2003 Marshall 1987X

Overview

I have wanted to experience the “NMV” Marshall magic for a while, but always known I would have to ruin a part of that experience by installing a master volume mod - heresy I know, but it is almost required to realistically use the amp in a home studio or even for gigging musicians who might hope to use an amp like this at a small bar gig, or even larger shows that use a PA system.

So when this amp appeared used online for a great price, despite the added and unknown knob replacing one of the normal channel input jacks, I jumped on the opportunity. There are many variants of master volumes used on these amps, and I figured since this one didn’t replace the presence knob or add any new holes to the front panel, I could sort it out when it arrived.

Upon initial inspection, and with help from the Marshall forums, the added potentiometer was connected to the board between R23 and R24 - a “cross line” volume control. I found it incredibly sensitive and not all that effective - making the amp sound intensely distorted at low settings and then deafeningly loud with very little travel between those two settings. There was also a faint feedback squeal present, perhaps this would’ve gone away if I turned the control up louder but I’d already decided this volume wouldn’t be usable to me in my home and promptly removed it. This volume is visible in one of the pictures below before I took it out.

Additionally, the previous modder disconnected the normal channel completely, so even input 2, which still had a jack in place, did not work, and neither did the popular technique of jumping both channels - definitely not what I had in mind when I wanted this amp.

Since this amp has an effects loop, I started using a volume box pedal I have (essentially just a volume pot with input/output jacks) to reduce the volume. This worked extremely well, and after researching common master volume modifications online, I thought - why don’t I just install a pot just after the FX loop before it returns to the phase inverter?

I removed the previous mods and using some test cables, also tried connecting input 2’s jack back to pin 2 on the V1 tube to restore the normal channel - which worked great. The grid leak resistor was still in place, so I’d only need to add the input grid resistor back in place (factory would’ve been two 68k resistors in parallel, probably when the volume pot was installed they just cut these out when they removed the jack). I used a 47k resistor here because that’s what I had available to me - should be close enough to make me happy and prevent radio noise here.

After experimenting with the “one wire” mod, and really enjoying a bit of the lo-fi, over the top gain that produces, I decided I should combine my new master volume with this as well. While not perfect - certainly not properly EQ’d, with way too much bass passing the first drive stage, among other issues - I found it to be pretty entertaining to play and a pretty unique sound that none of my other amps could exactly replicate, it would be a nice card to have in the deck. I added a push-pull pot in place of the original mod’s pot, and configured it so when pulled, it engages the mod and cascades the gain from Normal channel’s V1A into V1B in series. This only takes affect when plugged in to Input 2, and engaged or not, Input 1 remains the bright channel only. I thought this sounded best, although I’m tempted to reverse it - V1B into V1A, so I can leave the guitar plugged into Input 1 instead, but I’m not sure how that would change the tone. Something to try next time I have the chassis out. It’s also worth noting that the stock 4700pf bright cap is still in place in this amp, and with the bass introduced with this mod I’d recommend leaving that stock - anything lower would likely be very dark sounding.

The master volume control is simple, and I’m surprised more people haven’t tried this method since it’s such a clean and reversible installation - of course, it would only work on modern reissues with the added FX loop so that may be part of it. The pot is wired in just after the FX loop’s wire back to the main circuit (the FX loop is just before the Presence control), and sends any cut signal to ground. This works great - just as well as having the volume in the loop, but right on the front of the amp.

So the real question is - does this take away from “plexi” experience? If I wanted a master volume, why wouldn’t I just buy a 2204 instead, which has factory master volume and cascaded gain stages? Honestly, I still think this amp is totally viable with these mods. Sure, the 2204 is an extremely similar circuit, and the gain stages in that amp are better voiced since it was designed that way to start, but that amp cannot do the the same tones the 1987x can do. I didn’t break any of the original functionality - the two channels can be run alone, or in parallel to get that famously tweakable sound with the channels jumped together with a patch cable - now with the added benefit of getting those tones at a lower volumes or an additional series gain mode that can’t be emulated by a 2204 or similar MV amp. Of course, power amp distortion is a big part of the classic plexi sound, but this is a great compromise especially for my home studio.