1977 Marshall JMP 2204 Mods & Schematic Questions

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1977 Marshall JMP 2204 Mods & Schematic Questions

Postby WS65711 on Sat Apr 17, 2010 5:21 pm

I have a 1977 JMP Model 2204 MV that I just bought (ebay) about 10 days ago. I took the chassis out of the cabinet for the first time yesterday and discovered that apparently at least one or more modifications has been made to this amp at some time in the past. Everything on the front and rear panel of the amp, and all of the components on the topside of the chassis appear original from what I can tell, with the exception of the "Treble" pot which is obviously a replacement.

I am hoping that someone here can tell me from the picture what this mod is (if it has a name) and what it is supposed to accomplish. Then I can determine if I want to keep the amp "as is" or return it to orignal specs (we do like the current sound, but would like to know what the mod(s) are there for). I am also attaching a link to the full resolution picture (too much resolution to place into the post), and other links to a pictures of an amp the same model and year as mine that I found on the internet. Any help or guidance anyone can provide will be greatly appreciated. :)

Image

Link to Full size image: http://av-pix.com/2010/Marshall_JMP/JMP ... llsize.JPG

Link 1 - picture of another amp: http://www.artesguitars.de/gear/picture ... _p2ngr.jpg

Link 2 - picture of another amp: http://www.artesguitars.de/gear/picture ... 7_p2gr.jpg
Last edited by WS65711 on Mon Apr 26, 2010 3:33 pm, edited 1 time in total.
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Re: 1977 Marshall JMP 2204 Modifications ?

Postby WS65711 on Wed Apr 21, 2010 1:34 am

demonufo wrote:Image
The amp pictured (gigant0r's amp) is a 1987 non-master volume amp. Note the four inputs.
A 2204 has only two inputs. However, if your 1977 2204 has that cathode bypass cap in the same position, chances are your 2204 is the early (non-cascaded) version that came out in late '75 and was changed to the much more popular cascaded circuit (like the 2203) sometime in early 1977. This early version isn't particularly great sounding by any stretch of the imagination (mine was just plain crap, until I modded it to late 2204 specs)

The easiest way to tell which circuit you have is by checking the resistor on v2 tube socket. If it is 330Kohms (instead of 100Kohms) then chances are you've got the early version. Also the input sockets will have a 1Mohm resistor on each if it is the early circuit. This would be only on the hi input on the later cascaded circuit.

It's not that difficult to change the circuit over to the later version. Steve Miller (SDM) over on the metro forum posted a how to somewhere.
There are a few components changes and a few wires need to be moved, but nothing major.

The correct schematic for your amp, is the Unicord 2204 schematic, dated 11/11/76. You may get a couple of variances in resistors on the power/biasing stage depending on the power tubes you have fitted, as that schematic shows 6550's, but with 220K bias splitters (standard for EL34's). 150K or less would be better for 6550's.


demonufo..... Since I posted here a few days ago I've learned a little more about the amp that I bought (from the ebay seller). He told me that when he bought it, he took it to a technician that told him that there had been modifications made to the preamp section of the amp. So he (the seller) paid the tech to "put it back to factory specs". Below is the ebay picture of my amp (which was taken BEFORE the tech "put it back to factory specs"). Notice that the capacitor, and those two resistors that form a "V" on the board (see pic in my original post) were not present in the ebay picture. I'm now assuming that if the seller's story is true, the tech mounted those resistors like that to avoid the time/effort to remove the board to solder them properly to the board.
Image
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Re: 1977 Marshall JMP 2204 Modifications ?

Postby WS65711 on Wed Apr 21, 2010 12:17 pm

demonufo.......... My intention is to put the amp back to original factory condition, but with all components mounted properly on the board as they should be (not hanging off the top of the board). :roll:

I'm not 100% clear from your response.... if my amp originally didn't have the capacitor (as evidenced by the ebay picture) that would make it cascaded, or non-cascaded version?

Thanks for the confirmation about the proper schematic. That is the one I had found on drtube.com and I was pretty sure that it was the right one for my amp. Again to be clear.... does this 11/11/76 schematic depict the cascaded or the non-cascaded version of the 2204?

Is the link below the SDM early/late conversion you're talking about?
http://home.comcast.net/~mamp17/Unsorted%20Pics.html
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Re: 1977 Marshall JMP 2204 Modifications ?

