Plexi Mods - Marshall Amp Modding Secrets!

‘Cerrem Mod’

This mod is what Eddie Van Halen used on his Marshall during the early club days, and supposedly NAILS the guitar tone from the first Van Halen album. will not be held responsible for any damages which should occur from you performing this modification on an amplifier, we do not endorse or recommend this modification as it is expected to put a lot of wear on your amp’s power tubes, and when they go the could also take your amp’s output transformer with them. The article was done by ‘mr.twistyneck‘, and you can now access further details on his site at

IMPORTANT!!! When I use this mod, I run my Marshall head at around 90 to 92 volts, using a Variac. If you’re crazy enough to attempt this, I suggest you do the same.
“What in the hell is Cerrem’s Mod?” you ask. Before I answer, let’s get a few things straight:

Van Halen’s self-titled first album has THE guitar tone.
Don’t argue with me about that opinion – you’re welcome to your own.
This killer guitar tone is often referred to as the “Brown Sound”, and is really confined to VH’s first album. After that, the tone changed.
Since the day I started playing guitar in 1985, I have wanted to recreate this tone.
A member of the Plexi Palace Bulletin Board posted a Marshall amplifier mod that he maintained Eddie used. Since the mod didn’t have a name, I named it after him. Thus we have “Cerrem’s Mod”.
Do this mod to your old Marshall and you’ll probably fry it. You’ve been warned. I take no responsibility for whatever you do, nor do I guarantee that it’ll work for you, nor do I advocate that you do it since it involves extremely high voltages and things burning, and in general, chaos.
All that being said, let’s continue!
I have tried a Mesa-Boogie, a $2500.00 Guytron (which actually sounded pretty killer), a master volume Marshall 100 watter, more pedals than I can count, and lastly an attenuator – a THD Hotplate. I would say that if you want to get a nice brown sound, just take a Marshall Super Lead, stick a Hotplate between it and a 4×12 cabinet, turn all the knobs on the Marshall to “10”, and then use the Hotplate to lower the volume. It sounds killer.


One must never give up one’s quest for one’s favorite tone! And thus it came to be that in the fall of 2002, Cerrem was nice enough to spill the beans about a resistor mod that Eddie Van Halen used to get that killer tone on VH1. The thread went on forever – basically it involved using an extremely heavy duty resistor to throw your amp into a different state of being. Here’s the mod in layman’s terms:

Put a resistor across the output transformer primaries. If I’m wrong, please correct me and I’ll revise it.

Well that doesn’t tell you much. Where are the output transformer primaries? Turns out use can bridge them at pin #3 of the two inner power tubes. WHAT? Okay, let’s say you take out the chassis of your Super Lead, flip it upside down and look at bottom of the middle power tube sockets. Do you see the little channel in the middle where the nub of the power tube fits in? The pin to the immediate right is pin #1. Next to that is pin #2. Next to that is pin #3. You’ll note that there is a wire connecting pin #3 of Power Tube one to pin 3# of Power Tube two, as well as another wire connecting pin #3 of Power Tube three to pin #3 of Power Tube four. Do you see how the power tubes are wired (at least in this respect) in two pairs?. Good. What you’re going to do is bridge those innermost pin #3’s with a resistor.

What it does.

It lowers the volume of the amp. I really don’t understand the technical jargon associated with this mod, so I just say, “It makes the two halves of the amp try to kill each other”. How much the volume is lowered depends on the value of the resistor.

Explaining the resistor part.

The OPT (output transformer) primaries are usually not tied together. Let’s say you just took a regular old wire and soldered pin #3 to pin #3 – you would short out the primaries and there would be NO volume. But let’s say you stuck a resistor across – some juice would get through, but not all. What this boils down to is that the higher the value of the resistor, the louder it gets. Makes sense, right? If the resistance was TEN BAZILLION K ohms, then it would be almost like there was no wire, hence the amp would be very loud. But put a 470 ohm resistor on there, and life gets very interesting….

To wit:

1.5K (1500 ohms) gives you great rock sound – but you’re still loud.
470 ohms gives you nice brown sound – a little bright, though. The amp is loud, but not loud enough to hear over my drummer – so I use my Hotplate for it’s line out function [It is not set to attenuate] and use a power amp to boost the volume through a second 4×12 cabinet.
100 ohms is just freaking sick. Imagine getting real feedback at bedroom levels.
The tradeoff is that you’re going to kill your amp. Now, I have been beating the hell out of my 1971 Super Lead (who is named Bill L.), and I haven’t killed it using a 1K resistor or a 470 ohm resistor. You can see the original setup at the very bottom of the page. According to Cerrem, this mod is very hard on power tubes – and should one blow or short out, it may take out your OPT as collateral damage. WHEEEEEE!!!! On a side note, all I ever blew using just a Hotplate was power transformers – not OPT’s. I thought to myself “Gee, I wonder what it’s like to blow an OPT?”

So I have yet to actually blow up my amp using Cerrem’s Mod, but I figure it’s inevitable. I don’t really care, since all of my Marshalls have been rebuilt several times and are players, not valuable vintage gear.

My first foray into Cerrem’s Mod involved me hanging a heat sink with a bunch of resistors mounted to it off the back of my Super Lead, and clipping large wires (that ran to the pin #3’s) that terminated with large alligator clips to whichever resistor I needed. I did several shows this way, and even recorded with this setup. It drove me crazy worrying about a clip falling off and shorting out the amp (I did that once at home, and blew the mains fuse).

Obviously this entire method sucked, so I decided to take another crack at it. I hated having two wires permanently soldered to the pin #3’s. I could have easily just mounted the resistors to the chassis of the Marshall, but I wanted a non-invasive setup. First, I constructed the tube sockets, and this was a real bitch. Suffice it to say that I bought the top and bottom parts of the sockets from – the white spacer in the pics is PVC pipe that I ground down to act as a spacer. Getting the eight wires from the bottom of the socket into the bottom piece ALONG with getting a wire soldered to pin #3 in each base all at the same time nearly drove me mad. It drove me to drink several times, which is why this project took so long.

I wanted to take the wires from the two middle tube bases and connect them to an external unit housing the resistor block. As it happens, I had a vintage Bulgin plug and jack sitting around, so I used these as my connectors. Golly, ain’t that cool?!!? Next, I gutted an old MBT light chase thingy, stuck in my resistor block [adding two resistors to the original five], mounted seven DPDT switches, and sonofagun it actually worked!. The seven resistors are mounted such that one, some, or all can be used. I can get combinations of ANY of the following:

75 ohm
100 ohm
470 ohm
560 ohm
680 ohm
1000 ohm (1K, in other words)
1500 ohm (1.5K)
There is no rhyme or reason to the values used – but I will tell you this – 75 ohm, 100 ohm or 175 ohms is STUPID freaking nuts in terms of sick distortion at low levels. And your tubes will glow. A lot. And the resistor block will get HOT. Oh, but the sound!!!

Related posts:

  1. ‘Cerrem Mod’ – Kevin O’Connor’s Take On The Mod
  2. ‘Cerrem Mod’ – Randall Aiken’s Take On The Mod
  3. Adding a pre-PI Master Volume
  4. “Cream Switch”
  5. V2a Cathode Switch
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