# 50 Watt Plexi, Help Me Understand Please!



## Mr. Nick

Im trying to build a 50 watt Plexi, the original with a master volume circuit. Unfortunately for me Marshall made the naming very confusing. Can anybody give me a link to an original JMP 50 watt. I'm going for the type randy rhoads used, except it needs to be the 50 watt version. He used a JMP 1959 MK II 100 watt. Im looking for the 50 watt version and i keep getting confused with all this JMP models floating around. 

Thank you all who help me
Nick


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## Len

You need a schematic?


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## Gunner64

There has to be tons of 50w slp layout diagrams out there. Find one..and then I would built the amp as stock first..add a master volume and R.R. mod after you get the amp running stock. I' m not an slp expert but I have built several clones of other marshalls and I've found it's better to get the amp as designed working properly before the add ons. Just my 2 cents..


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## slagg

Use this,tweak from there


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## johan.b

Original plexi never had a master volume..just a bright and a normal volume with different inputs 
Look for 1987x. Schematic. The "x" is for reissiue. And it's a copy of the ca 1972 version of the amp. Then add a ppimv type master volume and you should be close to your goal. To get even closer, look for "one wire mod" too, and you should be as close as you can come
J


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## slagg

This will give you an idea how Marshall altered the basic same amp


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## neikeel

https://doc-0g-as-docs.googleuserco...788/*/0By1xsHfamQ8iNkZ5S0hWT1RpUGc?e=download
This is link to a true 50w plexi layout.
Look on Steve's google drive folder there is lots more. Also there you will find my layout to have a NMV and MVversion in the same amp


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## Mr. Nick

Why would it


Gunner64 said:


> There has to be tons of 50w slp layout diagrams out there. Find one..and then I would built the amp as stock first..add a master volume and R.R. mod after you get the amp running stock. I' m not an slp expert but I have built several clones of other marshalls and I've found it's better to get the amp as designed working properly before the add ons. Just my 2 cents..


 be the 50 watt slp? Is the the same as the JMP?


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## Mr. Nick

Len said:


> You need a schematic?


Yes I'm just looking for he schematic of the circuit that was used 


Len said:


> You need a schematic?


Yes I'm looking for the Marshall Plexi JMP 50 watt schematic.


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## Gunner64

I'm not a plexi expert but from what I understand 50w plexi..1987...50w jmp..2204...


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## neikeel

The Marshall 50w Plexi evolved over the years. The JMP 50 started in 1967 with a shared v1 cathode and low filtering and high NFB. There was a PA model 1985, Bass model 1986 and a lead model 1987.
slagg posted the schem of Model 1989 from year 1970 which is a similar schem to the early models and that is the Organ. 
Most people who come from 'outside' think of a JMP50 as the Model 1987 Lead amp from 1969.
This is split cathode, as rectifier and tighter filtering and this is very similar to the later master volume 2204 amps post 1977 MY. Which simply inserts a 1M pot between the output of the treble pot and the feed into the PI. And cascades the two halves of V1 Hence my version which incorporates the two versions in switchable form.
basic schem for a 1987 is above.


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## Mr. Nick

I


neikeel said:


> The Marshall 50w Plexi evolved over the years. The JMP 50 started in 1967 with a shared v1 cathode and low filtering and high NFB. There was a PA model 1985, Bass model 1986 and a lead model 1987.
> slagg posted the schem of Model 1989 from year 1970 which is a similar schem to the early models and that is the Organ.
> Most people who come from 'outside' think of a JMP50 as the Model 1987 Lead amp from 1969.
> This is split cathode, as rectifier and tighter filtering and this is very similar to the later master volume 2204 amps post 1977 MY. Which simply inserts a 1M pot between the output of the treble pot and the feed into the PI. And cascades the two halves of V1 Hence my version which incorporates the two versions in switchable form.
> basic schem for a 1987 is above.


I found the 2204 master volume schematic, which seems to be the right one if I'm right. 
Am i right? i wanna make sure this is the right one before I order the parts.


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## danfrank

The problem with the 50 watt master volume schematic you posted is that it doesn't have the cascaded preamp like all other JMP/JCM master volume amps. This is an anomaly for the first year or 2 of the 2204 JMP amps. So, what do you want? NMV style 50 watter with a MV (like the schematic you posted) or an actual cascaded input (the norm) 2204 master volume 50 watter?
The cascaded input version will have more gain. The "hi" input will be pretty dirty and the "low" input will be very clean...

The 2 schematics you posted are the same amp except for the "master volume" potentiometer installed on your 2204 schematic. If you turn the master volume knob to "10", both of those schematics will make the exact same sounding amp, basically a 1987. Maybe I misunderstood and that's what you actually want?


