Electro Harmonix Big Muff Pi

Troubleshooting Example:

Noisy Transistor

I regret I have no detailed notes for this repair. The photos are from May 2012.

For whatever reason, I took a photo of the 1966 CTS pot. The pedal is not from 1966. Electro Harmonix just happened to make a lot of their pedals using a batch of pots that were made in 1966. Don't let this fool you. The code for CTS is 137, and then the year is then next two digits, and the week of that year follows next (not visible in photo).

Kit Rae covers pretty much all the info you'll ever need for the Big Muff Pi. Although there is much minutiae between the various versions of the pedal, the guts of everything up to the late '70s op-amp version are very, very similar in the end result. I'm sure you can find 2 that sound different enough from a player's perspective, but a casual listener to someone playing 2 set roughly the same would probably not notice much difference.

This similarity is due to a great circuit design from Bob Myer. Each transistor stage has plenty of feedback so that variations amongst transistors, temperature, and even the supporting components largely do not affect the final result. You can increase or decrease cap values to get more or less bass, and time can also do that to the caps - thus creating more variation amongst vintage units over time. I vaguely recall that this particular "Triangle" variation had a somewhat "trebly" response. But I even asked my coworker if he heard a difference, and the response was less than enthusiastic. It was just a Big Muff.

Probably the most common failure I have seen is a failed electrolytic coupling capacitor. Well, the most common failures are definitely broken or faulty switches, worn out jacks, broken battery snaps, dirty and/or broken pots... but those things fail for every pedal. Big Muff specifics - there really isn't much that comes to mind. For how sloppy they appear sometimes inside, they have a really good circuit design and seem to hold up pretty well over time.

What I find most interesting about the Big Muff Pi is the theory that you can trace the evolution of the Tube Screamer back to the Big Muff. This theory stems from a couple things. First, the Big Muff is the first known example of using diode shunt feedback in a guitar distortion unit. Diode shunt feedback for clipping purposes goes back to the vacuum tube days, but Myer was the put that idea to use in a commercially available fuzz box. After the Big Muff was a hit, it was widely copied. Maxon, who would eventually put out the Tube Screamer for Ibanez, was making such copies. The Tube Screamer's tonal success has been attributed to the diode shunt feedback amongst its other features. I checked out the evolution of Maxon/Ibanez schematics through the '70s, and I see a chain of small changes that go from Big Muff copy to Tube Screamer. Whether or not you see such a chain is another matter. But I like to think that the evolution exists.

Shots of a "Civil War" Russian Big Muff.

An extremely "road worn" Russian Big Muff Pi. The original stomp switch was dead, so a Carling was subbed in its place.