Is JCM900 using the same OT as the Plexi?

[soundguruman]

Thanks. It's a long list!!! I definitely try the step 3, 5, 7 and 8 first as one complain I have with the clean channel is not enough bass and chime.

I have been an EE for 30 years, modifying this should be a cake walk technical wise. I worked with surface mount 8 layer pcb for years!!! The question is whether I want to go through this or not. I do like the Plexi and I have a 73 SF Fender Pro Reverb. You can't beat the clean sound of the vintage Fender SF.

From looking at the schematic, there is just way way to many stages. I think the circuit can be simplified a bit. Tube circuits are interlaced with opamps!!! Nothing simple like the legendary amps like Plexi and vintage Fenders. I still believe in less is more when comes to tone.

The parts have the leads bent and folded down to the circuit board.
Do not try to un-fold the leads. This tends to damage the circuit tracks.

Instead, cut the 2 components legs. Heat the connection and PUSH the lead out of the hole, one at a time.
Then gently wick the solder out of the hole.

Put your new part in the holes. But don't bend the leads over. Leave the leads straight. Then solder it in.
This will allow changing the parts, without damaging the circuit tracks.

As you see, I have subtracted one stage of the overdrive, and lowered the gain at IC4b.

[J M Fahey]

*Other* Marshalls are "Marshalls" , this one is not.

The fact that you yourself suggest some 35 mods to make it sound, sort of gives me the reason, doesn't it?

And after 30 solid years of being the undisputed King of Heavy Rock amps, after JCM900 all kinds of "new kids on the block" started displacing it: Soldano - Mesa - Engl - Egnater - Peavey - (even Peavey !!!!!!!!!!!!!!!!!!!! ) - Framus - Bugera - Diezel - etc. , obviously to "fill a void".

So best Musicians in the World prove my point.

Today it's a *rarity* finding a Marshall onstage, much less than 50% of the time, while before JCM900 it was 100%.

And those are either older Plexis or modded JCM800 or newer JCM2000 or TSL onwards, where they made tubes work again.

But what would you know?

[soundguruman]

*Other* Marshalls are "Marshalls" , this one is not.

The fact that you yourself suggest some 35 mods to make it sound, sort of gives me the reason, doesn't it?

And after 30 solid years of being the undisputed King of Heavy Rock amps, after JCM900 all kinds of "new kids on the block" started displacing it: Soldano - Mesa - Engl - Egnater - Peavey - (even Peavey !!!!!!!!!!!!!!!!!!!! ) - Framus - Bugera - Diezel - etc. , obviously to "fill a void".

So best Musicians in the World prove my point.

Today it's a *rarity* finding a Marshall onstage, much less than 50% of the time, while before JCM900 it was 100%.

And those are either older Plexis or modded JCM800 or newer JCM2000 or TSL onwards, where they made tubes work again.

But what would you know?

Click link to see over 90 touring headliners currently using Marshall, on stage:

Starting with: Angus Young, AC/DC and ending with Billy Gibbons, ZZ Top

or is Billy using PV these days? hahahahahahhhahahhhahahhahahahhahahaahahahahahahahhahaha hahha!!!!!

http://www.marshallamps.com/artists/all/

Yes it's "so rare" to see Marshall onstage...maybe in South America...(?)



You do have some type of mental disorder regarding Marshall amps...

But you forgot that the amps that "fill the void" are mostly copies of Marshalls...
Mesa copied Marshall, and Bugera copied Mesa....and on and on...

So apparently the influence is still relevant there somewhere.

Peavey? They just sell to bands at dealer cost...to make Peavey visible on stages.
And the musicians buy the Peaveys and use them,
Cause they don't want their Marshalls beat up from touring.

"I don't always shred on stage, but when I do...I prefer Marshall."
Keep Shredding, my friends.

[Alan0354]

The amp has "no" bass. It's like a high pass filter.
It's a mistake.
#8 is the most important, to get the bass back. You can use 2uF thru 6.8 uF, whichever you like. To me, 6.8 gave me what I considered "Marshall" sound.
I would not go higher than 6.8, it would start to be "too boogie like." The low end will become too muddy.
However you gotta do #5, or else the clean channel will "never" be clean...