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## ampmadscientist

neikeel said:


> The Marshall 50w Plexi evolved over the years. The JMP 50 started in 1967 with a shared v1 cathode and low filtering and high NFB. There was a PA model 1985, Bass model 1986 and a lead model 1987.
> slagg posted the schem of Model 1989 from year 1970 which is a similar schem to the early models and that is the Organ.
> Most people who come from 'outside' think of a JMP50 as the Model 1987 Lead amp from 1969.
> This is split cathode, as rectifier and tighter filtering and this is very similar to the later master volume 2204 amps post 1977 MY. Which simply inserts a 1M pot between the output of the treble pot and the feed into the PI. And cascades the two halves of V1 Hence my version which incorporates the two versions in switchable form.
> basic schem for a 1987 is above.



*You MUST add 1K screen resistors to this circuit.

If you don't add the resistors, the screens will draw too much current - and the fuse will keep blowing.*

This is because: the design of the output tube was changed. Newer EL34 draws much more screen current, compared to the original EL34.


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## ampmadscientist

Mr. Nick said:


> Im trying to build a 50 watt Plexi, the original with a master volume circuit. Unfortunately for me Marshall made the naming very confusing. Can anybody give me a link to an original JMP 50 watt. I'm going for the type randy rhoads used, except it needs to be the 50 watt version. He used a JMP 1959 MK II 100 watt. Im looking for the 50 watt version and i keep getting confused with all this JMP models floating around.
> 
> Thank you all who help me
> Nick



*It's almost useless to put a master volume on this Plexi.*

This is because the plexi preamp is CLEAN. The plexi depends on cranking the amp to full volume - this overdrives the output tubes.

Plexi Distortion = output tube distortion. There is very little preamp distortion in this design.

BUT
if you modify the preamp...to 2203....then a Master volume will work pretty good.

PS: use PPIMV...Trainwreck design. Use a pot for the PPIMV which has a *high voltage insulation rating*.


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## ampmadscientist

danfrank said:


> The problem with the 50 watt master volume schematic you posted is that it doesn't have the cascaded preamp like all other JMP/JCM master volume amps. This is an anomaly for the first year or 2 of the 2204 JMP amps. So, what do you want? NMV style 50 watter with a MV (like the schematic you posted) or an actual cascaded input (the norm) 2204 master volume 50 watter?
> The cascaded input version will have more gain. The "hi" input will be pretty dirty and the "low" input will be very clean...
> 
> The 2 schematics you posted are the same amp except for the "master volume" potentiometer installed on the 2204 schematic. If you turn the master volume knob to "10", both of those schematics will make the exact same sounding amp, basically a 1987. Maybe I misunderstood and that's what you actually want.



Very good point. Spot on, also.


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## ampmadscientist

Mr. Nick said:


> I
> 
> I found the 2204 master volume schematic, which seems to be the right one if I'm right.
> Am i right? i wanna make sure this is the right one before I order the parts.



Yes, this schematic has the screen grid resistors...good.

The only thing is: depending on what type tube.....you will need to make minor changes to the bias circuit.

AND you want to use PPIMV instead of the stock master volume.


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## Mr. Nick

ampmadscientist said:


> Yes, this schematic has the screen grid resistors...good.
> 
> The only thing is: depending on what type tube.....you will need to make minor changes to the bias circuit.
> 
> AND you want to use PPIMV instead of the stock master volume.


The only thing is it looks like it does not have the 4 inputs, it only has 2. Why is this?


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## Mr. Nick

So overall I'm just incredibly confused. I just want a 50 watt marshall that has that classic marshall sound like a plexi, and that has a master volume mod and fx loop. Can anybody point me i the right direction?


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## slagg

Marshall 1987x,Add 1 wire mod,add PPIMV ,done.


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## ampmadscientist

Mr. Nick said:


> So overall I'm just incredibly confused. I just want a 50 watt marshall that has that classic marshall sound like a plexi, and that has a master volume mod and fx loop. Can anybody point me i the right direction?



Well don't be confused but...study these things for a while and you can learn it. Don't try to get it all in one day.

Plexi you will need to turn it all the way up to get a good sound, and then it's too loud. (loud as shit) Never underestimate how loud this amp is...stand 10 feet away minimum.
(when you are old, you will need a hearing aid-this is not a joke)
Plexi was made for stadiums. That's how loud it is. (did I mention how loud this amp is? I under-stated.)

Your basement is not a 20,000 seat stadium....but that's how loud this amp is. (the police will visit when you turn it up...be sure of this...)

However, with the 2203/2204 circuit (which is a slight mod from the original Plexi) you will get a better sound at a LOW volume.

A. So, the Plexi has the output tube distortion, you have to crank it to get a rise out of it...very too loud.

B. The 2203/2204 (2 input jacks) has the preamp distortion, and you can use a master volume, better sound at lower loudness level.

*And the difference between the 2 input and the 4 input:
The 2 input has 2X as much gain.
The 4 input does not have this double gain preamp. It's just like a Plexi basically, clean preamp.*

Now think this over: If you want true Plexi sound, it's going to be much louder. And then, there are several ways to compensate for the over-loudness.
Attenuator
Fluxtone Speaker
Iso Cabinet
Adjustable magnet speakers...
Speaker Emulator
etc...etc...etc...

