The original Musitronics-produced Mu-Tron III Envelope Filter was created in 1972 by Mike Beigel, and was the first envelope filter-style pedal on the market. The filter was designed to turn the volume profile of each note plucked on a guitar (or struck on a keyboard or other instrument) into a vowel-like sound, by gradually opening up a filter to let through more treble (or bass, depending on the setting) and then closing it to let through less.
For example, when set to the Low Pass setting, low-end would be unrestricted, or pass through. But treble frequencies would be treated differently. At the lowest point fo the volume waveform of each note the player would hear an almost all bass sound, while peak of the volume waveform of each note, all frequencies would be permitted to pass through. The result is a “wahaw”-like sound that was so unique as to catch the ears of many of the eras most innovative instrumentalists, including Frank Zappa, Bootsy Collins, Stevie Wonder and, of course Garcia, who became one of the pedal’s most prolific users.
After ceasing production, the pedals became harder to find, and while several attempts were made to copy the Mu-Tron’s magic (including the EXH Q-Tron, which had input from Beigel himself), none quite had the magic of the original until Beigel himself started a company of his own and released the Tru-Tron 3x in 2014. The Tru-Tron 3x quickly sold out and, without additional production runs, prices on the used market soared.
Trey first added the Tru-Tron 3x to his rig for the 2015 Fare The Well shows with the “Core Four” surviving members of the Grateful Dead — a necessity given Jerry Garcia’s association with the pedal. To the delight of many fans, the pedal stuck around when Trey brought out his rig for ensuing Phish tours, before disappearing in Summer 2017. It briefly re-appeared for the last show of TAB Fall 2017 tour, and something tells me it’ll be back in the rig by New Year’s.
Trey’s Mu-Tron sound has a unique growl to it, a bit more Zappa than Garcia. Listen to a sample and read more in the thread below:
Note: Trey has at least two of these pedals, one of which is branded as “Tru Tron” while the other is branded with the original “Mu Tron.” Apparently the new company had an intellectual property issue in getting the rights to the original name for US production so that name was only used for European models.