Saturday, April 25, 2015

Rocking in the Plexi world

Plexi Drive for classic plexi tones
1962 was the year that really changed the face and sound of rock'n roll. Not because the Beatles started gigging in Hamburg and not because Maestro issues the first fuzz-tone. More than anything, it was because Marshall first released their JTM 45 30W model. This early rock monster was heavily based on the Fender Bassman but using a 12AX7 tube for pre-amp and used KT66 tubes for the power amp driving a closed back 4X12'' celestion speakers cabinet. This little differences from the original Bassman had a huge impact on the sound and made Marshall a quick predecessor for the older Bassmans and VOX AC50 which were the power horses for early rock'n roll outfits in the UK.

Again, not directly related to the world of fuzz but definitely a cornerstone on the quest. Fuzz, as always, is just fingernails on a chalkboard without the right amp to smooth it out. Marshalls with treble boosters, Fuzz Faces, Octave fuzzes and more.

The JTM name is the acronym for Jim & Terry Marshall. The 45 is for the 45W series which was the upper limit at the time after the 30W series and before the 50W and 100W appeared.

The real deal JTM 45 head MKII

In the mid-60's The JTM 45 became the staple sound source for blues rock bands across the world due to its long sustain, crunchy leads and edgy cleans. It had this edge over Fender and Vox and continued to develop, giving birth to legendary offsprings such as the Marshall Bluesbreaker made famous by Clapton's sound with the Bluesbreakers on the Beano album (hard driven by his Gibson ES-335 and a Dallas Rangemaster), and the powerfull Marshall 1959 Plexi Superlead. It can be regarded that with every generation of Marshall amps, guitar music evolved, from blues to rock to hard rock to metal and so on. The amps and styles are that much interlaced between them.

Number One - The Original 1962 model which the JTM 45 was based upon. 23 units sold and changed history. Today resides in the Marshall museum.
Hendrix used the JTM as his main amp between 1968 and 1969. Angus Young of AC/DC relied heavily on the JTM 45 for his signature guitar tone on early AC/DC albums and live concerts (though he also used the 100W Superlead). Early Gary Moore and Early Peter Green sounds also rely heavily on the JTM 45/100 as the source for their glass shattering solos. It seems that regardless of whether it were humbuckers or single coils, the JTM delivered great clean tones and super dynamic crunch. It takes pedals very well especially if those have a low cut like the Tube screamer or any treble booster. With the bass heavy Fuzz Face it seems strange that Hendrix could get what he got out of this pair,but I guess he did, didn't he? The strat definitely sounds better than the Gibson using this setup.

Hendrix, Page, Yardbirds, The Who, Blackmore and Angus Young.
All of them used a lot of Marshalls and at some point during their early life and their better days played a JTM 45.

The Plexi name came from the plexiglass panel which covered the front side panel on the early models since 1962. In 1967 this panel was replaced with a brushed aluminum panel but the name "plexi" remained as the one-word-description to describe that signature tone to this very day.

Some really great info can be found around the web and here are a few examples:

Anyway, Since I first built the Runoffgroove Thor I have been trying endlessly to nail those early rock tones using various pickups and various rig setups. I did succeed getting a lot of that crunch I wanted but something still kept me up at nights.
1. I couldn't get those clean glassy tones from the Thor, probably because it was a high gain pedal meant to emulate the Superlead 1959 tones.
2. It had some hiss which made it hard for me to use it as the last link in the chain using fuzz pedals or other overdrives.

I had a 2 year tour around every forum known to man, played a lot of MIAB (Marshall in a box) pedals and spent many YouTube hours in order to really understand what pedal I want to build for that particular crunch. I ended up with several candidates all of which try to emulate the Plexi mid-gain overdrive and eventually decided to build the Wampler Plexi-Drive. The reason was two fold: 
A. I had more experience building the Plexi-Drive having built it in the past for a friend and it sounded great what I tested it against the Thor, although not as powerful, of course.
B. It uses standard components like J201 FETs and biasing them using trim pots made it easy to build without too much voodoo.

So, after waiting about a year after, finishing several other DIY projects I came around to this project and after a few evening selecting parts, soldering, assembling and paintings it was done.

Now the Plexi-Drive has a younger brother called Plexi-Drive Deluxe which is even more flexible and has more tone control. The schematic is still not around so I decided to give my Plexi-Drive some mods after searching the forums for some help.
Gut shot of the Plexi Drive based on the TagboardFX layout with biasing trimmers for the JFETs
The first mod I did was lowering the bass boost cap which emulates the 4X12'' cabinet. I ended up using a 1nF cap instead of a 2.2nF which makes the bass less heavy and more transparent. I guss it's a matter of taste and you can also use a 3 pole switch for more than two modes but for me it is enough.
The second is a 1uF cap bypassing the 1st gain stage by pulling the J201's source down to the ground. Although this seems like reducing the amp's gain, it actually boosts the gain up by increasing the volume and preventing the 1st stage from clipping the signal.

Now Brian Wampler indicated that this pedal is somewhere between the lower gain 18W tone and the mid gain JTM  45. I really couldn't tell the difference never having played any of the real deal amps but I can tell you for sure that this is the pedal that I was looking for when I started this Marshall quest. I wanted clean sparkly tones at lower GAIN settings and strong crunch at high GAIN settings with the TONE control compensating where needed. Usually this means that lower GAIN demands higher TONE. The LEVEL control gives you plenty of boost way way above unity and the extra switches give you some more versatility. Different pickups and different boosters really give you a lot of options here.
Mug shot of the modded DIY Wampler Plexi-Drive
I tried to give the pedal the same clean looks which the amp is known for with black and gold colors. I called the bass switch "Cab" and the 1st stage bypass switch "Boost" and now I have a modded Plexi-Drive giving me some great vintage classy tones. Finally I decided to go with a red LED which gives it some extra vintage Marshall looks.

 It sounds great with other pedals in the chain. A treble booster or an overdrive are great to drive it harder and it takes up fuzz pedals very good and probably best with single coils.

So there you have it. A really really great pedal with minimum noise, hiss and unwanted artifacts.

Below you will find 2 tracks that for some reason the Mixcloud shows only one and you can choose "next" to move on to the next one. The first one is to demonstrate the pedal with strong PAF humbuckers and the second with single coils. I play around with the volume knob, various pedal settings, bridge and neck pickups and even some wah or a treble booster in front on a few cases.

You can find the layout I used here:

The schematic is shown on the Revolution Deux page:



  1. Doron - thanks for this post. I'm wondering if there's a way to cut the bass, rather than boost it, via the toggle. I like the non-bass boost position for lower volume playing but things get a bit woofy at gig volume. Any ideas on how to mod the bass boost position to be a bass cut?

  2. Hey Doron, nice post.
    I bet you also checked out the ROG Thunderbird? How does it relate to the Plexi-Drive? Have you changed your opinion on what's the best MIAB pedal?
    I ordered a 1776 PCB and also wnated to build a PLexi-Drive with trimmers, but I thought maybe you can shed some light here?


  3. Thanks.

    I am just about to post the ROG Thunderbird post. It's a great overdrive and has much more gain than the Plexi-Drive. I would recommend the Plexi-Drive for low-mid gain overdrive with a very good crunch and great signal-to-noise behavior. If you are looking for pure hard hi-gain Plexi sounds, the Thunderbird will be more up your alley. It's noisier too due to the high gain but it's also much louder. Much much louder...

  4. I,ve bias drain at 3,5 but no hi gain sound,only light crunch,so what's the real source voltage neded?

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