BUT a master volume does NOT work very well on a stock plexi...because the preamp is CLEAN.
(the lower the master is in a stock Plexi, the cleaner the amp gets.)

That is why we modify a Plexi into a 2203/2204 (2 hole) preamp...to get preamp distortion.......THEN we can use the master volume....and get a better sound at a low loudness level.


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## slagg

And now you know.Nothing sounds like a Cranked plexi period.


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## Mr. Nick

Holy shit it all makes sense now. Thank you for giving me a clear overview on the differences. Could you elaborate on the difference between the 2203/2204?


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## Mr. Nick

ampmadscientist said:


> Well don't be confused but...study these things for a while and you can learn it. Don't try to get it all in one day.
> 
> Plexi you will need to turn it all the way up to get a good sound, and then it's too loud. (loud as shit) Never underestimate how loud this amp is...stand 10 feet away minimum. (when you are old, you will need a hearing aid-this is not a joke)
> 
> However, with the 2203/2204 circuit (which is a slight mod from the original Plexi) you will get a better sound at a LOW volume.
> 
> A. So, the Plexi has the output tube distortion, you have to crank it to get a rise out of it...very too loud.
> 
> B. The 2203/2204 (2 input jacks) has the preamp distortion, and you can use a master volume, better sound at lower loudness level.
> 
> *And the difference between the 2 input and the 4 input:
> The 2 input has 2X as much gain.
> The 4 input does not have this double gain preamp. It's just like a Plexi basically, clean preamp.*
> 
> Now think this over: If you want true Plexi sound, it's going to be much louder. And then, there are several ways to compensate for the over-loudness.
> Attenuator
> Fluxtone Speaker
> Iso Cabinet
> Adjustable magnet speakers...
> Speaker Emulator
> etc...etc...etc...
> 
> BUT a master volume does NOT work very well on a stock plexi...because the preamp is CLEAN.
> 
> That is why we modify a Plexi into a 2203/2204 (2 hole) preamp...to get preamp distortion.......THEN we can use the master volume....and get a better sound at a low loudness level.


 I forgot to quote you for my last question. I don't know if it makes a difference though on wether or not it notifies you.


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## Gunner64

2203 is 100w 2204 is 50w


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## neikeel

Mr. Nick said:


> So overall I'm just incredibly confused. I just want a 50 watt marshall that has that classic marshall sound like a plexi, and that has a master volume mod and fx loop. Can anybody point me i the right direction?



Don't be confused.
Read my post above!
The MV models that are ok are post 77, as I carefully referred to. The one in the first schem you pointed to is the first 2204 from 2976 which is a pretty horrible version and to be avoided.
AMS always rants about screen resistors and most people agree that they should be added but that is no reason to bin the rest of the schem. He also cannot cope with a non-cascaded pre-amp which is a very blinkered view, IMO. A plexi to me is the gold standard Marshall sound when pushed into output tube distortion by whatever method you choose!

Model 1987 JMP50 Lead is the 69 version with 4 inputs i.e. a plexi
Model 2204 JMP50 post 77 is 2 inputs, not a plexi.

You can build the 69 version as per the Metroamp instructions and fit the PPIMV (whatever sort you want) and the Steve Miller No loss loop. All the parts are available from Valvestorm and the schems, layouts and assistance is available on the Metroamp site.


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## ampmadscientist

I always rant about screen grid resistors....
because I don't want your amp to blow up....

FYI- 90% of techs do not know about this problem. The problem is constantly mis-diagnosed as "bad capacitors," or "bad output transformer."

Most players do not know: the design of the tubes was changed. The change causes the screen to draw more current...thereby creating the need for the resistors.
So, somebody learned something today ! (maybe not you, but somebody)

The model numbers are a bit confusing, because the 2X preamp (2 hole) and the normal preamp (4 hole) can have the same model numbers...thereby creating the confusion.


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## ampmadscientist

slagg said:


> And now you know.Nothing sounds like a Cranked plexi period.


The cranked Plexi is the standard by which all others are judged....it's the benchmark.
But- It's just a little too loud.


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## Mr. Nick

neikeel said:


> Don't be confused.
> Read my post above!
> The MV models that are ok are post 77, as I carefully referred to. The one in the first schem you pointed to is the first 2204 from 2976 which is a pretty horrible version and to be avoided.
> AMS always rants about screen resistors and most people agree that they should be added but that is no reason to bin the rest of the schem. He also cannot cope with a non-cascaded pre-amp which is a very blinkered view, IMO. A plexi to me is the gold standard Marshall sound when pushed into output tube distortion by whatever method you choose!
> 
> Model 1987 JMP50 Lead is the 69 version with 4 inputs i.e. a plexi
> Model 2204 JMP50 post 77 is 2 inputs, not a plexi.
> 
> You can build the 69 version as per the Metroamp instructions and fit the PPIMV (whatever sort you want) and the Steve Miller No loss loop. All the parts are available from Valvestorm and the schems, layouts and assistance is available on the Metroamp site.


