The Harmonic Percolator was a guitar effects pedal, a distortion stompbox produced in Milwaukee, Wisconsin in the 1970's. This effect's unique ability was to filter out odd-order harmonics, while allowing even-order harmonics through (if you're confused by the even/odd order harmonics business, check out this and this). It's claim was that even-order harmonics are more aurally pleasing than their odd counterparts, and hence by filtering the odd harmonics, a more musical distortion could be produced. Ironically, of those who used the Percolator religiously, Steve Albini of Big Black may have been the most famous. This is ironic because Albini used the unit to produce a sound that has been refered to as a "refined grind" (indeed it is a very harsh, almost square wave distortion).

They are somewhat rare today, as there were only a small number produced. They have also been difficult to reproduce, largely due to their use of Germanium transistors. Being that Germanium has been nearly replaced by the more consistant Silicon (Germanium transistors are inconsistant, even unreliable due to the nature of the element, and are known to 'leak', or bleed voltage), finding replicate or similar transistors is a difficult affair . One company, which deals largely in Theremins, claims to have produced a small number of exact replicas, using almost entirely new old stock (NOS) parts. Some originals still float around eBay and used music shops, but can be quite expensive (expect to pay upwards of $175 for originals or replicas).

Another option is the DIY route. There are a few websites (centered around guitar effects projects) that host schematics, said to be at least mostly accurate. Many of them use alternatives to the Germanium transistors (2N2904 and 2N2906 silicon transistors seems to be the most popular), which make it a cheap project. Whether or not these designs produce exactly the same tone as the original did, the principle is there, and it is at least an intriguing concept. With the gain set low, the box gives a smooth overdrive, a supplement to your amp's own sound, and cranked, it's pure rough industrial grind.


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