You could make this a permanent modification if you like the sound, but a far more useful solution is to make one of the amp pots have a second function as a SPST switch, and wire this mod to it. Just replace any of the tone control pots with a push-pull pot that has the same resistance value and taper, connect it up as normal, then use the two contacts of the switch to interrupt the mid pot’s ground connection as follows:

You don’t have to make the mid pot the one with the switch, but do make sure its the mid pot’s ground that you’re interrupting when you install this mod.

The results are what sounds like an increase in gain and a slight difference in feel, and obviously the tone controls become redundant.Those looking for an EVH brown sound like tone might find this mod particularly useful, and its nice as it is very simple to perform, and easy to engage. Here’s a soundclip courtesy of  Kevin Andrigo:

soundclip
text and images Copyright © Richard Baines 2010

Some people associate the 56k/250pf with the “classic Marshall tone” as it was used by Eric Clapton on his early recordings, The Who when they first got their hands on Marshalls, and even Eddie Van Halen’s “magic Marshall” is said to have used this tone stack. However, it’s also been said that until Marshall started using the 33k/500pf combo, Jimi Hendrix had all his amps worked on to put this combination in. Even more important though is what sounds best to you, and what better way to find out by being able to switch between the two with the flick of a miniswitch. Therefore, I’ve worked up these diagrams to make installation of the switch a cinch. Use whatever wire colours you want, I’ve just used whatever seemed to make the diagram clearest.

Method 1

First off is the most obvious way of doing it, and with each setting you will only be using one resistor and one capacitor. Some people might prefer this setting in case they’re using carbon comp resistors and want to maximise resistor distortion, or want to keep the circuit physically as well as electronically the same in each setting. The downside to this method is that you may get a popping noise when flicking the switch, as for a split-second the switch is breaking the contact between the resistor/capacitor and the rest of the circuit.

click the thumbnail to enlarge the diagram

Method 2

The second diagram shows the other way you can switch between two tone stacks, by adding components in parallel with the tone stack on the board. For this to work, you must be using 56k/250pf as the tone stack on the board, and use 82k and 250pf for the parts mounted on the switch. By putting the two resistors in parallel (56k + 82k), you reduce the value of the resistance to 33k, and by putting the two capacitors in parallel (250pf + 250pf) you achieve the capacitance of 500pf. Because in this method there is always a resistor and capacitor connected to the rest of the circuit, you eliminate any switching noise.

click the thumbnail to enlarge the diagram

Copyright © Richard Baines 2006

Let’s start by naming the wires. Each wire should have it’s code silk-screened next to it’s turret, from W1 to W4.

W1:    Red wire, connects to 10w 47k resistor.
W2:    Green wire, connects to ground.
W3:    Brown wire, connects to circuit board where treble pot middle lug wire went.
W4:    Black wire, connects to treble pot’s middle lug.

Basically the effects loop is inserted in-between the treble pot’s middle lug and the capacitor on the board which the middle lug was connected to (therefore it replaces the wire from the treble pot’s middle lug to the circuit board).

It’s advisable to twist together the green and black wires, and connect the green wire to the back of the treble pot or if you have a ground buss system, a point on the ground buss that’s close to the treble pot. For an even tidier job, you can also use cable ties or rubber bands to keep the two wires together. Run the two wires under the circuit board along the bottom of the chassis.

The red wire (W1) connects to a 10w wirewound 47k resistor. The other side of the resistor then connects to the voltage supply for the phase inverter and preamp, on the higher voltage side of the 8.2k/10k voltage dropping resistors where they meet the choke and screen voltage supply. On the below diagram I’ve circled the point in red for where the resistor connects to the circuit on a 100w Marshall 1959.

On the Marshall 1959slp, 1987x etc there is a special location for this resistor on the main circuit board, as you do not want this resistor hanging around in the air; it will be carrying between 300v and 400v, depending on how high your plate voltage is. If you don’t have anywhere on the main circuit board to put the resistor, I’d advise on creating a new small circuit board to mount it and run a wire from the 8.2k resistor to the resistor.

Copyright © Richard Baines 2006

Also, adding a low parallel resistance across the primary of the OT makes the amp’s output source impedance look very low and very resistive, which changes the damping factor and flattens the frequency response, eliminating the natural bass and treble boost you get from the speaker’s impedance curve. The end result is a flatter, midrangey tone.

Randall Aiken
http://www.aikenamps.com

Consequently this means if you’re looking to acheive that tone from Van Halen 1, and aren’t using this (potentially harmful) method (and are using something like a PPIMV instead), then it would be a wise idea to disconnect the negative feedback loop in your Marshall. This would be a 27k, 47k or 100k resistor that connects to either the 4/8/16 ohm taps for the ohmage selector switch, or to the speaker jack.

I looked at the text above that described the mod Cerrem originally described. It is a pretty standard trick for controlling gain of differential stages.