Im slightly lost. Can you give a link to the right schematic?


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## neikeel

This is the correct schematic for a 1969 Marshall JMP50 model 1987:




You just need to add the 1k 5w screen resistors and you are good to go. 
Here is a link to the Metro chassis layout:
50W CHASSIS DIAGRAM.pdf
Parts list:
50 WATT Parts List.txt
Parts order list:
http://www.valvestorm.com/50watt Kit
Build instructions:
50_WATT_KIT_INSTRUCTIONS.pdf
Prerequisites:
Click here for more info

Afraid that is as much as I can for now. If you are serious and start building more help can me provided


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## Mr. Nick

neikeel said:


> This is the correct schematic for a 1969 Marshall JMP50 model 1987:
> 
> 
> 
> 
> You just need to add the 1k 5w screen resistors and you are good to go.
> Here is a link to the Metro chassis layout:
> 50W CHASSIS DIAGRAM.pdf
> Parts list:
> 50 WATT Parts List.txt
> Parts order list:
> http://www.valvestorm.com/50watt Kit
> Build instructions:
> 50_WATT_KIT_INSTRUCTIONS.pdf
> Prerequisites:
> Click here for more info
> 
> Afraid that is as much as I can for now. If you are serious and start building more help can me provided


And just so were clear that DOES have the master volume already in it?


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## Mr. Nick

neikeel said:


> This is the correct schematic for a 1969 Marshall JMP50 model 1987:
> 
> 
> 
> 
> You just need to add the 1k 5w screen resistors and you are good to go.
> Here is a link to the Metro chassis layout:
> 50W CHASSIS DIAGRAM.pdf
> Parts list:
> 50 WATT Parts List.txt
> Parts order list:
> http://www.valvestorm.com/50watt Kit
> Build instructions:
> 50_WATT_KIT_INSTRUCTIONS.pdf
> Prerequisites:
> Click here for more info
> 
> Afraid that is as much as I can for now. If you are serious and start building more help can me provided


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## Mr. Nick

Wait dosen't the 1987 not have a master volume mod, and won't I have to turn it up unbearably loud to get that nice marshall sound?


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## Mr. Nick

ampmadscientist said:


> I always rant about screen grid resistors....
> because I don't want your amp to blow up....
> 
> FYI- 90% of techs do not know about this problem. The problem is constantly mis-diagnosed as "bad capacitors," or "bad output transformer."
> 
> Most players do not know: the design of the tubes was changed. The change causes the screen to draw more current...thereby creating the need for the resistors.
> So, somebody learned something today ! (maybe not you, but somebody)
> 
> The model numbers are a bit confusing, because the 2X preamp (2 hole) and the normal preamp (4 hole) can have the same model numbers...thereby creating the confusion.


So the 1987 schematic that newkeel linked below is the one that I will have to turn up unbearably loud to get good sound>=?


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## slagg

Dude,"Master Volume" is a add on.Original "JMP 50" *DID NOT HAVE A MASTER VOLUME! You can add it but then it's not a true "Plexi"curcuit. If you want the Randy Roads start with add ing the PPimv and the "One wire mod" and go from there.There is no such stock amp you are asking for. RR's amps were moded.*


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## Gunner64

Or build the 800 2204.


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## slagg

^^THIS^^


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## danfrank

I'm starting to think that the MF is being visited by a more than usual amount of trolls! Maybe I'm just being paranoid... LOL!

Either way, I'm getting a laugh at reading some of these threads...


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## slagg

The best Marshall tone I've ever got was a old JTM 50 cranked to 3 o'clock="8" thru the Bright channel with eq around noon, thru a quad of Greenbacks. IT had it really going on IMO.Oh, and a Les Paul.


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## Gunner64

That 2204 layout I posted kills. RR, Priest, Scorps AC/DC..ect.ect..80s/classic rock tones out the ass..by the time your done modding a plexi build your more or less going to have the same thing no?..all you smarter than me guys?.. To the op..no offense man if you can't understand what the guys here are telling you maybe you should buy and amp already built. Again No offense but these things got some juice inside them and they bite hard enough to kill you if you are not somewhat schooled in what not to do and why...just sayin.


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## danfrank

Gunner64 said:


> To the op..no offense man if you can't understand what the guys here are telling you maybe you should buy and amp already built. Again No offense but these things got some juice inside them and they bite hard enough to kill you if you are not somewhat schooled in what not to do and why...just sayin.



I was thinking the same thing...


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## neikeel

danfrank said:


> I'm starting to think that the MF is being visited by a more than usual amount of trolls! Maybe I'm just being paranoid... LOL!
> 
> Either way, I'm getting a laugh at reading some of these threads...



