Adjustable transistor bias mod on a Big Muff

[CA_Dan]

How can I acheive this mod on a current big muff? I am trying to emulate what Earthquaker Devices did on the Dirt Transmitter. "The bias control affects the voltage to the transistors". Not sure which transistors he refers to. If all of them, would just varying the input voltage acheive the effect? I've seen a voltage bias used on a basic germanium fuzz, but couldn't find anything out there for silicon fuzz pedals.

Earthquaker Devices Website

Youtube Demo

[11 Gauge]

Originally Posted by CA_Dan

How can I acheive this mod on a current big muff?

Nice and easy, that one is.

You're just replacing the resistors from the emitters of the transistors, which then connect to ground. They're pretty easy to locate in a Muff, as they are low value resistors (less than 1K, as low as 100 ohms).

...The initial boost and final recovery stages of the Muff are probably okay as is, and those transistors are actually a little higher in value (the one on the recovery stage might be as high as 3.3K). You can probably ignore those two. It's probably the middle two that are being targeted - the stages with the clipping diodes. If you intentionally misbias them, you'll get all of that gating and sputtering sort of stuff happening.

If you're doing your own thing, this obviously gives you extra options. You can replace the emitter resistors with trimpots, or you can temporarily wire pots to separate transistors. I'm assuming that the pedal in question uses a dual ganged pot to misbias a pair of transistors with a single control.

...You can also destabilize the transistors with pots in place of either the resistors that go from base to ground, or the collector resistors as well. Or you could put a "starve" pot on the positive voltage into the pedal, and just keep cutting the juice until it sputters. Lastly, you could replace the two 470K resistors in the feedback loops of the two middle transistor gain stages with 1 meg pots (or a dual ganged unit), and adjust the transistors until they go into cutoff, to your liking.

That's the cool thing about BJT transistors - there's plenty of ways to misbias them to get the sputtering stuff.

[CA_Dan]

Awesome - thanks 11 Guage. I was thinking the middle two emitters. Thanks for all the options and ideas!