A push-pull output stage is what you might describe as a “paraphase” stage instead of a differential stage. Paraphase stages typically receive two out-of-phase input drive signals, and producing two out-of-phase output signals. Differential amplifiers are very similar, but can be configured with single inputs or outputs. In the usual situation, the paraphase gain stages share a cathode resistor that behaves a bit like a current source, maintaining some semblance of balance in the output. These stages – like the paraphase driver in a Marshall Major – are resistively loaded.

You can reduce the gain of the paraphase stage (or any differential amplifier) by reducing the plate load. One way to do that is reduce the value of both plate resistors. Another way to do it is to strap a resistor across the plates, i.e., plate to plate. The load can also be a transformer which allows the paraphase outputs to be “added together”. The plate to plate load is what the total output stage sees, so if it is paralleled with another impedance, then the gain of the output stage is reduced.

To achieve significant signal reductions means going an order of magnitude lower than the starting impedance. A 4k or 5k OT might provide the optimum impedance to get 50W of output. Making this 400 to 500 ohms in total intuitively _increases_ prospective power output. However, we are not subbing in a lower-z OT, we are paralleling it with a plate to plate power resistor.

Indeed, the tubes will be working harder, with most of the power expended in the low impedance path instead of the parallel stock path. Intuitively, we might figure that about 90% of the power will be wasted in the resistor with 10% in the OT, which would give us a nice loudness reduction. What really happens is that the tubes DO try to deliver more power – not 10-times as much, but a significant amount more than they otherwise would try to deliver with just the OT – and that extra power is just wasted as heat. The reports from JP and others above verify that the tubes work very hard, to the point of running with red plates and sometimes burning out. Tubes are relatively inexpensive, so you might figure this is not a bad compromise. The danger lies in what else might be damaged when a tube fails: you could lose an OT, a PT, or both.

In lower power circuits, like a reverb driver, this method works very well. I use such an approach in my push-pull reverb driver to protect the transformer. However, the impedance ratio of the original circuit to the refined circuit was not as extreme as in the Cerrem example.

Although this is a very simple mod, there are risks involved. I would recommend you DO NOT DO IT IN ISOLATION. To reduce the risks, add some fast-blow fuses in series with each tube. This assures that no single-tube failure can take out the transformer.

The problem with fusing just the CT is that the fuse value must be high enough to accommodate the peak signal current for normal operation. In parallel output stages, this current might be quite high resulting in insufficient protection against reduced tube count failure modes.

Reduced audio output power is achieved, but the tubes may actually be working harder than if there was an attenuator between the amp and speaker. The sound is changed a bit, but not as much as an attenuator would do, and tube life is seriously reduced. Power Scaling would work a lot better without changing tone AND improve tube life AND give continuously variable loudness.

Sometimes, simple approaches work very well, but this one is fraught with potential hazards.

The dreamers among you might recognise above, another possibility – an expensive one: have the OT equivalent of a variac. Such a device would vary the effective plate load to the tubes thus vary the output power. The only realistic way to do this, comes from the welding world. Old variable output AC/DC power sources for stick welding had a variable reluctance core where a piece of core material is cranked into and out of the windings. This varies the resistance in the magnetic path, varying the output voltage and power that can pass through.

Have fun,

Kevin O’Connor
http://www.londonpower.com

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.

But…..

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 www.ampwares.com – 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!!!

How Does It Work?

Electronically it’s very simple. All it consists of is tapping the signal coming from the preamp before it gets to the phase inverter, reducing it in size, and then sending it back on it’s path to the phase inverter. This is done by inserting a 1M pot (logarithmic taper) between the centre lug of the treble pot, and the place on the board that the centre lug used to connect to. You can connect it up in different ways, but the most common place is to insert it here, sending a wire to the left lug of the MV pot from the centre lug of the treble pot, connecting the middle lug of the MV pot to the circuit board, and sending the remaining lug of the MV pot to ground.

Here’s a diagram:

click the thumbnail to enlarge the diagram

As you can see, all you have to do is replace the wire I’ve coloured in bright blue with the orange and green wires with a pot in between, and a ground wire coming from the pot. Where you put the new master volume control is pretty much up to you, you could replace an input jack that you never use with it, or move the presence pot to the back of the amp (in a speaker jack hole) and put this in it’s place.

How Does It Sound?

The effectiveness of this master volume depends really on the balance of gain in your amp. If your amp uses gain mostly in the preamp, it’ll most likely sound great. If you use a lot of gain from the phase inverter and output tubes, it probably won’t sound anywhere close to how your amp does when cranked. Since a SuperLead or SuperBass draws most of it’s gain from the later stages, this mod isn’t really that popular these days with them, and most people tend to use a post-phase inverter master volume, Power Scaling, or an attenuator.

However, if you’re planning on reworking the preamp of your SuperLead or SuperBass for more gain, this mod will probably come in useful – even if it’s not used on it’s own to attenuate the amp’s volume, it will be handy in stopping too much gain in the preamp from overloading the phase inverter.