Postby WS65711 on Wed Apr 21, 2010 5:32 pm

demonufo.......... The amp is currently all back together in the cabinet (so that my son can use it). He's the guitar player, not me. But I dug through the pictures that I took on Friday when I had the chassis out. This one (attached) looks like the V2 resistor is in fact a 330k value. It also appears like it might be a newer resistor (not carbon composition)? I'm almost beginning to wonder if the amp had previously been modified to the "newer" version (or was originally the newer version) and the service technician downgraded it to the old style? From what I can tell, the 11/11/76 Schematic shows the "older" version (it shows the resistor as 330k).

Image

Link to Full size image: http://av-pix.com/2010/Marshall_JMP/V2_ ... llsize.JPG
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Re: 1977 Marshall JMP 2204 Modifications ?

Postby demonufo on Wed Apr 21, 2010 9:16 pm

Okay, let's go through this. Those red resistors on the board aren't carbon composite's but Piher Carbon Films. CC's have sharper ends, more barrel shaped. I think your tech got hold of the schematic for the early 2204 and reversed the circuit from the later style which it may have originally been. Bit of a major cockup in my opinion. Those orange drops aren't exactly the most Marhally sounding caps either.
The resistor on V2 is a modern carbon film. The resistor on the far left of the board appears to be a CC. That's not original either.

I think your amp was originally the cascaded, later version of the 2204 which didn't have the cathode bypass cap in that position.

The 11/11/76 schematic is the non-cascaded early version.

Yep, you found the right how to for that amp, but due to the bizarre nature of the mods it might be a little more difficult to follow.

Tell the seller, his amp tech is a numpty! :lol:
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Re: 1977 Marshall JMP 2204 Modifications ?

Postby WS65711 on Wed Apr 21, 2010 9:52 pm

demonufo.......... Do you know if a schematic for the JMP 50watt 2204 with the newer cascade arrangement exists anywhere on the internet? If there is none, I will do my best to make one as part of this exercise. :wink:

It looks like my son (17 years old) is going to have to go back to playing on the '82 JCM800 4210 Combo for a few weeks . . . :cry:
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Re: 1977 Marshall JMP 2204 Modifications ?

Postby Martin L on Wed Apr 21, 2010 11:35 pm

That non-cascaded 2204 schematic from 1976 does cause a fair bit of confusion. The post-76 cascaded 2204 preamp is pretty much identical to the 100W 2203 here >http://www.drtube.com/schematics/marshall/2203u.gif<, at least up to the output of the phase splitter. I say pretty much identical, because there are minor differences from year to year, e.g. value of the presence cap and pot. The big differences between 50W and 100W are confined to the output stage and power supply.

Hope this helps you...
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Re: 1977 Marshall JMP 2204 Modifications ?

Postby demonufo on Thu Apr 22, 2010 6:04 am

That's about right. There are plenty other schematics around for the 2204 though. Any other schematic should be correct, just that Unicord one that is out. You'll find that the JCM800 ones are correct. Same with the 2203. The main thing that changed between the two periods is component types, not values.
On later JCM800's you might find a capacitor added onto v1 to cure oscillations, but I don't see those on the schematics.
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Re: 1977 Marshall JMP 2204 Mods & Schematic Questions

Postby WS65711 on Sat Apr 24, 2010 8:12 pm

demonufo & Martin L -

As promised (or as threatened :lol:) I've completed a preliminary (For Reference Only) version of the redraw of the Unicord 11/11/76 Schematic for the JMP 2204 Master Volume. I have drawn thousands of electrical schematics in my lifetime (it's part of my job), but only a small percentage have been "electronic" schematics like this one. Almost all have been "electrical" I/O type diagrams for Automated Control Systems utilizing PLC's and relays mostly. I had never produced a schematic that utilized Vacuum Tubes (Valves) as best as I can recall.

