Is JCM900 using the same OT as the Plexi?

[Alan0354]

I have a JCM900 Dual Reverb 100W small combo that I don't really like. I am thinking about converting it into a single channel 1959 Plexi. My question is whether they share the same output transformer.

I am hoping I can use both the PT and OT as they are all Marshalls. I am planning to put in either terminal strips or turret board to build the circuit. Anything I have to watch out?

[soundguruman]

I have a JCM900 Dual Reverb 100W small combo that I don't really like. I am thinking about converting it into a single channel 1959 Plexi. My question is whether they share the same output transformer.

I am hoping I can use both the PT and OT as they are all Marshalls. I am planning to put in either terminal strips or turret board to build the circuit. Anything I have to watch out?

No, it does not use the same OT, but similar.
You can still use 900 OT for plexi build, no problem.

What model number JCM 900?

There are some pretty cool mods for a 900, that I can pass on to you.
Makes it sound much more like a plexi.

No, I would not start over from scratch. I would mod up the existing 900.

[Alan0354]

No, it does not use the same OT, but similar.
You can still use 900 OT for plexi build, no problem.

What model number JCM 900?

There are some pretty cool mods for a 900, that I can pass on to you.
Makes it sound much more like a plexi.

No, I would not start over from scratch. I would mod up the existing 900.

Thanks

I have to go to the shed to look at the exact model. Today is raining( for a change), I don't want to go to the shed. I'll check if tomorrow. It is a tiny little 1X12 combo.

[soundguruman]

1X12 is coolest of all.

[Alan0354]

Rain stopped, I got it in the house. It's a model 4101. Serial # 931668502. I believe it's a 93 model. I want to verify the preamp schematic is JMP52-02.

[big_teee]

This one looks to cover all models.

http://www.el34world.com/charts/Sche..._100w_4100.pdf

[Alan0354]

This one looks to cover all models.

http://www.el34world.com/charts/Sche..._100w_4100.pdf

Thanks, I got this exact schematic, but you notice it has two versions of preamp and 3 versions of power amp. Which one is which. I did serial number search, and I seems to see JMP52-02 is the preamp, but I cannot verify the power amp and power section.

[daz]

Thanks, I got this exact schematic, but you notice it has two versions of preamp and 3 versions of power amp. Which one is which. I did serial number search, and I seems to see JMP52-02 is the preamp, but I cannot verify the power amp and power section.

I hadtwo 1-12 900 combos and they were nite and day. I think they vary wildly because the one had no issues i could, it just didn't sound good at all. The second one was maybe the best live marshall i ever used back when i was gigging all the time. And i had a jubilee and 800's and JMPs. That 900 cut thru the mix better than any and sounded perfect. Never a bad room for it. Great amp. The other one was horrible. Both were the earlier el34 models, not the later 5881. I dunno if they just did a number of revisions to improve the sound or what, but they were the same exact model and sounded worlds apart. I used the same tubes in both too.

[soundguruman]

Rain stopped, I got it in the house. It's a model 4101. Serial # 931668502. I believe it's a 93 model. I want to verify the preamp schematic is JMP52-02.

try wait, the circuit board can be damaged and it is best to have a pro technician install the modifications.
so consider not doing it yourself, but rather, let an experienced person work on it.

This is how I usually do it:

1. R35 is 10K. Replace R35 w/ 68K - prevents the input from overloading, distorting. Takes some grit out...I use 1/2W metal film, but it's your party.
2. C 8 is 470pf. Change C8 to 100 pf. -- C7 is 22 pf. Change C7 to 100 pf. The hi upper mid is a bit harsh and this is mellowing it out a bit. I use silver mica, but it's your choice...
3. C25 is .047 uF. Change C25 to .1 uF.I used a big honk'en Mylar cause it sounded cool. This gives the clean channel a bit more bottom end.
Yeah, this amp just has too harsh a high frequency, so this helps balance it out a little more.
4. Bypass R54 with a 500pf capacitor. I used silver mica. this is helping to find a less muddy tone for the clean channel. Helped the definition and made the highs more sparkly and less harsh.
5. Remove LEDs 1-4. This stops the clean channel from clipping, and it's completely clean, like a twin reverb...no longer breaks up in the clean channel...
6. C34 is .001 uF. Change C34 to 250 pf. It's kinda like the bright switch for the clean channel, but not too bright. I used silver mica. Again more sparkly in the high end...
7. Take out C33. This is intended to kill noise in the clean channel, but it's too muddy. And the clean channel doesn't need it anyway.
8. Bypass R2 with a 6.8 uF 25 Volt or higher volt electrolytic capacitor. The + of the cap will face the cathode of V1A.
Basically, there is a mistake in the design which kills the bass response of the amp. This is going to allow V1A to pass low frequencies. Suddenly the amp is going to sound like a Marshall, when before, it did not. Again, this resolves the high frequency harshness of the amp. The Lows will be balanced, much better with the highs.

Now I am going to change the overdrive EQ and gain. This makes the amp sound quite a bit sweeter, more like an Old Marshall.


9a. R43 is 4.7K. Change R43 to 10K
9b. R7 is 4.7K. Change R7 to 6.8K.
9c. R37 is 22K. Change R37 to 10K.
9d. R38 is 10K. replace R38 with a wire jumper.

Now we are up to the tonestack...

10. R13 is 220K. Remove R13 and leave it out. This resistor makes the amp too polite. Removing R13 makes the amp more aggressive. And, 13 is an unlucky number anyhow.

11. Remove C11 and replace it with a wire jumper. The voltage on the input of the tone stack is now about 110-132 volts DC.

