An old pic, the filters were all renewed long ago. The chassis is dated 4-14-70 for "Test" The front panel is metal (not perspex/plexi)
I got a 50W 1986 head many years ago, and it became the OEI "mule" as well as one of my favorite recording amps. It's still home to one of our original 50W production OTs.
[ You can tell the OTs from the very first batch of 50pcs because I had them wax dipped rather than varnish. I only stopped because it was a ton of work to clean them off with a heat gun before I could sell them! ]
The 1986 is the rhythm or bass head, the the 1987 is the 50W "lead" head. And the 1959 of course is the 100W head because that all makes perfect sense. (?)
The 1987 has a very bright channel which is a bit too much for me. But the 1986 is a bit too dark for me. The 1959 is more legendary than either of them. What to do?
I played around with a bunch of minor changes over time and I found a spot I kind of like so I thought I would share where I wound up. The reality is all of them, 50W bass, 50W lead, 100W... whatever, are all very close. So I could mix and match a bit, and it's all very reversible. No holes were drilled, that is for sure. Changing out coupling caps is a nice way to eq your amp, to help tune it to where you need it. And it's easy to reverse when you change your mind.
The amp kept it's stock power supply, bias and basic circuit topology. No cascading gain silliness. You can still do that great very slightly dirty clean tone. I'd hate to lose that.
Output Section:
I added 1K 5W screen resistors; melting screens kill more Marshalls than probably anything else.* (see below - "EL34 screens")
I added 1 ohm 1% bias resistors to the EL34 cathodes to measure bias.
I changed out the 220k grid resistors to 100k per the Ken Fischer suggestion.
I changed out the .1uF coupling caps from the phase inverter to .02uF to tighten things up a little. Another Ken F suggestion.
You can see the 1 Ohm 1% 3W wire-wounds on the cathodes for checking bias, the 1K 5W screens and the new 5.6k grid resistors here. It's not a pretty amp, it's been through a lot! But it has run reliably for me for decades at this point. Those are strings of 1N4007, like a Fender would have, since the original bridge rectifier failed. I used the single screw mount to hold down a terminal strip, and then I covered them in shrink tubing.
Pre-Amp:
The tone stack is from a 100W SLP. More mids focused.
The 2nd stage already had some "tropical fish" caps on the the cathode; I left those alone.
The mixer is stock and there is no bright cap on the volume. (also stock for a 1986)
The first stage got the separated cathode circuits from the 100W, with the bright channel getting a .68uF bypass cap.
You can see most of the changes here. A dual master volume was removed, unfortunately it was covered in silicone and very hard to clean up well. I added a 250pF to the existing 250pF in the tone stack to get to 500pF, and made an odd slope resistor value out of a pair of resistors. (I think it's 39k but not 100%) The tropical fish caps were already there in the cathode follower. I am assuming they are .68uF together but I didn't measure them. The 100k output tube grid resistors and .02uF caps were from an old article by Ken Fisher about getting more reliability. It does seem to focus the amp a bit more and it has a bit more dirt. Maybe someday I will completely re-build this to be pretty, and fix that dent, but I'm having too much fun playing it right now. :)
Old amp issues; this poor thing had been blown up before. Some past tech had destroyed the brass mounting screws, so I wound up scraping off any traces of arcing. It's ugly but works just fine; I have beaten on this thing for over 25 years like this. It's also hard to find this size socket. There are 3 or 4 different diameter octal sockets which is pretty annoying when you need to fix something properly.
What I do with it:
To put anyone's mods in perspective, it's good to have some idea about what they are going for. I like the classic rock tones these amps get by themselves, I generally don't push it with pedals so that was never a consideration. I could (and have) but it was never a design goal. I do jumper the channels a lot though, I like mixing and matching for gain and EQ. One channel is still darker, maybe better for a Tele or other bright single coil, and the bright channel goes the other way and really livens up a humbucker. The best overdriving guitar I probably have right now is a '76 SG with a pair of Seth Lovers in it. My 2003 Les Paul R9 with a pair of Seymour Duncan Antiquities is a very close 2nd. I definitely like the older "PAF" style pickups that are less midrangy and not as hot. I feel like the amp does a better job without so much signal on the front end. The guitars both have 50's wiring with VIP pots (highly recommended!) If you use overwound/hot pickups, they will be more midrangy and you may want to adjust the EQ on your amp, and so on. Make sure you play out with whatever mods you do so you can hear the changes in context. They may sound great at home and just not work on stage.
Net net here is where I wound up after playing and tweaking. YMMV, but I have left it alone for a few years because overall I am very happy with where it landed.
EL34 Screens (referenced in 'Output Section' above)
Why do EL34's get a bad rap? In a nutshell because the screen grid (aka G2) has a relatively constant voltage on it, but the Plate (which it is in front of) does not. When the plate swings below the screen grid, the grid will attract more of the current flow through the tube. Now the grid is simply a bare wire that is swaged in place around a pair of posts. There is nothing really holding it in place other than its own dimensional stability. It can (& does!) get red hot, and when it does it physically sags and it can short to other parts of the tube. You can watch this happen in your own amp if you like - you can see the screen grid through the openings in the plate. Have a friend play hard with the amp up loud and you'll see the grid turn red. Long story short, running these amps on high voltage with no screen resistors is a bad idea. The original beam power tubes in the 5F6-A Bassman don't have this issue. They place their G2 wires directly in-line with their control grid wires and then use beam forming plates to keep the streams focused. This allows them to run higher currents. The screen resistor lowers the voltage on the screen grid based on the current flow (V= I * R again) and helps prevent too much heat on the grid.
Adjunct - the 4x12 cab that Trace Davis found for me.
The 4x12 is a big part of the sound (no, really? lol) of these things and since my head did not have a matching cab I went looking. Fellow amp dog Trace Davis found this one in upstate NY, I think it's a 1972 but based on its various features it could be '72 or '73. It has a birch back, basketweave grill, no corners, large central beam, black levant, plastic handles (72+) and is loaded with 75Hz Greenbacks. (Not pre-Rola as far as I can tell, they don't say, "Thames, Ditton Surrey" on them, but they also don't say Rola... but the logo also looks more recent. They were probably replaced at some point, probably in he 90's). Anyhow, the two of them together are an experience. I just wish I could haul this out for more gigs! Everyone should have the experience of standing in front of a stack at least once.
The big central support that goes from the front to the rear. I was testing it open back here which is why the cable & jack are velcroed in place.
Heaven.
References:
1987 Schematic: https://schematicheaven.net/marshallamps/jmp_lead_50w_1987.pdf
1959 Schematic: https://www.thetubestore.com/lib/thetubestore/schematics/Marshall/Marshall-JMP-Superlead-100W-1959-Schematic.pdf
The Trainwreck Pages (thanks to Rob Robinette for hosting this searchable version!)
See P#31 for Marshall reliability mods
Aiken Amps 4x12 cab frequency sweep: https://www.aikenamps.com/index.php/frequency-response-of-a-marshall-4x12-cabinet
Disassembling an EL34: https://youtu.be/AqSBzjddJ4c G2 is the outermost grid when he removes the internals. You'll see it does not line up at all with the control grid. (not good or bad, but it has unintended consequences in Guitar amps)
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