There are 4 basic mistakes in the design
1. no bass
2. clean is not clean...
3. effects loop design is a mistake.
we can go into the effect loop later. for a variety of reasons, it has to be modified, or it will never work correctly. I'll explain later.
4. Reverb on these always sounded awful. Need a mod to clean up the reverb...have not decided on that yet.

Took just over a year, to iron out all the mods. Got feedback, as musicians tested the sound...over and over, and gradually evolved into what is here.

In this state, after completing the above as written...
This 900 is the best sounding 900 I have ever tried. The overdrive sounds killer, it shreds. It has killer "HUNK" in the low end, like a Plexi.
Really smooth overdrive, really tasty sounding overtones...sounds more like a Marshall, than a Marshall does.
The clean is really really clean.
I love the way it sounds now. It's just how I like it.

Sounded so much better than the original, there was no comparison.

Thanks

1) No bass is the main complain on the clean sound.

2) I am planning to take the effect loop out and go direct from V1b to V2a or even eliminate V2a all together depending on the gain from effect sent to return.

3) I don't even know why they put the LED 1-4 in the clean channel. For touch sensitive playing to walk between clean and compress by picking force?

I have to open up the amp and look at the pcb to tell the quality of the pcb. Does the pcb has PLATED THROUGH HOLES? Fender DRRI does not use plated through holes, it's very easy to damage the pcb and you have to remove the pcb to do major work like this. That's what I don't like about pcb amp, not that they are unreliable, they just much harder to mod.

[Alan0354]

Fender and Marshall is used by like 60% to 70% of the famous musician. They are just apple and orange. For clean, Fender is about the best. For crunch and distortion, Marshall is the standard where everyone measure against. Fender distort totally different, it does not have the raw and rude sound of the Marshall that defines the crunch.

[soundguruman]

Thanks

1) No bass is the main complain on the clean sound.

2) I am planning to take the effect loop out and go direct from V1b to V2a or even eliminate V2a all together depending on the gain from effect sent to return.

3) I don't even know why they put the LED 1-4 in the clean channel. For touch sensitive playing to walk between clean and compress by picking force?

I have to open up the amp and look at the pcb to tell the quality of the pcb. Does the pcb has PLATED THROUGH HOLES? Fender DRRI does not use plated through holes, it's very easy to damage the pcb and you have to remove the pcb to do major work like this. That's what I don't like about pcb amp, not that they are unreliable, they just much harder to mod.

A. All you need is to bypass R2 with a 6.8 uF cap. + of cap faces cathode. You don't even need to take the board out to do that. (see how I tacked it on the resistor?)
Your bass will be back in action. Bass for days dude.

B. LED 1-4 causes the clean channel to break up. You may like it that way...some people do like it.
or take 1-4 LED out. Then The Clean Channel will be totally clean - with no distortion at all.


Click on this photo. You can see where LED 1-4 has been removed.
You can see R2, and the 6.8uF I piggy-backed onto it.

C. No, the holes are not plated thru. It's easy for an experienced person.

D. IT IS NOT necessary to take out the effects loop!
*I do not recommend this*
This amp will sound KILLER just by making a few little MODS to it.
No, you don't need to re-design it, to make it sound great.

[soundguruman]

Fender and Marshall is used by like 60% to 70% of the famous musician. They are just apple and orange. For clean, Fender is about the best. For crunch and distortion, Marshall is the standard where everyone measure against. Fender distort totally different, it does not have the raw and rude sound of the Marshall that defines the crunch.

That is correct.
Marshall and Fender are the workhorse amps of the pro sound industry.
There is no substitute for the tone they produce.

Nearly every professional concert has these amps on stage - it's a "given."

NO, professionals worldwide have NOT stopped using Marshalls. That's total BS.

[Alan0354]

That is correct.
Marshall and Fender are the workhorse amps of the pro sound industry.
There is no substitute for the tone they produce.