I was starting to get the same feeling! Hence my last line above:


> If you are serious and start building more help can be provided


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## ampmadscientist

Gunner64 said:


> That 2204 layout I posted kills. RR, Priest, Scorps AC/DC..ect.ect..80s/classic rock tones out the ass..by the time your done modding a plexi build your more or less going to have the same thing no?..all you smarter than me guys?.. To the op..no offense man if you can't understand what the guys here are telling you maybe you should buy and amp already built. Again No offense but these things got some juice inside them and they bite hard enough to kill you if you are not somewhat schooled in what not to do and why...just sayin.



Since there is (almost) no difference between Plexi and 2204, except for the 1 added preamp stage. The tube to do this and the circuit space/parts is mostly already in the amp. You just re-wire what is already there to start with....

The 2204 "IS" a modified Plexi....and we are just doing what the factory did, to create the same result.

If you have a plexi to start with...then, why heck not?

I install a switch, to *change between Plexi and 2204,* which comes in handy!
This push / pull switch is part of the channel gain control.
No-you do not need to drill holes, or permanently alter the amp to do this.





For the master volume, I use Train Wreck convention...





And for master volume pot, use a pot w/ a *high voltage insulation rating,* such as PEC.
(bias voltage will be present on the master volume pot - therefore use a very well insulated, highly reliable pot)


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## m1989jmp

ampmadscientist said:


> Since there is (almost) no difference between Plexi and 2204, except for the 1 added preamp stage... ...



I have to disagree, plexi with V1a and V1b in series has more gain than JCM800, so it's definitely not just a matter of an extra gain stage.

In other words, if you plug into LO input on a JCM800 (bypassing that ''added'' gain stage) you absolutely can't get that ''plexi'' sound.


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## slagg

Love the "crossline" Master.


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## Kris Ford

m1989jmp said:


> I have to disagree, plexi with V1a and V1b in series has more gain than JCM800, so it's definitely not just a matter of an extra gain stage.
> 
> In other words, if you plug into LO input on a JCM800 (bypassing that ''added'' gain stage) you absolutely can't get that ''plexi'' sound.


THis is true..my OWM/RR modded Super Bass has WAYY waaaaaaaaaaaaaay more gain than my '78 2203.


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## neikeel

We are wandering off topic but the cascade (1 wire or pukka Marshall way is more a question of component values for how much gain you get.
The whole reason I posted the layout of my 2 in 1 early in the thread as you can have 1987/2204 in the same amp without holes and with just a PP switch in one of the inputs.
Can't post the image directly as I am yet to find a replacement for photo bucket where all my pics are!! Try this media through the forum gallery!












Neil Special



__ neikeel
__ Aug 18, 2017


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## slagg

All this great info and this thread will end with nothing.


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## ampmadscientist

The 1 wire mod is basically a mickey mouse attempt...
Use the 2204 schematic instead.


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## neikeel

ampmadscientist said:


> The 1 wire mod is basically a mickey mouse attempt...
> Use the 2204 schematic instead.


Personally I agree, which is why I prefer the mod in my post above which is totally different!


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## Mr. Nick

slagg said:


> Dude,"Master Volume" is a add on.Original "JMP 50" *DID NOT HAVE A MASTER VOLUME! You can add it but then it's not a true "Plexi"curcuit. If you want the Randy Roads start with add ing the PPimv and the "One wire mod" and go from there.There is no such stock amp you are asking for. RR's amps were moded.*


I know the original did not have the master volume. Im just getting lots of pointers towards different circuits, some people saying it has the master volume, some people saying it does not. I just like my information straightforward and I'm getting lost in all the numbers and models.


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## Mr. Nick

danfrank said:


> I was thinking the same thing...


I Understand how to put these together and how to safely discharge the amp. Im just getting different pointers in different directions with different amp schematics and I'm getting lost in it all.


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## Mr. Nick

ampmadscientist said:


> Well don't be confused but...study these things for a while and you can learn it. Don't try to get it all in one day.
> 
> Plexi you will need to turn it all the way up to get a good sound, and then it's too loud. (loud as shit) Never underestimate how loud this amp is...stand 10 feet away minimum.
> (when you are old, you will need a hearing aid-this is not a joke)
> Plexi was made for stadiums. That's how loud it is. (did I mention how loud this amp is? I under-stated.)
> 
> Your basement is not a 20,000 seat stadium....but that's how loud this amp is. (the police will visit when you turn it up...be sure of this...)
> 
> However, with the 2203/2204 circuit (which is a slight mod from the original Plexi) you will get a better sound at a LOW volume.
> 
> A. So, the Plexi has the output tube distortion, you have to crank it to get a rise out of it...very too loud.
> 
> B. The 2203/2204 (2 input jacks) has the preamp distortion, and you can use a master volume, better sound at lower loudness level.
> 
> *And the difference between the 2 input and the 4 input:
> The 2 input has 2X as much gain.
> The 4 input does not have this double gain preamp. It's just like a Plexi basically, clean preamp.*
> 
> Now think this over: If you want true Plexi sound, it's going to be much louder. And then, there are several ways to compensate for the over-loudness.
> Attenuator
> Fluxtone Speaker
> Iso Cabinet
> Adjustable magnet speakers...
> Speaker Emulator
> etc...etc...etc...
> 
> BUT a master volume does NOT work very well on a stock plexi...because the preamp is CLEAN.
> (the lower the master is in a stock Plexi, the cleaner the amp gets.)
> 
> That is why we modify a Plexi into a 2203/2204 (2 hole) preamp...to get preamp distortion.......THEN we can use the master volume....and get a better sound at a low loudness level.