Copyright © Richard Baines 2006

Over the lifespan of the SuperLead and SuperBass amps, Marshall at various points in time (sometimes randomly) added a bypass capacitor to the 820ohm cathode resistor for V2a. The reason for this could have been tone shaping, which is the main reason you’d probably want to add this switch, but it was more likely used to block extraneous noise entering the signal path through the cathode. The two capacitor values used intermittently by Marshall were 330uF and 0.68uF. The 330uF bypass cap was used mainly before and during the transitional period of late 1967 to early 1968, whereas the 0.68uF capacitor was used mainly afterwards up to somewhere in 1969. It’s not particularly common to see the 0.68uF on the metal panel Marshalls that followed, and most definitely not the 330uF.

So what do the bypass capacitors sound like?

It’s generally thought that 0.68uF seems to add more crunch to the tone, as it sets the stage at full gain, except it starts to roll off frequencies below 285Hz.

The 330uF bypass capacitor on the other hand sets every audible frequency at full gain for the stage, thus making the tone (EQ) similar to without any bypass capacitor at all, yet thicker and slightly gainier sounding.

The best thing really though is for you to judge for yourself, and this switch lets you do that at any given time on the fly, even when the amp is on. There’s not much voltage running through the cathode so you can use just about any mini toggle switch for it to work well.

click the thumbnail to enlarge the diagram

If you use an on/off/on SPDT or DPDT switch, you will be able to select between .68uF and 330uF bypass caps, or no bypass cap at all. You can still wire this up with an on/on switch, but without the middle switch position, you will always have to have one of the two bypass caps engaged in the circuit.

Copyright © Richard Baines 2006

There ARE small tone altering and/or gain altering mods.
I do have pic’s so I CAN physically see that the amp has a split cathode arrangement.
…the cap on v2(330uf) cathode.
…its a 2.7k/.68 and the resistor looks like the stock part (but I don’t think it is) but the cap is one of those yellow square type from the 70′ Marshall’s. From the circuit card It looks like it came stock with this arrangement. The post that is pressed in looks stock too which tells me more than anyone could. In the pics the serial # is 12301.

His treble cap was a round shaped ceramic that was a 250pF that said MURATA … With 56K on the tone circuit feed..

One of the 470K mixer resistors was bypassed with a round hollow tubular MURATA cap 500pF… If memory serves me right on the value, or it was a 330pF..I will check my notes….

The real kicker, his phase-inverter “get-rid-of-the-FIZZIES” cap was a 100pF instead of the normal 47pF ….and this my friends is how the “brown sound” with that added compression happens..

Oh, BTW those 820 ohm resistors were carbon-comp and drifted in value up about 1.1K and make the amp much more gainy and warmer, since these re-bias the 12AX7 valves in a bit more non-linear region..

I am pretty sure the feedback resistor was a 47K …I will have to check my notes…

The filter cap in the center of the board was a dual 16uF grey RS cap…

The screen filtering was 2 DALY 32uF light-blue caps in series… The voltage doubler were 2 100uF DALY royal-blue caps…

Rear cap on top of chassis was a royal-blue HUNTS 32uF or 16uF…need to check notes..

The value of the coupling cap between V1 and V2a….022uF.

At least that was what it was in 1980…

1.The first 820 ohm resistor (carbon type) measured a little over 1K. It was bypassed with a blue 330uF resistor. can

2.His treble cap was a 250pF Murata flat ceramic one. The cap across his 470K was a Murata hollow round 330pF. cap

3.His second stage 820 ohm (which also measured a little over 1K, was also bypassed with the exact same type 330uF blue coloured cap that was on the first cathode resistor.

4.The filtering caps for the middle of the board were grey coloured RS caps that had dual 16uF values.

5.The screens were 2 blue caps in series (DALY 32uF’s).

6.The voltage doubler were two big blue caps (100uF DALY’s). The one on the outside was a blue Daly – and was a single 32uF.

7.The feedback resistor was a 47K of unknown origin.

8.His power transformer was the smaller one of that era. His OPT was also the smaller one with 1.5″ stack.

The amp was either left stock into a load resistor or, a big Ohmite (or other) power resistor was placed somewhere in the circuit to cut the power of only the output stage meaning Sylvania 6CA7′s were the only valves to hold up to this.

If this is true – doesn’t this kinda dispel the whole ‘special’ 67 slp 100 myth …as almost all amps of that year had pretty much identical configurations (with the small exception of the 330uf cap on the second preamp stage?

Dankuwell ! (dutch for many thanks)
Plaap

Assuming your JTM 45/100 or similar early 100w Marshall amp has the 56k/250pf tone stack combination, first you need to locate this on the circuit board. It should look something like this:

or this:

I’ve circled the two components in black. They should be a 56k resistor (bottom circle) and a 250pf capacitor (top circle). Change these out to a 33k resistor and 500pf capacitor respectively.

If you want to be able to switch between these two tone stacks, I’ve drawn up two diagrams in the general/other mods section that shows the different ways in which you can wire up a simple switch to change between them.

Text copyright © Richard Baines 2006