Modifying/rebuilding a Marshall 1987x

[alfi27]

Hi fellas,

This is my first post in here, and I might as well come clean straight away: I'm a noob when it comes to amp building and modding, but I am very eager to learn. So I thought that my already modded Marshall 1987x, which is a really good but not great amp, could be a just as nice starting point as any. What I'm initially trying to achieve is simply a 1987 to 2204 rebuild, which is hardly a rebuild considering how similar the circuits are. However, I am not even close to capable of reading those complicated circuit diagrams that just show how the signal travels, and I don't know how to translate PTP layouts to PCB. If anyone has, or could make some sort of step to step guide to convert it to a 2204 that would be absolutely great!

Part two, is that once that has been achieved I want to go a bit further. I'm thinking a knob for the NFB (shouldn't be too tricky), and also some sort of switchable (not necessarily footswitchable) gain boost in the realms of a certain J. Arredondo. Although I'm sure it would be cool to try a permanent Jose mod, I do love the stock 800 tone and I don't want to do this because I think a stock 800 is lacking in gain or anything really, it's more to have something similar to a switchable built in boost or overdrive pedal for more modern and face melting lead tones. I think Ceriatone's Chupacabra and Yeti are good examples of what I'm thinking, I haven't actually tried any of them but I love the idea of the Era-switch.

Looking forward to hearing from you guys, cheers!

[pompeiisneaks]

You mentioned something about a PTP layout to PCB? Most marshalls weren't PTP, they were turret or eyelet board, not sure which off the top of my head, but basically PTP means you directly connect components together or maybe use a few terminal strips to make sure components have stability, but there's no board involved. Also making any PCB takes a lot of skill, time, and then ordering them is pretty pricey unless you're doing them in bulk. It may cost upwards of hundreds of dollars for a one off PCB. (setup costs, etc) unless you're going to etch your own.

As for converting, there's likely someone out there that has a 2204 layout already there for you, google around for one, and you'll see how to build it. You'll still need a turret board or eyelet board done, you can do them yourself or have someone like Hoffman Amps build you one pretty quickly, you just need to take the time to repeat the layout on his app on his site www.hoffmanamps.com

I've used them, and they're great.

This sounds like a pretty ambitious first start, so you may want to just try a few simpler mods to your amp first, to get your feet wet, but it's up to you.

Welcome to the forum, I'd help more but I'm not well versed in marshalls, been more of a Vox, Fender type guy myself.

~Phil

[alfi27]

Thanks man! Even though I didn't write it, I did consider to convert it to PTP, but that's unnecessary for now. What I meant by that was that I struggled to understamd how PTP layout diagrams translate to PCB, not quite sure how to explain this in other words if that's unclear. However, yesterday I had a little breakthrough and I managed to understand at least a lot of it, as the wires from the inputs, tube sockets and pots are obviously the same...

That's a reasonable concern, I did market myself as a complete novice which is not entirely true. I've probably wired up or modified close to 30 guitars, one pedal and one Orange Tiny Terror amp (with an included step to step guide). I'm not particularly worried that I will have issues with the execution part of it, the only thing I can imagine is connecting a wire to the wrong place or something like that.

I ordered the parts last night so it will be interesting... If everything goes according to plan we can focus on part two of the project from now on! Just something I considered when I was thinking about the PTP conversion, to add the diode circuit from a Silver Jubilee (not rhythm clip) on a switch. Obviously that would be really gainy compared to an actual Jubilee, that some people who removed the diode circuit found to be an almost completely clean amp, which a JCM800 certainly is not... I know next to nothing about diode circuits, but would it be possible to put a level/gain knob before/after the diodes, or would it be more beneficial to set up some sort of mix knob and running the diode circuit in parallel?

[pompeiisneaks]

Hopefully someone else that's familiar with the marshalls and diode clipping can chime in, sadly I'm not. Good luck on the build, and show us some pictures of the progress, we all love amp build progress reports!

Glad you could make sense of that. One of the most invaluable skills in building amps is being able to convert a schematic to a layout and vice versa. Takes time and patience and it's not super easy to teach.