Anyway, I plan to use this drawing (after review and constructive criticism from others on this forum) to produce a modified "new" version schematic that will be correct for the "Later" version 1977 cascade configuration. If anyone has a sharp eye and can point out any error(s) in what I've done it would be appreciated. Also, I plan to add the pin numbers for the tubes on the schematic. If anyone happens to have them marked down somewhere and can provide me with that information it will save me some time . . . :sleepy:

http://av-pix.com/2010/Marshall_JMP/Pre ... MP2204.pdf
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Re: 1977 Marshall JMP 2204 Mods & Schematic Questions

Postby WS65711 on Mon Apr 26, 2010 3:41 pm

This morning (after having spent the time to redraw the 11/11/76 Unicord schematic) I stumbled across the 2/28/78 Unicord schematic in the attached link. Can anyone tell me if this 2/28/78 schematic is the proper schematic for the "Later" version 1977 50W Master Volume amp? It would seem that a schematic with this date would be too early to be a JCM800 version schematic, wouldn't it?

http://www.seattle-attorney.com/guitar/ ... ematic.pdf

In the forum where I found this schematic, the poster claimed that he bought a new 2204 in 1981, which had been produced in 1980. He said that this 2/28/78 schematic was a part of the Marshall documentation that he received with the amp.

http://groups.google.com/group/alt.guit ... b69b8fac2c
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Re: 1977 Marshall JMP 2204 Mods & Schematic Questions

Postby LD50 on Mon Apr 26, 2010 6:00 pm

This is correct as the better sounding amp (IMO) :wink:
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Re: 1977 Marshall JMP 2204 Mods & Schematic Questions

Postby demonufo on Mon Apr 26, 2010 6:59 pm

Interesting, I've never seen that particular dated schematic before, but yes that is the correct circuit, going by the inputs.

Note that both these schematics are for 6550 operation though, going by the 150K bias splitters on V4 and V5. These would normally be 220K for EL34's.

The JCM800 schematic is exactly the same BTW, there were no circuit changes between the "proper" (if you'll allow me to call it that! :wink: ) JMP 2204 circuit and the JCM800 version. The only addition you see cropping up occasionally on the JCM800 version is a 100pF cap on V1 to prevent oscillation. Something which good lead dress easily avoids. Shielded cable on the input stage avoids it better still.
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Re: 1977 Marshall JMP 2204 Mods & Schematic Questions

Postby WS65711 on Mon Apr 26, 2010 7:33 pm

Thanks guys. Hopefully I did something good (useful to this forum :cheers: ) by unearthing that 2/28/78 schematic. I do know that the guy (Jim) who had originally posted it actually scanned it to pdf himself. And it's hosted on his website. I can't find anywhere else online that this schematic exists, and believe me I've been chasing down every lead on these JMP's for about the past 2 months. I've emailed Jim to find out it he has any other useful information he can share . . .

I am aware of the difference in the resistors for the EL34's as opposed to 6550's. My amp originally had 6550's, but had been converted to EL34's sometime in the past. I need to double check that the proper resistors are in place. I kind of wish that I would have stumbled onto this "new" schematic before I spent time redrawing the 11/11/76 version, but oh well. Maybe I'll still take time to complete the "later" version as well someday . . .
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Re: 1977 Marshall JMP 2204 Mods & Schematic Questions

Postby WS65711 on Fri Jun 25, 2010 8:55 pm

We started taking this amp apart today to change out the large electrolytics and prepare to perform the early-to-late 1977 conversion. I also want to change out the previously replaced Treble pot which has a "D" shaft and is apparently the incorrect value as well. The markings on the existing pots are as follows:

Presence: 5k LN AB 7637 CTS
Bass: 1M LG AB CTS 7716
Mid: 25k LN AB CTS 7718
Treble: (Replaced) 220 LIN SPEC 72A (Schematic calls for 250k)
Master: 1M LG AB CTS 7716
PreAmp: 1M LG AB CTS 7716

All appear original except for the treble pot. Should the proper replacement be linear taper or audio taper? The MetroAmp site indicates audio taper, but I just want to be sure.
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Re: 1977 Marshall JMP 2204 Mods & Schematic Questions

Postby demonufo on Fri Jun 25, 2010 11:49 pm

Treble mid and presence should all be linear. 220K is a pretty reasonable approximation of 250K as well IMO, but if it looks out of place, and you can get better, then get 'er out of there.
George has a tendency to use Audio tapers because on the amps he measured, the pots were closer to log taper than linear.
Nothing wrong with that theory, since he is trying to recreate the sounds of vintage plexi's, and he is quite right to copy to that degree, but the originals should be linear in these three positions.
Choose whichever you feel comfortable with.
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