# Let’s talk JCM 900’s ! (Diode removal easy mod) Clip added



## Louis

Edit !!.......For the mod , page #2

Hi guy’s !!

I would like to know if some of you like the JCM900 ?,.......I only played one in the 90’s and don’t remember how it sounded.

Some say it’s trash !...some say the Dual Reverb is the worst one ,....some say the SLX is the best 900 of them all etc......and then comes the clipping diode thing but Jubilee’s have them also .!

Johan made a few good demo’s ,....one comparing the 900 to a 800 and one with a vintage 1971 cab loaded with Pre Rolas , must say the 900 through the 71 cab sounds pretty good !

Let me know your thoughts !


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## Budda

They are easy to find, grab one and check it out.


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## pipestone62

I don t know why the bad rep. I have 100 watt mkiii JCM 900, great amp.
The footswichable dual volumes is great, wish my jubilee had that.
I’ve tried slx JCM 900 and like it too.

Here,s a good link on mkiii:
Erik's Corner (the blog): Marshall JCM 900 MkIII


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## pipestone62




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## keto

There’s a MKIII for sale here but the price, oye vey...I bought my 50 watter MKIII head for $500 a couple years ago. Not my personal favourite Marshall I’ve owned but very good ... was however my son’s favourite, he likes a little less gain than I do, so I gave it to him.

In case you get one, there’s a trick.....if you don’t use the fx loop, bridge it with a patch cable. I forget the technical explanation, but it sounds BIGGER, and you can hear the difference immediately at any volume.


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## Louis

keto said:


> There’s a MKIII for sale here but the price, oye vey...I bought my 50 watter MKIII head for $500 a couple years ago. Not my personal favourite Marshall I’ve owned but very good ... was however my son’s favourite, he likes a little less gain than I do, so I gave it to him.
> 
> In case you get one, there’s a trick.....if you don’t use the fx loop, bridge it with a patch cable. I forget the technical explanation, but it sounds BIGGER, and you can hear the difference immediately at any volume.


Didn’t know that !!............Tx


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## Louis

This guy gets nice bluesy tones with his,


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## Frenchy99

I got the 4100 and I like it . Having the appropriate cab helps . I don' even understand the trash talk on it ...


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## Budda

keto said:


> There’s a MKIII for sale here but the price, oye vey...I bought my 50 watter MKIII head for $500 a couple years ago. Not my personal favourite Marshall I’ve owned but very good ... was however my son’s favourite, he likes a little less gain than I do, so I gave it to him.
> 
> In case you get one, there’s a trick.....if you don’t use the fx loop, bridge it with a patch cable. I forget the technical explanation, but it sounds BIGGER, and you can hear the difference immediately at any volume.


That fx loop trick works for most amps with loops. Its how I used to run my jsx. Essentially you get the full power of the amp. Not jumpering is a great way to keep the volume reasonable too.


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## Destropiate

I've never understood the 900 hate. Seems like the prices on these have gone way up in the last few years so maybe the stink is washing off these amps with time. I've never owned one but have played quite a few hi gain dual reverbs and always thought they sounded great. I should have bought one years ago when they were all over kijiji for 500-600 bucks.
I think a lot of the original stigma came from the diode clipping circuit. People said it was fizzy, can of bees tone etc. Any high gain amp I've played can get that can of bees tone if you want it to. Usually I've found it's a problem with not being loud enough and scooping all the midrange out as opposed to a problem with design. We all know the jubilees and 2210's have diode clipping and they didnt seem to get the same hate for not being "all tube"


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## Greg Ellis

Destropiate said:


> I should have bought one years ago when they were all over kijiji for 500-600 bucks.


THIS!

It really was not all that long ago, maybe 5 or 6 years back, when they were everywhere, and very cheap. $400-600 ish all day.


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## Frenchy99

Got mine dirt cheap... got the head and cab in trade for onw of my guitars ...


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## Budda

Greg Ellis said:


> THIS!
> 
> It really was not all that long ago, maybe 5 or 6 years back, when they were everywhere, and very cheap. $400-600 ish all day.


Same with jcm800's and 5150's


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## ezcomes

keto said:


> In case you get one, there’s a trick.....if you don’t use the fx loop, bridge it with a patch cable. I forget the technical explanation, but it sounds BIGGER, and you can hear the difference immediately at any volume.


I've read that the 900's are bad for their FX loop connections failing...best advice I have read is to always keep a cable in it...it can't fail if theres a cable in it permanently


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## KV242

I've got a mkiii 2100 model and while it's not my favourite Marshall, it still sounds great and I'm not likely to sell it any time soon. I picked it up about 3 years ago for $500. It's a darker character and more gain than a JCM800 2203/2204, and nowhere near as loud. The 900's have voice of their own in the Marshall line up. I don't experience the "can of bees" tone with it. It's got killer crunch and can melt faces. I've played 4100 Dual Reverb's, and they aren't bad either. But I found the tone to be smoother with the 2100. I've not tried an SLX, but I'd like to.

And I as well jumper the effects loop.


I like this video comparing three JCM900 models.


