Marshall Guv`nor: a Pedal that sounds like a Tube Stack

 

To be more accurate, it sounds like a valve stack; after all, it is a British pedal imitating British Marshall amplifiers.

Don't tell anyone because it's a secret: when creating his JTM45, Jim Marshall actually copied the circuitry of a 1959 Fender Tweed Bassman 5F6-A.

And Mr. Marshall also equipped his first 50-watt combo amplifier, a 1961 JMP Tremolo “Bluesbreaker,” with four 10-inch speakers. The iconic 2x12" version was made a year later.

Tolex instead of tweed, a horizontal arrangement of tubes, and ring-shaped loudspeaker magnets—the differences end there.

But why does the British Marshall sound so different from the American Fender? Is the difference really just the Celestion speakers instead of the Jenhsen ones with their U-shaped magnetic core?

The stronger magnetic field of British loudspeakers compresses the sound's dynamic range to a significant extent. One can compare the waveforms recorded with the Shure SM57 microphone from the 12" Celestion G12M Greenback and Fender Mustang II (V1) loudspeakers, as well as from the 8" Orange Voice of the World PPC108.

But the spiciest part of the Marshall sound is literally based on the usage of British valves instead of American tubes.

In the post about a simple DIY tube guitar amplifier, we already noted that the sound of a particular tube depends on the geometry of the grids and anode.

And if we compare 12AX7 preamp tubes with ECC83, we will see and hear that they are all very different. This is determined not by an American or European stamp but by the design of this particular unit from a particular manufacturer.

It's a completely different story with the EL34 and 6L6 power amplification output tubes. Their sound difference is on another level. Short and thick American 6L6 can be described as high fidelity with a large headroom; long and thin English EL34 is, on the contrary, a growling and roaring powerhouse due to the fact that it tends to limit and distort the signal.

Does this mean that if you swap the speakers and tubes in the Tweed Bassman and Bluesbreaker combos, the Fender 5F6-A would become a Marshall JTM45? Not really, no.

In the universe of guitar amplification, nuances matter: the cabinet's open or closed back wall, the fine selection of component values, and the frequencies to which the tone controls are set.

And, of course, the sound is influenced by the anode supply rectifiers and the supply and output transformers. And according to some experts, even the material of the chassis matters: the windings of transformers have considerable leakage inductance, and their alternating magnetic field interacts with the metal of the chassis.

Then there's another question: is it possible to emulate all these effects without real tubes, bulky loudspeakers, and heavy multi-pound transformers? A guitar amplifier is a complex yet still measurable device; it's subject to the laws of physics and can be mathematically evaluated.

There is no doubt about the possibility of recreating a certain sound by other means. The question is how complex the circuit with transistors and ICs will be sufficient to obtain a convincing effect.

The idea to replicate the iconic sound of the Marshall JTM 45 and JCM800 in a pedal body and create the world's first amp-in-a-box, oddly enough, first came to none other than the employees of the Marshall company itself, led by the head of the Development Department, Steve Grindrod.

Guitarists worldwide owe a debt of gratitude to this man for developing the legendary Marshall JCM800 and Vox AC30.

Today, the godfather of British Sound owns his company, Steve Grindrod (Albion) Amplification. He produces Pendragon guitar amps and Gypsy Boy acoustic amps and works with various charities. And in 1988, his efforts led to the creation of the Guv’nor pedal.

In 1992, the pedal was re-released under the name Drive Master. It differed from the original only without an effects loop jack.

The circuit looks like a regular distortion pedal with two operational amplifiers. The first is included as a non-inverting amplifier; the signal is supplied to the non-inverting input. And the second operational amplifier is used in the inverting connection.

The drive regulator potentiometer is configured to control the negative feedback of both op-amps. This is a brilliant design by Steve Grindrod.

By comparison, Orange tube and transistor amplifiers typically use dual, linked potentiometers to control the gain of the two stages simultaneously.

Red LEDs are used as limiting elements in this Marshall amp-in-a-box. This makes it possible to adjust overdrive and compression using the drive knob in a wider range and get a sound from light blues overdrive to hard heavy metal distortion.

And, of course, the standout feature of this pedal is the gorgeous three-way tone stack, tuned for that signature Marshall sound. It has even more components than the tone stacks of many other tube amps.

This happens because the pedal does not have as many gain stages, each with timbre-shaping frequency-dependent circuits, as a large amplifier does. The tone stack has more work to do in the pedal than in the amp head or combo. At least, that's what Steve Grindrod thought when developing The Guv'nor.

However, in 1992, when developing the Blues Breaker pedal, it turned out that the distortion sounded great with just one tone knob, a simple treble-bleed circuit.

Three new pedals were developed. Drive Master was a reissue of The Guv'nor. Blues Breaker featured soft clipping and lower gain.

The Shred Master, on the other hand, was a pedal with maximum gain and a Contour knob, which allowed to emphasize the mids for a vintage sound or, alternatively, make a mid scoop and get a modern sound.

As any successful pedal deserves, The Guv’nor was soon copied by other manufacturers: Daddy O from Danelectro, Crunch Box from MI Audio, Riot from Suhr, and Angry Charlie from JHS.

Different versions of these pedals may have a three-way tone stack or a single-tone knob, a switch or presence control in the form of a knob or trim pot inside the body, and a limiting diode switch that sets the compression depth.

The version I put together has four knobs: gain, volume, tone, and presence. I enjoy how this pedal sounds with different guitars. In the video, I play a 2011 Gibson MM Explorer.

Suppose you want to add Marshall-style voicing to your rig and get some bright British crunch. In that case, you should consider purchasing or building one of the pedals based on the Marshall Guv`nor circuit.