. We will see if this one actually gets delivered and if I decide to keep it. Would really like to find a nice 800 but I literally just play in my spare room so it seemed foolish to spend that much at this time.

Hey, this SLX ('93) has EL34's. Later ones usually had 5881 unless they were converted, and then very very late ones would be back to EL34 again. Cheers!

Sorry - I don't think the hotmod will fit on the 1x12, the tube sockets are extremely close together. If there were a way to rotate the hotmod by 90° it could work though, just not sure how to accomplish that.

Awesome story! I’m a big fan of the Mk III. It’s the only Marshall combo I still have and I wish I’d kept the head I had of it. I also had a Laney AOR50 but sold it before I started making videos. It had a lot of gain but very loose low end - but also really cool pull switches to shape the tone. Definitely a unique amp but I couldn’t quite get along with it - which may change now that I have more cabs. I think it just didn’t sound good with V30’s to my ear because that’s all I had for a long time

Hey, check here: www.drtube.com/marshall-jcm900/ The very first one is the 2100 Mk III preamp, if you look at the second link down "CD0109 Issue 9 JMP51B PCB, Marshall, 1990" Power amp schematic on a separate page below that "CD0192 Issue 10 JMP50C PCB"

Hey, I don't think they really help the low volume. Some amps that have two volumes active at the same time, you can manipulate them a bit to get the tone at lower volumes - I use that trick on my VHT Pittbull and others a lot. These amps though, each master volume doesn't have any effect on the other, and they are both identical tapers and pot values. So if you are using "Master A" and turn it down to lower volume, it's the same exact tone if you used "Master B." That also means regardless of which master you are using, if they are both set to "5" and you footswitch between them there's no tone or volume change either. Hope I explained that clearly haha cheers

@@totallyradguitars959 yes, that does indeed help/make sense. Thank You

I've had a few comments about my SLX being lower gain so it's possible I have some component or tube out of spec. It still sounds really great - thick and full to me, but one of these days I'll get it up on the bench and check it out again. What I will say though is that your 4210 - I have the head versions (2205 or 2210) - is my all time favorite Marshall sound. Direct, great boosted or unboosted, I don't care about the diodes.... just a beast of an amp. Working on a video for them right now actually.

​@@totallyradguitars959 looking forward to watch this video with your 2205 and 2210. :)

Hey, it’s a ‘93 with el34’s

I've never seen a 900 Mk "2." Mine is a very early one with the circles around the knobs - maybe yours is even earlier and it got a re-used or mislabeled silk screen plate?

Yes definitely, I think the Jubilee was a hit and this was their way of taking the success of the 2203/2204's and bringing in some of the experience they had with diodes in the Jubilee and 2205/2210 amps. The biggest difference is the Jubilee used some LED's for clipping, just like a JCM900 DR ChA, the Mk III is all more "traditional" diodes, so the clipping is a little harder. The Jubilee also has a little asymmetry to the clipping on the lead channel which gives it its unique tone, plus some different EQ/voicing. I still have a 2555 Jubilee, probably need to do a detailed video on it (my original Jubilee video doesn't talk about the circuit and is definitely a bit unpolished!)

@@totallyradguitars959 I just watched your older video comparing these to the Jubilee. Would love an updated overview of the Jubilee to understand how it changed the 2203 circuit and why the EQ is so responsive compared to other Marshalls. Loving your videos!

A lot of people lift the first leg on D1 but that's just an input clipper, not what the "sensitivty" knob controls (that's the second group you mention, between V1B and V2A). Not sure what you mean that they aren't anything like a JCM800 - the 900 Mk III is almost exactly a 2203 preamp with a few different values to change the voicing, and diodes crammed in between gain stages 2 and 3 with a level knob. Maybe "polite" isn't the right word to use, but the 900 Mk III with diodes off is smoother and lacks the grittier, angrier sounding mids that a 2203 has. You can get a different flavor of that grit back a bit by adding in the diodes, but the amp alone is just a smoother-voiced 2203 circuit.

Thanks for the reply. I may get an SL-X next, because they aren't too expensive and gig with that a while. Right now my main gig amp is a PRS HDRX 20 which surprisingly loud and is way better sounding than my Granger 1987. The amps in your video sound really nice.@@totallyradguitars959

A few people have told me mine has less gain than theirs too. Something I've been meaning to look at internally forever but I'm happy with the way the amp sounds and haven't had it opened up in a long time, other priorities.

Had a hermansson modded one was just like a sharper tighter Marshall some might live them but not even close to wizard impressive regardless

@@josephbutler148 I like them about the same .