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## pipestone62

One thing to consider when valuing used tube amps is condition. 
The cheapest 100 watt used Marshall head I ever purchased was $ 600.00 and I had to put $ 600.00 in parts and repairs in to it to get it up to 100%.


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## Business

I've said it before but, you can pretty much just forget about Channel B (lead, with clipping diodes) and crank Channel A (what most people use as a clean channel) and you'll get a JCM 800 type tone with a little bit less gain. Just use a boost for more gain 

Regarding the loop: the bypass circuit is crappy. Patch a cable, mess with the loop volume knob and you can find a sweet spot


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## Louis

Business said:


> I've said it before but, you can pretty much just forget about Channel B (lead, with clipping diodes) and crank Channel A (what most people use as a clean channel) and you'll get a JCM 800 type tone with a little bit less gain. Just use a boost for more gain
> 
> Regarding the loop: the bypass circuit is crappy. Patch a cable, mess with the loop volume knob and you can find a sweet spot


I’ve read that they are some easy mods to remove the diodes in the preamp statge .

Also , never thought Robin Trower used those
on this tour !


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## sorbz62

I have used JCM900s live many times, mainly 4100 heads and both channels are awesome once the amp is cranked. Many people use an OCD or a Tubescreamer into a valve amp so what is the difference? They are fantastic workhorse amps.


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## sorbz62

Business said:


> I've said it before but, you can pretty much just forget about Channel B (lead, with clipping diodes) and crank Channel A (what most people use as a clean channel) and you'll get a JCM 800 type tone with a little bit less gain. Just use a boost for more gain
> 
> Regarding the loop: the bypass circuit is crappy. Patch a cable, mess with the loop volume knob and you can find a sweet spot


So, you mean do not use Ch2 as it uses clipping diodes but use Ch1 ....with a boost!!! Haha!

The effects loops are also good but tend to get fugunk up. If you clean the jacks often then you should get a nice sounding loop.


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## danielSunn0)))

Speaking of 900's, figure I'll share mine... 
100watt SLX, el34 version.


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## Business

sorbz62 said:


> So, you mean do not use Ch2 as it uses clipping diodes but use Ch1 ....with a boost!!! Haha!


Not all overdrive pedals use diodes
And circuits using diode clipping aren't all created equal


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## butterknucket

I had a Dual Reverb combo back in the 90's and thought it wasn't bad.


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## 55 Jr

I've got a 2501 50 Watt (MKIII combo).
Great grab and go Marshall.


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## Louis

danielSunn0))) said:


> Speaking of 900's, figure I'll share mine...
> 100watt SLX, el34 version.
> 
> View attachment 176529


Great !!
Is your top cabinet wearing a skirt ?


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## Louis

Ok so a few weeks back I bought a 1991 very clean JCM900 4500 DR with EL34’s , got home and something bottered me so I started reading
about a mods that would take the diodes off and have the amp behave like a tube amp .

I found this mod posted below and took the amp to P81 Amplification and Patrick did the mods plus a few tricks of his own and it’s night and day !!
The amp for some reason is a lot quieter with more note definition and a lot warmer ,can pull some nice brown sound at low volume.
Now the gain comes from V1 and not from
diodes !................So here it is for you 900 owners , the mod is quick and easy so shouldn’t cost you guy’s much , two hours at the most.

Below you have the mod and explanation on what it does , I didn’t do number #8 and my tech did something different with D10 but don’t remember what it was.

...........................................................................
******* JCM900 Dual Reverb mod *******

1] C27, (4n7)snipe one leg off
This removes the weird, post-overdrive, pre-EQ, shaving off some of the fizz, restoring mids, and giving a subtle bassboost. It doesn't do anything for more gain or more articulation. It does help the tonestack to react more like a traditional marshall.

2] C35, (220pF) to 100pF
Restoring some of the treble response, pre-overdrive. Better for pinch-harmonics, articulation and tone in general. Subtle but worth it.

3] R42, (10k) piggy back another 10K over it. This provides more gain, especially over 500Hz. Great for more saturation, without making it muddy.
4a] D10, (diode) snipe one leg off:
-Prevent the diodebridge to clip the signal to +/- 2 Volts. This alters the clipping threshold, loosing some overdrive. It lowers perceived gain. Could make the opamp clipping more audiable.

4b] R60, (470k) Pickybag 47K over it
-Lowers output signal to the FX-send, and downstream components. Tweak this if perceived volume is too low or too high.

4c] R13, (220k) snipe one leg off
-Removes global feedback around the tube-gain-stage. Now this tube is driven very hard (4a) and acts as a traditional gain stage in vintage Marshall's. It makes the tonestack act like a traditional Marshall tonestack, and its clipping behavior is like an old marshall. But since the clipping treshold of this tube-stage is HIGHER then the diodebridge, you will loose overdrive. It lowers perceived gain.

5] Change C9 (220nF) to 100nF, or even 47nF. Lower the pre-clipping bass response, to prevent mud at high gain settings. 100nF or 47nF completely depends on you pickups and playing style.