Nearly every professional concert has these amps on stage - it's a "given."

This is my opinion only:

Fender is a lot more general purpose, you can still get good distortion. Marshall is more narrow minded, but it's crunch is legendary. For musicians that are up there, they can play their own style, they can afford to have a one trick amp that do the job exceptionally well like the Marshall. But for us mortals that need to play all different styles, Marshall is a little too specialized.

I bought a 73 Pro Reverb, so I want a Marshall Plexi. But if I have to live with one amp only, I likely would choose the Fender with good clean sound and make do with the distortion with pedal. For the longest time, I use A/B pedal to jump from a clean amp to a distorted amp to get the best of both worlds. But then the question is whether you want to lug two amps every where? As working musician know, you do have to play soft music that has nothing to do with crunch!!! That's just comes with the job.

[soundguruman]

Many do not realize that Marshall is also a very clean amp.

[Jazz P Bass]

Ho Hum.

[Alan0354]

Many do not realize that Marshall is also a very clean amp.

There might be other Marshalls that I don't have. I owned a Plexi triple stack in 74 and this JCM900, I love the crunch of the Plexi. But the clean sound is not very good. But The crunch is legendary. That's the reason I am seriously thinking about make this a Plexi. Then I can use an A/B pedal and get Fender clean and Marshall crunch. The next thing is to lower the power of the Marshall by switching tubes and/or attenuator.

But this is a future project, I am gathering information while I am working on my Bassman build. I am going to fix the JCM900 and do some of the mod you suggested. I am not going to do anything on the reverb as it's not important to me.

I like to hear what you do to the effect loop. My first impression is to eliminate it all together and eliminate all the SS opamp in the effect loop circuit.

[olddawg]

That JCM 900 combo is a reasonably desirable amp if it is any decent condition. If you are not an experienced tech, I would suggest you sell it and apply the cash towards buying a good Plexi kit if a Plexi is what you really want. Once you start modding, it can become someone's worthless failed project rather quickly. Someone will buy it right now, as is, unmolested. I don't care for them but there are lots of people who love those JCM 900 combo amps.

[cyclone]

those things (jcm900) have the bugzapper clipping circuit built into the preamp, and they break all the time. thats why they are 300$ at the store and a beat JCM 800 is 500$

[soundguruman]

We took 4 LEDs out of the preamp. That stops the clipping.
And FYI they DO NOT "break all the time." More likely, they are abused...

A bad speaker cable, the wrong impedance speaker, etc...that's what makes it break down.

Once we did a few mods to it, it was just as good as any other Marshall.
The overdrive was smooth and creamy, the clean was sparkling clean, and the effects loop worked perfect with several pedals at once.
My little mod gave it killer low frequency response and it grinds like a chainsaw.

So, it's really not bad at all. I would rather play this than any Boogie...
Since it had a low power triode mode, we could play it just about anywhere.

I am a very picky guitar player, and I have a very definite idea about how a Marshall should sound.
There are a few design mistakes but once those are ironed out, it's great. In fact, I would keep it.

Effects Loop:
What we did find, is that between front panel input effects -- and effects loop, there needs to be two separate effects power supplies.
Otherwise you get a ground loop, and low frequency hum.
But with isolated effects power supplies, the hum was gone.
That was the only bug that cropped up, and easily solvable.

[Bjorn218]

I don't want to hijack the thread as I think its very interesting, but want to ask a question about the 900.

My bass player owns a 4100 head and some times at rehearsal I use it instead of my gear.

The question I have is that his is no where near as loud as I know Marshalls can be, are the JCM900's output normally lower than say an 800 or JMP?

For example, When I use my JMP 2203, rehearsal volume on the dial is around 3.5-4, The 4100 to get the equivalent volume the amp must be at 7-8. This 900 also doesnt have that stereotypical Marshall volume jump. Where it goes from too low to way too loud less than halfway between 1 and 2, its a gradual increase through the travel of the volume pot. Is this normal in these amps?