~Phil

[strelok]

I'd suggest doing some reading to get a better understanding of what's going on. This is a nice place to start:

http://valvewizard.co.uk/Common_Gain_Stage.pdf

The 1987 and 2204 circuits aren't much different from each other, and can be converted without needing a new board or anything. Just a few resistors and maybe a cap or two. I was never a huge fan of the diode clippers. If you want even more gain a boost pedal is usually better or a post phase inverter master can help a lot too. You can also bypass all the un-bypassed stages for additional gain, that might be the easiest way to wet your feet. But do your reading first, the page I linked will make what I said and the differences between the to circuits easier to digest. And always make sure the amp is uplugged and the filter caps are discharged before making any changes!!!

[alfi27]

I'd suggest doing some reading to get a better understanding of what's going on. This is a nice place to start:

http://valvewizard.co.uk/Common_Gain_Stage.pdf

The 1987 and 2204 circuits aren't much different from each other, and can be converted without needing a new board or anything. Just a few resistors and maybe a cap or two. I was never a huge fan of the diode clippers. If you want even more gain a boost pedal is usually better or a post phase inverter master can help a lot too. You can also bypass all the un-bypassed stages for additional gain, that might be the easiest way to wet your feet. But do your reading first, the page I linked will make what I said and the differences between the to circuits easier to digest. And always make sure the amp is uplugged and the filter caps are discharged before making any changes!!!

Thank you so much! Yeah that is what I figured last night after staying up until 3AM doing research, ordered the new components and they ended up at less than £25; would have been even less if they hadn't removed two of the stock caps and done some strange stuff when installing the PPIMV. Pure luck that I even managed to discover it...

Like I said I really love a stock JCM800 and it has more than enough gain when I use it with my Torpedo Live, but as I'm in the position of hating pedals and wanting to learn how to mod amps I figured why not at least look into some sort of built in pedal-esque switchable circuit, as all of the legendary pedals are diode and/or transistor based after all, right? Putting a circuit like that before V1 should have the same effect, although not quite as tweakable unless I make room for some extra knobs.

[alfi27]

So I got the parts, and even if I don't really have the time I couldn't wait until Friday... Everything is pretty straight forward, there is just one thing I don't get. As Marshall don't like to solder caps and resistors (except for bright caps) on the actual pots, I get a little confused although I think I got it right.

If you look at the wire connections that say BN6 and VL2, they're swapped on the 1987x and 2203x. I'm assuming that the additional resistor on the 2203x is the 4.7k to the 25k presence pot, and as I got the wrong pot (ordered 25k and got 5k) and according to several people it's only affecting the taper a bit, not the maximum and minimum positions. So I shouldn't worry, right? I also see that all of Ceriatone's 'hot rodded plexi' amps use 5k presence pots, so maybe it's even a better choice?

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[alfi27]

A little update: This project is by far the biggest pain in the ass I've experienced in a long time, the guy who modded this amp already is wanted, dead or alive... Probably have to desolder every single wire that goes to the PCB, the green and orange wire have been moved as you can see on the picture of my amp (picture in my previous post). Soldering them back in place is like soldering blindly and I've burnt a few wires (not all the way through the plastic/insulation luckily) from tilting the soldering iron a few mm too far one way or another, and it also seems that the holes are slightly too small for the wires?! Which are stock as well...

If the amp was stock when I started I would have been finished yesterday probably... If something is wrong with the circuit when I'm finished (IF I manage to finish it that is) I'm definitely just gonna order up a PTP turret board and get it overwith, much easier to build from scratch than cleaning up other people's mess.

[astro8585]

Haven't had time to digest all your individual issues but just by memory it's all in the first 2 or 3 preamp stages. I have a 79 pre jcm 800 2204 50w which I changed, years ago, to plexi type specs-apparently the opposite of what you're doing. The main thing is that the old circuit is the inputs in parallel but the MV/jcm800 has the first two stages in series or cascaded as often termed. Once you're into the first 2-4 stages(or a point within the first two preamp tubes) the rest of the amp is identical. I eventually pulled my pcb and redid it ptp which got rid of some weird background artifacts that were going on. You could probably find a prewired jcm800 ptp board to swap for your pcb if you wanted to go that route. It should be relatively simple to do one way or the other.