I have another question. So when you mod it into a 2203/4 circuit, you say "master volume" like it is included in the circuit. Is the master volume mod included in the circuit of the 2203/4?


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## Gunner64

Build the 800 2204..master volume...or build a 1987..add master volume..owm..whats the confusion then?.pretty straight forward imo..you have not just schematics but simple layout diagrams posted here for you. But I' m telling you from my experience my 800 2204 clone doesn't dissapoint.


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## Gunner64

Mr. Nick said:


> I have another question. So when you mod it into a 2203/4 circuit, you say "master volume" like it is included in the circuit. Is the master volume mod included in the circuit of the 2203/4?


Build the 800 2204. Master volume is standard.


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## Mr. Nick

Gunner64 said:


> Build the 800 2204. Master volume is standard.


Will do. thanks for the straightforward answer unlike some of these people.


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## Gunner64

Dude everyone was just trying to help. Nobody knows your level of experience so don't take offense to the technical aspects of some of that help..these guys know their stuff, and don't know how much of that stuff another guy understands. That said the diagram I posted results in a great classic/ 80s rock head. Good luck.


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## danfrank

Mr. Nick said:


> I Understand how to put these together and how to safely discharge the amp. I'm just getting different pointers in different directions with different amp schematics and I'm getting lost in it all.



Everyone here is just trying to help you out. I apologize if my comment offended you but nobody here wants you to get in "over your head" and you do something that gets you hurt or worse.

Ah! G64 beat me to it! LOL!


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## Mr. Nick

danfrank said:


> Everyone here is just trying to help you out. I apologize if my comment offended you but nobody here wants you to get in "over your head" and you do something that gets you hurt or worse.
> 
> Ah! G64 beat me to it! LOL!


Its no problem. Im just getting lost in the numbers lol. I just need(ed) a straight answer because this model number stuff is confusing. I also have a friend who builds tube amps who is helping me.


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## Mr. Nick

danfrank said:


> Everyone here is just trying to help you out. I apologize if my comment offended you but nobody here wants you to get in "over your head" and you do something that gets you hurt or worse.
> 
> Ah! G64 beat me to it! LOL!


The inly thing is i showed him the schematic of the 2204 and he said it looks like it does not have a preamp...


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## Gunner64

Build the diagram I posted of the 800 2204..its not a schematic. Has everything you need. .800 2204. Early straight jmp2204s had no master as was mentioned. 800 2204..is a master volume amp. The diagram is straight forward and idiot proof.(trust me I built it lol) You're making it harder than it needs to be.


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## Mr. Nick

Gunner64 said:


> Build the diagram I posted of the 800 2204..its not a schematic. Has everything you need. .800 2204. Early straight 2204s had no master as was mentioned. 800 2204..is a master volume amp. The diagram is straight forward and idiot proof.(trust me I built it lol) You're making it harder than it needs to be.


Thats all the straightforward info I need. Thanks a bunch for making it clearer for me!


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## Gunner64

Others may disagree, but thats what I did and I couldn't be happier with it.


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## danfrank

Gunner64 said:


> Build the diagram I posted of the 800 2204..its not a schematic. Has everything you need. .800 2204. 800 2204..is a master volume amp. The diagram is straight forward and idiot proof.(trust me I built it lol) You're making it harder than it needs to be.



Looking back, this is the first amp I ever built way back when. It sounded good but looked like a science fair project. I built it on a 24" X 12" piece of pine board! The electronics were in an aluminum chassis but the transformers were mounted on the board; not enough room on the chassis. I had a lot to learn yet. LOL!


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## Gunner64

I built mine in a mojo 800 chassis, with faceplates and all. It wasn't my first build so all is neat and tidy, but I had a couple experimental builds too lol..the end result is the bottom head on the smaller stack.


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## danfrank

@G64: I must be blind but what post was the diagram you posted? I didn't find it in this thread.


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## Mr. Nick

danfrank said:


> Looking back, this is the first amp I ever built way back when. It sounded good but looked like a science fair project. I built it on a 24" X 12" piece of pine board! The electronics were in an aluminum chassis but the transformers were mounted on the board; not enough room on the chassis. I had a lot to learn yet. LOL!