12a. Replace C37 with a 470 pf 500 Volt silver mica Capacitor.
12b. Replace C 38 with a .022 uF 600 Volt Orange Drop Capacitor
12c. Replace C39 with a .022 uF 600 Volt Orange Drop Capacitor

Well, you don't have to use silver mica and orange drops, but I liked the way it sounded.
You can use what you like...
The 3 capacitors, C37, C38, C39, should be at least 250 Volts or higher.

13. We skip 13 cause it's unlucky.

14. Now, because we made the clean channel cleaner, we are going to increase the sensitivity of the Clean Master Volume...
14a. Pin 2 of IC8 goes to ground. Cut that track, right in the middle...
14b. Install a 1 meg resistor between pin 2 of IC8 and ground.
Now, the settings of clean master volume and overdrive master volume will be "closer" together. There will not be such an extreame
difference between them...

15. Reverb Wires:
15a. The reverb send and return wires are located very close to the input jack.
15b. Reverse the location of the send and return wires, where these go thru the chassis holes, to the reverb tank.
15c. Move the location of the reverb wires, away from the input jack. Relocate the route of these wires, so they
are as far away from the input jack, as possible. Route these wires around the inside edge of the chassis, so they are no longer
parallel with the input jack. Make the separation distance, from the input jack, as wide as possible.

There will be a whistling oscillation in this amp, because the reverb wires are too close, to the input jack.
Move those wires away, from the input jack, to stop the oscillations.

You can see how I moved those reverb wires, away from the input jack.
This is necessary, to stop an oscillation loop, between the input and the reverb circuit.

Now there is one remaining issue:
The effects loop
The effects loop levels in this amp are incorrect, and will not work with a standard USA effects pedal.
The effects loop must be modified to work correctly.



2 be continued...shortly...

[Alan0354]

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.

[Alan0354]

I forgot to ask, the negative bias circuit is kind of strange, looks like Marshall is too cheap to put a tap to get -50VAC, instead, using a small value cap and tap off the high voltage. Using the cap as a voltage divider and get -50V with a rectifier diode!!! What's the deal?

Ha ha, before I can do any mod, I have to first fix it. It blew fuse, I never open it to even check. Instead, I had been using it as a cabinet for almost two years before I discovered how bad the speaker sounds. So I put it in the shed until today. that Celestion G12T-100 with the cabinet has a scoop mid sound where the bass is high with some chime. The mid is really bad.

[J M Fahey]

IC6a output is pin 5 .
It is also IC6b output, by the way.
It's roughly equivalent to TL072 BUT a switching version, pin 5 gets assigned to one or the other, depending on what voltage is applied to pin 1.

As of a 50V bias tap, there's nowhere to take that from, since the PT has a single HV winding, rectified by a diode bridge, so every turn of it is always positive with respect to ground.

And the reason to make an SS front end was that in the early 90's new tubes quality was abysmal, and a large company like Marshall can not rely on NOS tubes for a massively produced amplifier.

And really it's not *that* many stages (for an SS amp, that is ).
First stage: IC1 a or b , they are never ON at the same time, is the input stage.
Provides gain and buffering for the next stage.
Second stage: IC6 a or b, the clipping stage.
Not much more than a glorified MXR Distortion+ or equivalent. Boring.
By the way, clipping is solid state all the way, courtesy of diode bridge W005 plus extra diode.
And symmetrical to boot. Boring.
A DS1 sounds tons better, because it was designed as a distortion pedal from the beginning.

And the tube V1 a/b does NOT repeat NOT distort even a teensy bit.

Basically it's there for looks

[Alan0354]

It seems like they just sandwich some tubes into a SS frontend. I am not sure whether any mod can safe this amp. That's the reason I am thinking about tearing it all out and put in the Plexi circuit. It's simple.

So is the way to generate the -ve voltage is using the +ve cycle to charge up the 47nF C15 so when the voltage start to go from +HV towards 0, the other side of C15 goes negative. C15 and C14 forms a voltage divider?

To be honest, to me, the cascade gain amp is just a distortion pedal in front of an amp. The disadvantage is you are stuck with that pedal no mater what!!! That's the reason I am out of the mood of any cascade amp. Get a tube pedal if you want tube distortion!!!

[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 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.

[soundguruman]

IC6a output is pin 5 .
It is also IC6b output, by the way.
It's roughly equivalent to TL072 BUT a switching version, pin 5 gets assigned to one or the other, depending on what voltage is applied to pin 1.

As of a 50V bias tap, there's nowhere to take that from, since the PT has a single HV winding, rectified by a diode bridge, so every turn of it is always positive with respect to ground.

And the reason to make an SS front end was that in the early 90's new tubes quality was abysmal, and a large company like Marshall can not rely on NOS tubes for a massively produced amplifier.

And really it's not *that* many stages (for an SS amp, that is ).
First stage: IC1 a or b , they are never ON at the same time, is the input stage.
Provides gain and buffering for the next stage.
Second stage: IC6 a or b, the clipping stage.
Not much more than a glorified MXR Distortion+ or equivalent. Boring.
By the way, clipping is solid state all the way, courtesy of diode bridge W005 plus extra diode.
And symmetrical to boot. Boring.
A DS1 sounds tons better, because it was designed as a distortion pedal from the beginning.

And the tube V1 a/b does NOT repeat NOT distort even a teensy bit.

Basically it's there for looks

Do you EVER stop whining?
wha wha wha!

Yeah, to you, it's a distortion plus...
To a musician, it's a Marshall.

Getting "quite a lot" of tube overdrive...despite the front end being solid state buffer and switching.
That's what the 900 is all about.
They used a solid state buffer, to decrease the tendency to oscillate and ring, which keeps it more under control.
So, it tends to be more stable, at high gain settings.

FYI: It does not sound like a "distortion plus."