Marshall experts - can you check out this 2204

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dstroud

dstroud

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So I pulled the trigger on this 2204 from GC for $1799 - I knew it was a gamble, but this thing sounds fantastic!! Seems maybe slightly gainier than the 2203X I had. But not crazy gain by any means. The sn appears to be from 1987. Looks like Drake transformers, filter caps seem to have been replaced in 2020 and theres a note inside the amp that says “modded power rail to match vertical input specs - fall 2023”. Here’s some pics - you guys think this looks pretty good? I have no intentions of changing anything, just curious to know what I got lol there is a wierd little jumper on v1, not sure what that’s about but I put a closeup on it.

edit - so apparently the long cable is nothing more than a shielded input wire that replaces the non shielded pcb version. certainly doesn’t need to be that long though lol amp is a keeper for sure!
 

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Not really an expert but I would say, not bad except for that hodious looking mod.
 
Bias caps were replaced thats good. Replaced filter cans are F & T thats good.
Power tubes are TAD thats good. The sheilded wire "modded power rail" off V-1 not totally sure could be a type of "one wire mod" for gain.
If it sounds good / plays good play it like no tomorrow. I wouldn't expect any problems.
Cool amp.
C4 / C5 / C8 / C10 coupling caps may have been swapped out which is prolly a good thing i have some similar like yours in my
Friedman JMP 2203 Dave replaced.
 
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Alpha Stryke said:
Bias caps were replaced thats good. Replaced filter cans are F & T thats good.
Power tubes are TAD thats good. The sheilded wire "modded power rail" off V-1 not totally sure could be a type of "one wire mod" for gain.
If it sounds good / plays good play it like no tomorrow. I wouldn't expect any problems.
Cool amp.
Click to expand...
yup just gonna play it for sure!! but curious what that long wire is lol Got my Blankenships for gigging, this was just for fun in the studio
 
dstroud said:
yup just gonna play it for sure!! but curious what that long wire is lol Got my Blankenships for gigging, this was just for fun in the studio
Click to expand...
The mod sez "modded to vertical input spec". Sombody prolly juiced the gain a bit.
Ive seen alot worse.
*edited my first post.
 
edit - so apparently the long cable is nothing more than a shielded input wire that replaces the non shielded pcb version. certainly doesn’t need to be that long though lol amp is a keeper for sure!
 
The 2 thick black leads with the green shrink wrap at the ends are shielded cable.
The one that loops around the left end of the chassis takes the signal from the input jack through the 68k R3 to the grid of v1 first gain stage.
The shielded cable on the right feeds the grid of the second gain stage on v1 from the pcb.
It looks like they have connected the shields of both cables together with the thin black wire and grounded the shields at the ground of the pre amp filter cap.
Thats a good mod to reduce noise.
I can't tell exactly what they have done to the power supply rail, but it would likely involve playing with the droppers between stages and the voltages to the plates of the pre amp tubes.
If it sounds good, then they got the voltages right, and you are good to go.
Overall nice amp, and congrats on a good score!
Glad you like it.
 
fusedbrain said:
The 2 thick black leads with the green shrink wrap at the ends are shielded cable.
The one that loops around the left end of the chassis takes the signal from the input jack through the 68k R3 to the grid of v1 first gain stage.
The shielded cable on the right feeds the grid of the second gain stage on v1 from the pcb.
It looks like they have connected the shields of both cables together with the thin black wire and grounded the shields at the ground of the pre amp filter cap.
Thats a good mod to reduce noise.
I can't tell exactly what they have done to the power supply rail, but it would likely involve playing with the droppers between stages and the voltages to the plates of the pre amp tubes.
If it sounds good, then they got the voltages right, and you are good to go.
Overall nice amp, and congrats on a good score!
Glad you like it.
Click to expand...
hey thanks for the breakdown!! yup time to just play it now :)
 
Some burnt up insulation on some of the preamp socket connections, but far from the worst I've seen. That's mostly aesthetic in itself, but it does indicate a less skilled technician/amateur completed the work.
 
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Hey Darren the amp looks pretty good. I agree with what Fusedbrain has pointed out. Don't worry about the length of the shielded wire on the high input, the guy was just routing it far away from the circuit board for lower noise purposes, you may want to secure that cable with those stick on cable tie pads they sell at the hardware store or some hot glue to hold the cable in place, painters tape ain't gonna cut it I don't see anything really objectionable unless you want the higher B+ back then you would have to remove those added resistors that the tech soldered to the Preamp Filter cap to lower the B+ to V1A and V1B, kind of a shit way to do it but it works at least they aren't hanging like a flying lead where they could touch the chassis, they are secure. It looks like the B+ to the power tubes might still be stock which would be 450-467DCV as it doesn't look like the guy messed with the B+ node to the power tubes but I can't be 100% certain from the pics.

Now to address the Power Rail mod. My 1984 2204 has a B+ of around 466DCV @ 120ACV. My 1972 Marshall 50 watt SL has a B+ of 400 DCV so about 50 DCV lower. The lower B+ will have less headroom, more chew and distort more than the JCM800 with the higher B+ as it applies to the preamp B+ and the power amp B+ . That's why the JCM800's were tighter, more headroom, more grind less chew more crunch and seemed to distort a bit less than the JMP's from 1977-1980 with the JCM800 line taking over in 1981.

Those JCM800's B+ voltage went up overall about 50DCV from where the JMP 2204's and 2203's were. My 1979 100 watt 2203 had a B+ of 454 DCV and an 1981 JCM800 2203 could have 490-520DCV B+, some people have reported 510-520 DCV.

If you like how it sounds that's all that matters, just check the bias of the amp and maybe see where the B+ is at on the power tubes.

Here's a schematic you can print out that shows the stock voltages of a 1983 JCM800 2204 amp, yours should be close.

https://mhuss.com/MyJCM/JCM800_2204.pdf
1783888430537.png
 

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Hey Darren I had a look around and it does looks like the tech did lower the B+ to the power tubes as well from what I can see. Here are some stock pics of a 1987 2203 like yours that you can compare things to. You are just going to have to get a multimeter and see where the B+ voltages are. It looks like in 1987 Marshall only used two B+ rail dropping resistors and on that 1983 Schematic they used three 10K dropping resistors on the B+ rail and that's what my 1984 2204 had with the stock board from what I can remember and most schematics for 1977-1980 2204's show three 10K resistors being used.

So on your amp the tech used a 12K 2% tolerance( hence the 5 band color code) for the first dropping resistor and then added two more 10K dropping resistors on the V1A and V1B filter cap nodes so three dropping resistors same as they used in 1977-1980 2204's except for the first that is a 12K probably to tweak the voltages lower to the JMP values. While your preamp B+ rail has JMP values I still think your main power tube B+ may be in the 460-470 range you'll just have to see what it reads when you check the bias of the amp.

https://www.kentonelectrix.com/1987-marshall-jcm-800-2203

Here are the stock pics.
1783892203289.png

1783892234024.png

1783892266313.png
 
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