RunOffGroove - Thor - Marshall Superlead Emulator

The Marshall 100W Super Lead amplifier is a legendary source of endless guitar tone, it's sweet break up and aggressive edge made this amp the choice for many rock and electric blues guitar legends - Eddie Van Halen had a few, as did Hendrix. So, if you're still wondering why you should be interested in playing through one of these check out this demo;

It's got serious tone! But let's be honest here - not all of us can afford one of these machines, let alone be able to crank them up to the volumes required to really get the most out of them. So, what can we do? Well, runoffgroove.com have given us an option - the "Thor" overdrive pedal.

Modelled on the Marshall Super Lead circuitry using the runoffgroove Jfet  Fetzer Valve "tube emulation" techniques the Thor aims to recreate all those great Super Lead tones in a controllable (and affordable) manner.

Let's look at the description from the runoffgroove website;

"Our primary inspiration was once again the Marshall Super Lead. Numerous old Marshall tracings and schematics were scoured for the definitive components that make up the famous sound. After trying many - if not all - of these arrangements, we have arrived at the values shown above.

The smaller Bright cap on the volume control is taken from the JTM and early Super Leads. This value allows a usable Bright setting without the harsh treble effect exhibited by the later amps. Beginning in 1969, the amps often had 5000pF caps that are almost always removed or replaced with a much smaller value.

The classic 470k/470pF filter preceding the second gain stage was chosen for its fuller sound. This is a later Super Lead filter with a lower break frequency than the older design, which gives a fuller sound.

The mu-amp section better simulates the odd order harmonics of the amp's Class AB push-pull distortion.

For those without a 4x12 cabinet, the added Bottom control closely mimics the bass resonance of the famous Marshall speaker arrangement.

All these ingredients result in a sound similar to the classic Marshall amp.."

And here's a demo of the Thor in action;

It does a nice job of getting you near those Super Lead tones. Admittedly it's never going to get you all the way - the sheer volume and smooth clipping of real EL34 output tubes add so much to an amp's sound it's impossible to completely recreate a screaming Super Lead with just a few jfets running at 9v but the Thor does a damn good job!

Ok, so it's time to check the schematic;

It's a nice little circuit; 2 common source jfet stages driving a mu-amp stage with a bass boost circuit on the end. To understand how they picked the values for biasing those jfet stages you can read the Fetzer Valve article on runoffgroove. It's got some great info on how careful biasing of a common source jfet stage can result in the jfet having clipping characteristics similar to a standard tube gain stage with the correct frequency response and harmonic generation, it's a great read and highly recommended if you're interested in jfets :-)

For those of you wanting to build the Thor here's a perfboard layout;

And a vero layout;

For reference and the original Thor article check out the great runoffgroove website here; http://www.runoffgroove.com/thor.html