​@@techdeathhippie6319maybe some models are better I had the slx with 3 gain stages or 4 can't remember not the highest one one back from that lol but anyway it was not even as good as my stock TSL. It was a bit underwhelming and disappointing and I had other issues with it but I know his work is good and a lot of people love it just was not a good experience for me wizard on the other hand are something else huge crushing and tear ur face off I really should have kept a wizard around now they are ridiculously priced probably never own another but my engl special edition gets me close and does more

@@josephbutler148 I have the 5 gain stage 800 and 4 gain stage dual rectifier . I love my wizards too . But these two brands definitely my top damage

@@techdeathhippie6319 try the diezel herbert mk3 destroys them all

Hey, I don't have a JVM anymore but I'm sure I'll get one again - I tried to buy one recently but the deal fell through. I will say that while I had the JVM410, the OD1 channel sounded nearly *identical* to my TSL. I did not like the lower mid scoop of OD2, which is the JVM2xx channel as well. Clean and Crunch channels were "just ok" - I didn't hate the amp or anything but my TSL was half the price and I felt sounded so similar that it wasn't worth keeping the JVM so I sold it. That said, I want another JVM410 so I can do a circuit analysis on it. It has some really cool switching depending on the channel and mode you are using. Cheers

You’re the second person to mention that my SLX seems to have less gain - when I got it a few years ago I checked the tubes and the values in the circuit to make sure they were right but now I’m second guessing myself! When I have a little spare time I’ll open it up again and give it a good run-through and report back if I find something out of order.

@6:17 "...And I looked in there and nothing is out of place" Yes I already had cross checked the amp with the schematic before making this video.

You're not the first to mention that but I'm not sure - just today I had the amp open, rolled all of the tubes, checked voltages internally, it's been recently recapped, nothing appears out of spec or out of order. It definitely has more gain than my 2210 or 2203.... not sure what else there would be to check. Most of the other ones I've heard on RU-vid sound just like mine too. Maybe it has something more to do with my recording or guitar but either way, I'm quite happy with the tone so I'm not going to mess with it any more haha

Have you used your SL-X 2500 with the legendary tones lynch mod ?

@@xvx6663 I haven't because the preamp tubes are closer together on the SLX than the 2203. The base on the Lynch mod can swivel and I have been told it can swivel enough to go into the SLX and it will work, but when I got it about halfway there it felt too tight to go any more.

@@andrewspade7969 I asked because I also have a SL-X 2500 and I used the lynch mod on it and it made the amp tighter. Yeah you can rotate the lynch mod while it’s connected to the tube socket to make it fit Rigth in between the other tubes. I did not even know, the guy that made the lynch mod told me how to do it over email. I was just wondering if you had tried it and if you also liked the sound. Only thing is that i think the extra tube the lynch mod provides may have burned the Br3 in the circuit and now my amp won’t play so I need to get that part replaced with something that will be able to take all that heat. I been on forums and asked questions and they all said that eventually the Br3 malfunctions and needs to be replaced and that most likely the lynch mod caused the issue in my SLX. This just happened too… it worked good for months and just last week it stopped working. I’m gonna take my amp to tech to have him work on it and see if it was the Br3. But man does the lynch mod sound good with the SLX.

It is not an 80% solid state preamp, I think you might be confusing the JCM900DR with the JCM900MkIII, which are completely different amps. In fact, the JCM900SL-X is an all tube signal path. If you look at the schematic section of this very video, you will notice that the JCM900MkIII schematic is nearly identical in layout to the JCM800 2203's schematic, with the exact same basic topology. I am not sure what you mean by low voltage vs high voltage. All tube amplifiers have lethal high voltage components within them, including both JCM800's and all variants of JCM900's. In case you confused voltage with wattage, I'd like to point out that the first popular Marshall was a tube rectified JTM45, as in 45 watts of output. Almost immediately after that, the amps were made in 50w and 100w varieties, starting in the late 60s all the way to the present day. There was never a time of "high wattage only" and then low wattage amps came later - they have been available both ways since inception.

@@totallyradguitars959 sorry for my original comment it was very vague i admit. i was no0t either talking about whether there was a time im talking strictly marshall.pre amp voltage definitely matters imo.just so your not confused by what i was saying.it matters heaps actually and when modding its a great place to start

@@totallyradguitars959 are you aware of 4 ohm tapping your amps.that was also part of earlier marshall mods and design.i tried to reply with a great link here from a really good builders site but it keeps getting removed weird,type in rob robinette marshall ,how a jcm 800 works.he covers what ive said in extensive detail .if your interested.

Hey, I’m familiar with that site (tons of great info I agree) and the voltages - ex. lower voltages to preamp tubes usually mean less headroom and more drive etc. Just pointing out that I think you meant the 900DR when saying it’s mostly solid state, because this video is about a different amp (the Mk III and SLX)

@@totallyradguitars959 nah i got that i know that model..nothing wrong with diodes mate.look at boogies total pre amp tube gain but lack that brite crunch of any diode based marshall imo..i love diodes but there solid state in the tech jargon of it all..cheers