As we all know, the Internet of Things is full of various modifications you can apply to vintage Marshalls. The small problem I’ve noticed is that the same mods are recycled over and over again. It’s hard to find truly unique approaches.
Now, generally speaking, a Marshall amplifier is not rocket science, and there’s only a finite number of things you can do with this design. In this article, I aim to contribute something relatively original to the family of Marshall amplifier mods.
But first, let me give you some backstory. A couple of years ago, I received an amplifier for servicing from a guy who’s the frontman of a well-known band around here. He used to play a Mesa Boogie Rectifier but, at some point in his career, came across a peculiar Marshall JCM800 2204 that had been modded somewhere in Germany.
This amplifier didn’t sound much like a traditional Marshall. Instead, it was absurdly fat and greasy, with exorbitant amounts of gain. In my opinion, it had far too much gain—you had to keep the Gain pot below halfway. After 12 o’clock, the amp would become unstable. Despite these "cons," by keeping the gain lower, the amp became a fiery ball packed with rock ’n’ roll.
I took a look inside this amplifier and noticed it was one of those designs where the cathode follower stage had been removed. That half of a 12AX7 was repurposed to implement a standard common cathode gain stage. Later, I discovered that Reinhold Bogner used this style of mod for some players back in LA.
It didn’t take me long to connect the dots—Bogner, being German, and this 2204, modded in Germany—this type of mod might have a nationality. Seriously, I suspect there was an old tech in Germany who developed mods like this, and Bogner either adopted it or perhaps even originated it. But does it really matter? No, it doesn’t. It’s a logical modification to make to a circuit and a fantastic way to achieve more gain.
But how important is the cathode follower in a Marshall circuit, really? As some of you may know, my philosophy is that—what? Everything makes a difference. And the cathode follower does a great deal in a Marshall circuit. It completely changes how the tone stack responds and affects the feel under the fingers, at least to me.
I consider having a cathode follower-buffered tone stack or a non-buffered one to be a very important design choice in every amplifier. And what I know for sure is that some people like it, and some don’t. We’re not really talking about whether it’s better or worse without it—it’s definitely different.
The story about "The Grinder" starts with me getting contacted by a friend of mine, a phenomenal guitar player, inviting me for a drink and saying he had a friend with him who wanted to talk to me about an old Marshall he wanted restored.
Soon after having a detailed conversation with the owner, I realized that he wanted the amp modded for a bit more gain. He wanted the amp to deliver something in the style of what Steve Lukather gets out of his Bogners as a core tone, using drive pedals with it.
The interesting thing is that the amplifier in question was actually a '68/'69 Plexi, which I didn’t know at the time. It seems to have been recapped in the 70s, so I initially thought it was a '72. In any case, the amp was cut with a grinder back in the 90s so it could fit into some rack, and they only used the power amp as a slave. Quite crazy, right?
Naturally, I started thinking about how I could deliver the sounds he wanted, without further additions of gain stages, and I came up with this design:
I don’t have the schematic for the power amp drawn out, but it’s a typical late 60s Marshall Super Lead power amp. Nothing special was modded there. And the power section is not listed because it’s all stock Marshall. Of course, this mod can be done to pretty much anything from the 60s Plexi all the way to the JCM800s. Obviously, it would sound a bit different because of the different transformers, choke, and power section, but it will be in the ballpark.
Ok, so, the mod. In general, whoever was doing mods to Marshalls, everything should be clear as day here. Not many things to explain.
The phase inverter is pretty much stock. I used a 47k negative feedback resistor connected to the 4 Ohm tap on the output transformer. Obviously, the Master Volume has been added (since I was modding a Super Lead).
The tone stack was left to be the classic Marshall thing, but you can see that the tone stack is not fed by a Cathode Follower, rather, it's fed by a standard Common Cathode gain stage, which gives you a lot of gain.
A very important note: You will see that there is a lot of attenuation happening in the form of Tee Attenuators. I highly recommend that you leave this attenuation intact, as the circuit has a lot of gain and it can quickly become unstable. I am a firm believer in proper gain staging and not over-blowing inputs of certain gain stages. That being said, to answer your question—yes, the gain in this circuit can be increased just by reducing attenuation before gain stages 2, 3, and 4. I just don’t recommend it. I designed this to have the healthiest amount of gain as possible. Just use a pedal.
Gain stages 2 and 3 are pretty easy to understand. I recommend keeping those snubber caps there. It gives the amp a nice polished distortion sound, and it also keeps it from oscillating.
A thing worth noting is that the treble bleed cap on the gain pot can be swapped for a 4.7n for a different gain feel on lower settings, but to be honest, I just kept the gain maxed out on this amp and went for heavy sounds.
In the first gain stage, the cathode resistor can be swapped for a higher value, it will change how the amp feels. I went with this because it offers a more compressed and smoother feel, which was requested by the owner.
Last important thing to mention is that I left the 4 inputs of the Plexi intact, but I used only 2 of them—and not in the way they are wired in the typical 2204/2203 circuit. I left the HI and LOW inputs, where the HI input would connect the two 68k resistors in parallel for an equivalent of 33k, and the LO input would just go through the 68k. This design is meant to be used with the 68k grid stopper or the LO input. I just left the HI input there to be available if the amp was played with a lower input signal for whatever reason. With all my guitars, the HI input was just too much, and the amp would squeal too much and be sensitive. This could have obviously been solved with another snubber cap (which I chose not to) or perhaps with a better preamp tube, but honestly, I just like how it sounded with the LO input.
There you go, an old Plexi brought back to life, welded together, modified, nicely packed, and I absolutely loved it.
I have an evil idea, since I really liked this amp— I will ask the owner to pick it up for a few days and record it once again. Maybe I’ll make some profiles for those profiler toys people are gigging around with.
And yes, there were some early YouTube videos on it with very raw sound samples, and a few more pictures I will link them down below.
Until next time, happy chugin'.
