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Formula No. 51 (Low-Watt Tweed Amp Emulation) Guitar Effects Pedal

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A compact pedal that harnesses the power of the 5E3.

The Formula No. 51 was inspired by old tweed amps, most notably the 5E3 circuit. The character of the 5E3 is very unique. It has very minimal power filtering, low plate voltages, elemental tone stack and does next to nothing to control the low end between gain stages, not to mention the underrated output transformer and speaker. The entire system runs inefficiently resulting in a loose bass, greasy mids, cutting treble, and incredible grit—none of these qualities were ideal or intentional back in the day. In spite of this, over the past five decades, the amp inspired some of the coolest, most sought-after guitar tones ever recorded.

Catalinbread has captured the unique qualities of these amps in the Formula No. 51. For this circuit, they felt cascaded JFET gain stages yielded the best result. They have a softer sound than MOSFETs. JFETs also have much more natural attack/decay than diode-clipped rigs which are pingy on the attack and fizzy on the decay. They chose capacitor types that enhanced the midrange and burnished the treble.

Volume controls overall output volume and is capable of overdriving the input of your amplifier for additional saturation. Having this much output on tap to boost your amp can effectively give you the same type of "channel volume interaction" found on the amps when you balance it with the Gain control.

Tone is a simple high-pass filter arrangement like on the amps that decreases in its effect near maximum Gain settings. Goes from smooth and buttery to extra crispy at full up.

The Gain control begins where a 5E3-style amp would be with its Volume on about 3 or so and goes beyond what a typical 5E3 circuit would deliver gain-wise. If you've ever played a real 5E3, you know that it doesn't really come alive until you open it up some—clean tones are in there, but you have to get to them via the guitar's volume controls. The Formula No. 51 is no different. Humbuckers and higher output single-coil pickups will overdrive the Formula No. 51 fairly easily, so if you are playing a guitar with these type pickups go easy on the Gain control at first unless you want to channel Neil Young out of the gate.


  • The famous tweed amp sound in pedal form
  • Goes from sweet and singing to ragged and raging
  • Tone circuit functions as a bright cap or treble rolloff depending on Gain
  • Great foundation for fuzzes and boosts

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