Seen the price of a vintage Marshall Plexi lately? Your three grand won’t guarantee a virgin circuit or great tone, but if real Plexi tone is what you’re all about, you can get there for a lot less, guaranteed. When TQR Advisory board member and Sheryl Crow guitarist Peter Stroud decided to make a Plexi from his late ’70s fawn 100W head, we enlisted the capable assistance of our main toneman Don Butler and the team at Mercury Magnetics, home of
the ToneClone line of authentic vintage-style replacement transformers. Mercury has blueprinted and reproduced nearly every vintage transformer made for guitar amplifiers, and they can rewind any blown transformer to exact, original specs and sound. So settle back and enjoy the following discussion and review of Peter Stroud’s successful conversion of a common (they built 1,000s) Marshall 100W into a stellar 100W Plexi.
TQR: What is the origin and general history of this Marshall amp?
Peter: I bought this amp about 10 years ago with the matching straight-front 4×12, 100 watt cab from a dealer down in Texas. It’s a ’77 Super Lead 100 watt 4-input. I was big into collecting the f awn-colored Marshalls at the time – a poor man’s custom color. Around the same time, I had learned how to work on my own amps from the standpoint of preventive maintenance and tone tweaking, so I converted the amp over to a cascade-style master volume circuit like a late ’70s head or JCM800. Eventually, I missed having the 4-input sound and put it back to its original wiring.
Don: This amp was a standard 100 watt, non-master Marshall from the late ’70s. Some mods had been done and then undone. A few coupling caps had been changed as well as a few resistors
Strengths: Semi-loud. Typical PCB Marshall from this era. Better quality glass board than what’s used now.
Shortcomings: Ceramic caps. This amp was voiced for brighter sound, less bottom, and quicker distortion onset that original ’60s versions didn’t employ. It still had a couple of the original filter cans. The amp sounded exactly the same no matter what guitar was plugged up to it – no tonal distinction between any instrument at all! Very over-emphasized, weird G string odd harmonics (most likely old filter cap related). I have to add from a personal note that this was the worst sounding Marshall I’d ever heard or played! In a word – no tone!
TQR: Describe the specific improvements that you wanted to make to the amp, both from a technical perspective, and sound-wise. What components were changed and why?
Peter: I don’t know why – from my monkeying around with it perhaps (laughs), but this Marshall just sounded like crap. Every time I plugged in I would end up turning it off – totally uninspiring. I replaced tubes and a cap here and there but resigned to the notion that it might be the transformers. I had replaced all of the power supply caps, but not the 50/50mf in the preamp circuit. It could have been that – who knows? But in any case, I would have thrown it on eBay had it not been for the color. Instead, I decided to make it a guinea pig. I’ve been messing around inside my amps again recently and have had a real desire for that 100 watt Plexi sound and the cleaner, fuller tone they deliver. This head was far from it! Having read the ToneQuest interview with Sergio about Mercury Magneticstransformers and poking around their website, I figured the fawn 100 watt would be the perfect amp for a conversion – change out the transformers for high voltage models reminiscent of the Plexis and wire it up exactly like a ’67. I was determined to demystify the whole Plexi thing once and for all for my own comfort. After seeing how Mercury Magnetics had “cloned” the power and output trannies of an old Plexi, I felt the missing link had now appeared. Then you stepped in and it mushroomed into a topic for ToneQuest, and a very deserved one indeed. Nothing better than to see if any Marshall can become a Plexi, especially since the latest prices on eBay for Plexis are more than I care to spend. I need amps that are bulletproof and easy to replace. I spoke with Sergioand Paul at Mercury and they eventually hooked me up with Don Butler. I was thrilled to have Don take over the job knowing his ear for tone and knowledge of Plexis. There were three CD’s I used as a tone reference: bootlegs of Cream in ’67 and Jeff Beck in ’68, and the Hendrix at Monterey Pop Festival album.
Don: Peter wanted the amp to sound like a mid-to-late ’60s Plexi 100 watter. More specifically – a ’67. I replaced all the little green (or were they grey?) ceramic caps with polyester tubular caps and replaced any resistors that needed to be changed with carbon comp types. Polyester caps were used in the ’60s in all Marshalls for the main tone and coupling caps. I know for the higher value caps in the pico farad range, Marshall generally used the ones from Lemco or RS that were a flat mica-looking cap covered in a baked on ceramic or wax dipped coating. I used silver micas for those in Peter’s amp. I like the smoothness and more transparent quality of silver mica. The original polyester coupling and tone caps that Marshall used in the ’60s were from a company in the Czech Republic called Iskra. I believe Phillips imported those to the U.K. Those are the ones everyone refers to as the mustard caps. I used Mercury’s Plexi 100 output tranny, 10H choke and 100W Plexi power tranny that provides the B+ with around 525V DC after rectification. In my humble opinion, Mercury makes the best sounding, most accurate and most reliable clone transformers on the planet.
I also used the FRED rectifiers in place of the little cheap-o IN4007 diodes for the rectifier. I like the way they smooth out the harshness of the treble frequencies and take out that mid range bump that’s common with a solid state rectifier in amps. They also seem to add more note clarity, harmonics, and detail to the sound and tone of an amp. I replaced the components, transformers and the remaining filter cans. I then converted the circuit to what was used in 1967 in terms of how it was wired and component values.
Really, it’s not much. On the first valve/tube you remove the wire coming from pin #8 and shorten it and solder it to pin #3. The cathodes on that tube were shared and went to an 820 W resistor with a 250mf @ 25v cap paralleled with it to earth/ground. The second valve/tube did not have a cathode bypass cap in those days – only the resistor. There was a .68 cap I removed from that spot. I also removed the .004 cap from the volume pot on channel #1. That cap makes the amp distort quicker, and usually at just “2” on the volume pot you’re out of clean headroom and well into distortion land. That’s not what Peter wanted or what the old amps were like. I also used two 0.1/ 600V caps coming out of the phase inverter vs. the .022/600V caps that Marshall started using in around ’69 to brighten up their amps.
On the later Plexis you will find a 500rf and 33K slope resistor on Lead models and a 250rf & 56K slope resistor on Bass & PA models. Most, if not all of the schematics I’ve seen for early 100 watt Plexis used the 250pf & 56K in the tone stack. Peter’s amp had the 500rf and 33K. I replaced the 33K with the 56K for more bottom and hit a mid point between the 500rf & 250rf and used a 390rf cap for the treble control. Not too bright and not dark. With Some Lead amps, you have to turn the treble off and even then it can still be too much. I’ve found that by using a 390rf you get a happy medium point where the treble pot is still useable. All I did was put the amp into what Marshall’s circuit was in 1967 and used the same values and type of components they used, with a few improvements like the FRED rectifiers.
TQR:What type and brand of tubes were selected and why?
Don: I used the Svetlana EL-34s, as I’ve found them to sound extremely close to what the old Mullard xf3s offered in tone, as well as reliability. I’ve compared them side by side in my old 50-watter and in my ’64 VoxAC-50 and it’s really close. Preamp tubes – I used the new Ei Elite gold pin 12AX7s. To me they sound very nice and close to old Brimars and Mullards. Don’t forget that the Phillips Electronics Corp. once owned Mullard and Ei among many other subsidiaries. Ei seems to have made an excellent attempt at creating a good sounding, reliable 12AX7 that they pre-test before sending out. My favorite 12AX7-type of valve/tube for Marshall’s are the Brimars, as they offer a bit of that aggressive old tone that Marshall’s are famous for.