Since I love to ever engineer things, and the amp chassis are too big for my enclosure(an old radio cabinet), I m going to to what I do best and Over-engineer it and design a two level turret board that Ill CNC myself so the components can take up less space, and measure all the components with a caliper and make a 2d layout on my CAD program.


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## Gunner64

danfrank said:


> @G64: I must be blind but what post was the diagram you posted? I didn't find it in this thread.


Ha. Its gone..the post is even gone...idk what happened to it..weird. Here it is. It can be googled. Its a triode electronics layout.


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## slagg

Mr. Nick said:


> I know the original did not have the master volume. Im just getting lots of pointers towards different circuits, some people saying it has the master volume, some people saying it does not. I just like my information straightforward and I'm getting lost in all the numbers and models.


Funny thing is if you really look at these amp models they are more or less the same with tweaks here and there to get were you need to be.


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## danfrank

Gunner64 said:


> Ha. Its gone..the post is even gone...idk what happened to it..weird. Here it is. It can be googled. Its a triode electronics layout.
> View attachment 42910



Thanks


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## Gunner64

Ahh its post 5 in the ops "cant decide which amp to build " thread. That explains it .lol..no one listen to me..I 'm stoopid..


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## Gunner64

Mr. Nick said:


> Since I love to ever engineer things, and the amp chassis are too big for my enclosure(an old radio cabinet), I m going to to what I do best and Over-engineer it and design a two level turret board that Ill CNC myself so the components can take up less space, and measure all the components with a caliper and make a 2d layout on my CAD program.


Keep in mind that you may have hum and noise issues from changing the basic configuration. Low volt and high volt wires crossing..heater wiring..input wiring..I'm not saying it can't be done but wire routing and lead dress are in my experience a huge influence on unwanted noise issues.


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## Gunner64




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## Mr. Nick

JA, Ich weiß. I would basically be splitting the board in half hamburger style, then stacking them on top of each other, with some sort of spacer giving them adequate distance apart. To avoid unwanted hum I would use some very good shielded wire, and place any components that give off a magnetic field out of phase with each other. Before I do all this I will collaborate with my amp friend, and he can tell me that the best placement is, and where to put certain comments to avoid hum.


Another idea I had was to lay all the resistors on one section side by side, and do the same with the capacitors. Basically group everything together by type and arrange it by size and color, then wire everything accordingly. It would look fantastic for people with OCD and it would take a lot more wire, but it would look really neat and organized.


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## Gunner64

Are you sure your not f#cking with us? You want to build a plexi jmp master volume 2204 split turret board resistors on one side and capacitors on the other marshall clone in a old radio cabinet...ahhh ok..lol.  you really do try to make everything difficult don't you?


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## Mr. Nick

Gunner64 said:


> Are you sure your not f#cking with us? You want to build a plexi jmp master volume 2204 split turret board resistors on one side and capacitors on the other marshall clone in a old radio cabinet...ahhh ok..lol.  you really do try to make everything difficult don't you?


Nein, its the german in me. I always feel the need to over engineer everything to perfection


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## danfrank

Mr. Nick said:


> To avoid unwanted hum I would use some very good shielded wire, ...


The best way to avoid hum is to get the grounding scheme right, avoid ground loops and keep the filament wires as far away as possible from everything else. Early Marshalls were not so good at this, a lot of it had to do with the way they wired the filaments. Another No No is using the chassis for grounding. The chassis must be grounded at ONE point but never used as a grounding path for electrical currents.


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## Gunner64

Filiments..absolutely. And I used a ground buss back to the cans on my 800 build. Not the chassis. This thing is quieter than my actual 2204, which is actually pretty quiet.


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## danfrank

Yeah, with good grounding techniques, it's amazing how quiet a guitar amp can be considering the amount it amplifies the PU signal.


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## ampmadscientist

Gunner64 said:


> Keep in mind that you may have hum and noise issues from changing the basic configuration. Low volt and high volt wires crossing..heater wiring..input wiring..I'm not saying it can't be done but wire routing and lead dress are in my experience a huge influence on unwanted noise issues.


Layout makes a huge difference...


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## ampmadscientist

Mr. Nick said:


> Nein, its the german in me. I always feel the need to over engineer everything to perfection


In tube amps, that's OK.


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## jman271

ampmadscientist said:


> *It's almost useless to put a master volume on this Plexi.*
> 
> This is because the plexi preamp is CLEAN. The plexi depends on cranking the amp to full volume - this overdrives the output tubes.
> 
> Plexi Distortion = output tube distortion. There is very little preamp distortion in this design.
> 
> BUT
> if you modify the preamp...to 2203....then a Master volume will work pretty good.
> 
> PS: use PPIMV...Trainwreck design. Use a pot for the PPIMV which has a *high voltage insulation rating*.