6] snipe one leg of R17(100k), bend it upwards and solder a 4n7 cap in series between the leg of R17 and the bit open spot were you sniped R17 off. This is a fixed depth mod. As the 4100 doesn't have a lot of feedback (at least mine doesn't, warn tubes?) , the effect is subtle. More deep bass, and a more giving, elastic, organic feel.

7] Pickyback a 4n7 over R38(10K). This shaves a bit more fizz. This is subtle, but welcome I think. This trick I got from the JMP-1.

8] If you did 4], but find the gain too low, consider putting a 4u7 to 22uF cap piggybacked over R2 (1K5). This increases gain and give more overdrive. Might introduce some fizz, so 7] might be very good in combination.


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## corailz

I’ve tried a JCM900 combo 2x12 50w at home once(can’t remember the model) , and i liked it. It was fizzy if pushed too much in higher gain territories, but it sounded very good when the gain was about 1/2 or a bit over and pushed with a OD pedal. 
It was not as good as my JCM2000 DSL50 for hugh gain stuff(This amp is a well kept secret, a gem), but if i can recall, the JCM900 had a reverb too and it was really quiet compared to the one on the DSL that was doing weird noises(Rattles from the tank) when playing at higher volumes, no matter if it was on or off. 


Envoyé de mon iPhone en utilisant Tapatalk


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## NorlinNorm

Louis said:


> This guy gets nice bluesy tones with his,


I Like!!
This is my jam preference!!


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## Louis

Fianally made a crappy Iphone clip ,
complete different amp , fizz is gone and there’s nicer note deffinition and some warm fat crunch!


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## Adcandour

Well, I took the FS ad down or my MKIII Model 2501.

I was playing quietly last night and was really happy with the tone. I was playing Black Diamond during the day and nailing it. A lot of clarity and a really nice classic rock tone

Today I had Steve Moratto add a choke transformer. Amazing. No tonal changes whatsoever - just a much nicer feel.

I'm starting to really love this amp. I'm considering adding some reversible mods that add a bit of sag, since my primary issue with this amp was how stiff it is.


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## Frenchy99

Here`s my 4100... Like it very much.

Just needed new tubes when I got it.


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## Louis

adcandour said:


> Well, I took the FS ad down or my MKIII Model 2501.
> 
> I was playing quietly last night and was really happy with the tone. I was playing Black Diamond during the day and nailing it. A lot of clarity and a really nice classic rock tone
> 
> Today I had Steve Moratto add a choke transformer. Amazing. No tonal changes whatsoever - just a much nicer feel.
> 
> I'm starting to really love this amp. I'm considering adding some reversible mods that add a bit of sag, since my primary issue with this amp was how stiff it is.


Ha !!........nice to hear !!!......found this mod here below , but really don’t know what it does ! still waiting for mine !!
Keep us posted on the next steps !!


Marshall member quote,
*******************************************
When I was younger everyone said there really aint nothing to be done about making the JCM 900 2500/2100sound improved. I disagree. Now the 4500/4100 are another story I have not played with so far.

Of course more can be done but I will finish at this point.

1.) R5 3k3 add .68uf bypass cap. (Do not decrease resistor value).

2.) R8 1k5 add .68uf bypass cap.

3.) D2 Remove single 1N4007 diode and replace with set of 5.1 volt zener's(1N4733). *Stripes (Cathodes) Outward.*

4.) R4 Plate = 320k.

5.) R7 Plate = 320k.


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## Adcandour

Louis said:


> Ha !!........nice to hear !!!......found this mod here below , but really don’t know what it does ! still waiting for mine !!
> Keep us posted on the next steps !!
> 
> 
> Marshall member quote,
> *******************************************
> When I was younger everyone said there really aint nothing to be done about making the JCM 900 2500/2100sound improved. I disagree. Now the 4500/4100 are another story I have not played with so far.
> 
> Of course more can be done but I will finish at this point.
> 
> 1.) R5 3k3 add .68uf bypass cap. (Do not decrease resistor value).
> 
> 2.) R8 1k5 add .68uf bypass cap.
> 
> 3.) D2 Remove single 1N4007 diode and replace with set of 5.1 volt zener's(1N4733). *Stripes (Cathodes) Outward.*
> 
> 4.) R4 Plate = 320k.
> 
> 5.) R7 Plate = 320k.


I'm actually trying to do the absolute bare minimum, so I don't affect too much. I'll be talking to Moratto today as well as Rich Gowan in Cambridge.

I'm trying to understand what makes a note trail off with an 'ow' sound (think 'A' and 'E' chords in the intro to "Everybody Wants Some" by Van Halen). That's the only thing I feel the amp is lacking.


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## Louis

adcandour said:


> I'm actually trying to do the absolute bare minimum, so I don't affect too much. I'll be talking to Moratto today as well as Rich Gowan in Cambridge.
> 
> I'm trying to understand what makes a note trail off with an 'ow' sound (think 'A' and 'E' chords in the intro to "Everybody Wants Some" by Van Halen). That's the only thing I feel the amp is lacking.


Apparently the reare three different circuit for the MKIII which make modding kind of puzzly !!
But once you find the secret sauce !..It Rocks !!

I’m subscribing to your surch to find out more and will do the same once I get mine in the house !!!’.....!


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