Marshall Guv'nor / Drivemaster / Equinox Overdrive PCB

Equinox - Marshall Guv'nor / Drivemaster Guitar Pedal PCB

$10.00

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Project overview

The Equinox Overdrive project is a clone of the Marshall Guv’nor / Drivemaster. It’s an overdrive with a 3-band tonestack, and it was among the first overdrive pedals to use this type of tone control, allowing for more amp-like toneshaping via bass, midrange and treble knobs. Because of this, you can get a very wide range of tones out of it and it is suitable for many different styles of playing.

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Marshall Guv'nor / Drivemaster circuit history

The Guv’nor was first released in 1988, Marshall’s first effect pedal since the Supa-Fuzz and Supa-Wah in the late 1960’s. The name was suggested by Jim Marshall after one guitarist tried a prototype and remarked that “that pedal’s just the guv’nor, innit” (which is British slang for “the best” or “the ultimate”).[1] It was specifically designed to be an amp-like pedal, allowing you to get a Marshall-like sound at low volumes or with different amplifiers.

The original Guv’nor had a post-drive effects loop that was engaged along with the pedal, allowing you to treat it like you would a second amplifier channel—for instance, putting EQ, delay or modulation in the loop so that your entire lead tone could be switched with one effect.

In 1991, the Guv’nor was renamed the Drivemaster, and the Shredmaster and Bluesbreaker pedals were released along with it. The only change was that the effects loop was removed; the schematic is otherwise identical. It was a very popular pedal at the time and remains well-regarded.[2]

These pedals were discontinued a few years later, and in 1998 they were replaced by a new line of compact pedals, including the Guv’nor Plus, Bluesbreaker II, and the Jackhammer, which is a renamed Shredmaster. Generally these new versions added features—for instance, the Guv’nor Plus has an additional “Deep” knob—and other schematic changes were made as well. These rereleases are generally viewed as vastly inferior to the originals, which now fetch pretty high prices on the secondhand market.

The only notable clone of the Guv’nor is the Danelectro Daddy-O. The most notable player of the Guv’nor was Gary Moore.

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Circuit analysis

Circuit blocks

Adjustable gain stages & filterHard clipping (LEDs)Passive 3-band tonestackVolume control

Gain stage

The first circuit block of the Guv’nor is comprised of two op-amp stages: an adjustable op-amp boost & filter, set up as a non-inverting stage which has a bass roll-off corner frequency of 723 Hz (same as the Tube Screamer), followed by an inverting stage with a treble roll-off corner frequency of around 1060 Hz. So there’s a bit of a mid hump, but not quite as sharp as a Tube Screamer where the bass and treble cutoff frequencies are both 723 Hz.

The Gain control is interesting—it’s the same variety as found on the Bluesbreaker and Shredmaster pedals, but I’ve not seen it elsewhere. Let’s break it apart.

First, a bit about op-amp gain: in a non-inverting op-amp configuration, the gain is a factor of the feedback resistor divided by the ground resistor, plus 1. In an inverting op-amp configuration, the total gain is a factor of the feedback resistor divided by the input resistor.

In this circuit, the Gain pot is actually set up as a voltage divider rather than a variable resistor. When resistance is added to the feedback loop of the non-inverting op-amp, it’s also subtracted from the input of the inverting op-amp—which in both stages increases the gain.[3]

So the gain is calculated like this:

  • With the Gain control all the way down, we have 2k2 & 0 in the non-inverting stage for a gain of 1, and then 110k input & 680k in the inverting stage for a gain of around 6.
  • With the Gain control all the way up, we have 2k2 & 100k in the non-inverting stage for a gain of 46, and then 10k input & 680k in the non-inverting stage for a gain of 68.

Even though the pot is linear taper, in this setup it behaves somewhat logarithmically, so it’s very usable across the range.

Clipping stage

The Guv’nor uses a hard-clipping configuration, but it has a very high threshold since it uses LEDs as clipping diodes. It’s not going to sound as compressed and gainy as a Distortion+ or OD-855. (On this project, I’ve included an optional second set of diodes if you want to experiment with something besides LEDs, but first read the “Volume control & circuit output” section below. The LEDs sound very good so I would recommend sticking with them.)

3-band tonestack

This is a strange one. The topography looks pretty similar to the 3-band tonestack used on Marshall amps, which was adapted from Fender. Actually, though, it’s a bit closer to a Big Muff tone control. The Big Muff control pans between one side that cuts bass and emphasizes treble, and one side that cuts treble and emphasizes bass. The Guv’nor tonestack expands on this so that the treble side has an additional control for Mids (which is a pretty common mod for the Big Muff—see my Halo project), and the bass side has an additional control that essentially blends another capacitor into the circuit, which adjusts the bass frequency response.

The end result is that the controls are all highly interactive: as you turn the Treble control all the way up, you also gradually reduce the bass control’s effect on the circuit. The Mids control is connected across both sides, so it has an effect at both extremes of the Treble range, though it’s slightly different on each side.

Volume control & circuit output

Pretty standard from here on out, but I actually want to talk about what’s not there. Passive tonestacks are lossy, meaning the overall level of the signal is cut during the process of shaping the tone. If you look at the Shredmaster, Big Muff or MT-10 Mostortion, you’ll notice a gain recovery section immediately following the tone stack.

Why not here? Well, in this case it’s because of the choice of clipping diodes. In a hard-clipping formation, the signal is clipped above the diodes’ forward voltage, which means the signal’s amplitude is hard-limited as well. But since LEDs have a very high forward voltage (around 1.8V, compared to 0.6V for a standard 1N914), in the Guv’nor the amplitude of the signal is high enough to still have good volume without a gain recovery stage after the tonestack.

This is another reason why I recommend not straying too far from LEDs as clippers in this circuit. Lowering the clipping threshold will have a direct effect on the available volume, and you may find that you don’t have enough if you use something other than LEDs. If you do use other diodes, then at the very minimum, stack two 1N914s for a threshold of 1.2V.

The other strange thing about the output section is the 470pF capacitor to ground, which cuts the treble by a fair amount. A common modification is to omit this altogether—after all, you’ve already got a very usable treble control, so just turn it down a bit if it’s too bright. You could also drop this to 220pF or 100pF if you don’t want to remove it altogether.

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Notes & references

  1. Maloof, Rich. Jim Marshall, the Father of Loud, pp. 163-164.
  2. Marshall Guv’nor Drive Pedal – 1988 – Planet Botch
  3. Mark Hammer, forum post on diystompboxes.com