Hello, I have a 50 watt plexi clone, (Ceriatone 1987x), with a ppimv with some res on it, are you saying if I go with a higher insulating rated pot I can lose the res? Will this make it sound better? thx!


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## Copperheadroads

Mr. Nick said:


> Nein, its the german in me. I always feel the need to over engineer everything to perfection


Build it the tried & true way .


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## bill bokey

jman271 said:


> Hello, I have a 50 watt plexi clone, (Ceriatone 1987x), with a ppimv with some res on it, are you saying if I go with a higher insulating rated pot I can lose the res? Will this make it sound better? thx!


No, the resistors on your PPIMV are there for two reasons. The 2M2 are safety resistors in case of a wiper failure (you can remove them if you switch to Frondelli PPIMV) and the 470K bring the pot's value down to around 250k (from 500K I presume)


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## jman271

Thx very much, so if what Im reading is correct, I can remove this current set up and install a dual 500k,(250k per), and wire it accordingly etc..I guess my question is since I have a ppimv if I go with the fond set up will the amp "sound" better? I hear of the PEC dual pot is a good one, $30? Insulated, etc, any thoughts? thx- 

this is a good diagram I found...https://robrobinette.com/5F6A_Modifications.htm#Frondelli_Master_Volume


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## ampmadscientist

jman271 said:


> Thx very much, so if what Im reading is correct, I can remove this current set up and install a dual 500k,(250k per), and wire it accordingly etc..I guess my question is since I have a ppimv if I go with the fond set up will the amp "sound" better? I hear of the PEC dual pot is a good one, $30? Insulated, etc, any thoughts? thx-
> 
> this is a good diagram I found...https://robrobinette.com/5F6A_Modifications.htm#Frondelli_Master_Volume



Some pots are rated, like 30 VDC max. My point being

People run the bias voltage thru the pot, but I think this may be a bad idea unless you know the voltage rating of the pot.
(voltage insulation rating)
Because if insulation is inadequate, bias voltage may short to the chassis.
Regardless of safety resistors, yes.

The safety resistors do not protect the amp from insulation failure of this pot.

That is why we use a "rated" pot, with a known insulation rating. (see: the manufacturer spec sheet)

Because of this insulation factor (bias voltage is connected to pot) I will usually use a pot rated 350VDC or higher.
I will verify the rating with the manufacturer spec sheet.
Because it is not worth (bias failure) blowing up some tube amp because of a cheap / un-rated pot, that's why.


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## jman271

this now makes complete sense, thx very much for your posting, I am learning so much by being on here!


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## Copperheadroads

Mr. Nick said:


> Nein, its the german in me. I always feel the need to over engineer everything to perfection


 You mean Over engineer ,over think . That's a slap in the face to all amp builders here as you are suggesting you can do it better than the tried & true way .


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## ampmadscientist

Mr. Nick said:


> The only thing is it looks like it does not have the 4 inputs, it only has 2. Why is this?



The amp w/ the 2 inputs has twice as much preamp gain.
That's the one you want.

They took the 2 input stages of a Plexi, and combined them in series to boost the gain.
Then it works good with a master volume at lower loudness.
(you don't need any distortion pedals)

The thing about Plexi is: it's too loud to be practical....but you don't need to buy one to find out.
That's why the 2204 was created - to give you more grind, without the extreme loudness.

This is why the 2204 became one of the most favorite sought-after Marshall amps.


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## ampmadscientist

neikeel said:


> The Marshall 50w Plexi evolved over the years. The JMP 50 started in 1967 with a shared v1 cathode and low filtering and high NFB. There was a PA model 1985, Bass model 1986 and a lead model 1987.
> slagg posted the schem of Model 1989 from year 1970 which is a similar schem to the early models and that is the Organ.
> Most people who come from 'outside' think of a JMP50 as the Model 1987 Lead amp from 1969.
> This is split cathode, as rectifier and tighter filtering and this is very similar to the later master volume 2204 amps post 1977 MY. Which simply inserts a 1M pot between the output of the treble pot and the feed into the PI. And cascades the two halves of V1 Hence my version which incorporates the two versions in switchable form.
> basic schem for a 1987 is above.



Except in this schematic-
the screen resistors need to be added.


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## Thomas Steven

i heared the 50 watts plexi is more gainy compare to the 100 watts version


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## neikeel

Thomas Steven said:


> i heared the 50 watts plexi is more gainy compare to the 100 watts version



Not really, if you go for the same spec (like for like preamp) pretty much the same, they are slightly more compressed and tighter (higher filtering in the later models).


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## Thomas Steven

there's quite a few peoples who tells me that, dont know why? it doesnt make sens right?! they both are plexis , maybe there is a model for the big one and another for the small one (50 watts is not that small lol)


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## neikeel

Thomas Steven said:


> there's quite a few peoples who tells me that, dont know why? it doesnt make sens right?! they both are plexis , maybe there is a model for the big one and another for the small one (50 watts is not that small lol)



Time for you to do some research and reading sir!


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