Keeley Super AT MOD

The Keeley Super AT Mod is a limited edition overdrive pedal created by Robert Keeley and guitarist Andy Timmons, released in 2023 and limited to 1,000 units worldwide¹²³. Inside is a modified Boss BD-2 Blues Driver circuit, and the pedal carries two voicings on a toggle switch: the original Keeley Phat mode and a new AT mode, Timmons' reinterpretation of the Phat Mod, voiced for humbuckers with a tighter, more transparent low end⁴³.

Keeley describes the design goal as making the circuit sound and respond like a tube amplifier in a stompbox, with low-gain heat and crunch that cleans up nicely¹². On Premier Guitar's demo, John Bohlinger called it "a 3-knob stomp with two modes of harmonically rich distortion"⁵.

What it does: Andy Timmons' take on the Phat Mod

The Super AT Mod is the second pedal Keeley built in collaboration with Andy Timmons, and it captures what a JHS Pedals demo called "the more subtle overdrive side of Andy," as opposed to a big distortion sound⁴⁹. Timmons had used Robert Keeley's mods of the Boss BD-2 for a long time, and the signature pedal grew out of that relationship⁸.

The core of the pedal is a further modified Blues Driver. Delicious Audio describes it as a refining of a circuit that started as a mod Robert Keeley did to Bluesbreaker pedals and developed into the Phat Mod¹. The Timmons mod accentuates both the tone control and the gain creation, which in Bohlinger's demo made the circuit feel more amp-like⁵.

Keeley says the topology is similar to classic California tube amps, with JFET transistors substituted in place of tubes, giving the pedal what Gearnews called "an amp-like feel" and a very touch-sensitive overdrive suited to expressive playing⁴².

The controls: Level, Tone, Drive and the AT/Phat switch

The layout is the familiar three-knob Blues Driver arrangement, plus a small toggle for the two modes⁴².

The mode switch is the pedal's main feature. Where the earlier Super Phat Mod's toggle switched between Flat and Phat EQ modes, the Super AT Mod offers Phat and Timmons' own AT mode².

AT mode, Phat mode and pickup choice

The two voicings are aimed at different guitars. AT mode features lower mid-bass frequencies so humbuckers sound tighter and more transparent; Guitar World describes it as voiced specifically for humbuckers, carving out some of the low-mid and bass EQ range¹⁷. Phat mode keeps the original Phat Mod circuit with a little extra low end, giving single coils more depth, body and power¹⁴.

Andy Timmons has stated directly which to use with which guitar:

Asymmetrical clipping with diodes and LEDs

The clipping stage is where the AT mod does its work. In AT mode, the clipping diodes use a combination of regular diodes and LEDs to produce asymmetrical tube clipping¹. Gearnews describes the same design: a Blues Driver circuit that clips asymmetrically based on regular diodes and LEDs⁴.

The reasoning, as Delicious Audio explains it, is that LEDs clip or distort the signal at a higher voltage than regular diodes, and asymmetry enhances the tone by giving the ears something more interesting than a basic square wave¹. Bohlinger put it more simply in Premier Guitar's demo: the mod "makes clipping asymmetrical, essentially one half of the sine wave is not clipping as much"⁸.

Asymmetrical clipping also occurs naturally in tube amps: tube biasing, output transformers, and the balance in a push-pull output section can all cause it, which is part of why the design is described as tube-like¹. Guitar World and MusicRadar characterize the clipping as asymmetrical JFET transistor-based, with MusicRadar likening the tone profile to a Californian tube amp²⁷.

The lineage goes back further. Keeley's original BD-2 mod changed diode D3 to a 1N4002 for asymmetrical clipping, adding second-order harmonics, a modification documented on the Guitar FX Layouts project page for the Blues Driver¹⁰.

History: from Keeley's Blues Driver mod to a limited edition

Robert Keeley began modifying the Blues Driver at customers' requests after he had done a Tube Screamer mod, as he recounted on That Pedal Show¹¹. That work grew into the Keeley Phat Mod, a modified version of the Boss BD-2 Blues Driver¹², and later into Keeley's own Super Phat Mod pedal, the unit that forms the basis of the Super AT Mod⁷.

The Super AT Mod's clipping is a return to Keeley's original clipping style, a 20-year-old modification first performed in the Keeley Blues Driver Phat Mod¹⁴. Released in 2023 as a limited edition of 1,000 units worldwide, handmade in Edmond, USA, it shipped in a metallic red enclosure similar in design to the Super Phat Mod's³²⁷.

On pricing, the press disagreed at launch: Premier Guitar's demo and Guitar Pedal X gave $199, while Guitar World gave around $249³⁴. MusicRadar listed it at £219 in the UK, and Gearnews cited an MSRP of EUR 259 with a Thomann launch price of $215.00⁴².

Guitar Pedal X's reviewer compared it against a modded BD-2 and found the Super AT Mod, in both modes, "somewhat better honed to Andy's style of playing," cutting through a little easier but sounding ever so slightly thinner overall³.

What came after

The creative partnership between Keeley and Timmons continued, with the Muse Driver following as a 4-in-1 drive pedal¹³. Writing at Guitar Pedal X in January 2024, Stefan Karlsson said he expected the Muse Driver to replace the Super AT Mod in his own best-of-year list, though that was the writer's view rather than a Keeley statement¹³. Keeley has since carried the Phat Mod circuit forward in other forms, including the Keeley Tube Drive, which combines the Super Phat Mod circuit with a high-voltage 12AX7 tube preamp.

Review and demo videos

Premier Guitar's First Look demo with John Bohlinger covers the pedal's background, the asymmetrical clipping mod and how the Timmons voicing changes the tone control and gain creation⁵.

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A second Premier Guitar video goes deeper on the clipping design and Andy Timmons' long use of Keeley's BD-2 mods⁸.

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JHS Pedals' demo places the Super AT Mod alongside the other Andy Timmons drive pedals and shows the more subtle overdrive side of his sound⁹.

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