Pretty much everyone has a go to classic pedal. Mine is the Big Muff.
A pal of mine used to have a proper Black Russian Big Muff with a modded tone stack which he played his bass through. It sounded class. I had a chance to steal it, but I didn’t cos I’m nice like that. I walked away with gut shots.
So when I first started breadboarding I naturally went to the Muff circuit (If you like Muffs check out the Kitrae site. It’s got everything you’ll ever need to know about Muffs). Here’s a schematic!
For some reason that eludes me now I started off with the values for a tall font green Russian and messed around with it as much as I could. It’s a great circuit for playing about with.
I popped in a couple of MPSA13s for Q1 and Q4 (Q is schematic shorthand for transistor, so Q1 = the first transistor, Q2, the second, etc.), mainly cos I thought that would make it really ballsy. My guitar is tuned to B (because a couple of friends beat me to baritones and I didn’t want to look like a big old flabby hairy copycat). I think these two factors led to it being a little too boomy. I found that the sustain wasn’t as long as I’d hoped either and the decay was pretty sharp and sounded crap. I think the signal was getting a little squashed because the transistors hadn’t been biased properly and there was too much boost on the low end which ended up smushing the other frequencies and a loss of clarity.
I tried a couple of different tone stacks using the tone stack calculator to try and boost the mids, and finally settled on the one below. I wanted a switch to boost the mids too, so I put two resistors in parallel (12K and 27K) and an SPDT switch in between so I could flick between 12K and 39K. If you’ve ever bypassed a Big Muff tone stack you know how much volume you lose with it in there. I was trying to avoid that, so I tried to get the mid position of the tone knob to be as close to 0dB as possible.
Here’s what I tried (the different values of R2 are to represent the switch being on or off):
It looks good right!? The problem is it doesn’t do as much as it looks like it should, largely because my bottom end wasn’t in check, but generally as a tone stack in this circuit it was almost entirely pointless. there’s no point putting a knob on the top of a pedal if it isn’t worth tweaking. I’ve since changed the tone stack but I’ll get round to that in another post, with samples of both.
I also played around with optional hard asymmetric LED clipping on a DIP switch just before the output, I cut this out in the end because it wasn’t adding much, a tiny bit more dirt. Also the clipping diodes in the Big Muff only clip certain frequencies, set by the capacitors by the diode pair above Q2 and Q3. It’s best to clip the top end of a signal and those small value capacitors do exactly that, they make sure you clip the treble end. So this probably only added to the muddiness. I don’t remember, but that makes sense. it didn’t work in this case, but I’d try it again with a less gainy circuit to add more bite in the future. The idea came from a wonderful article by Aron Nelson which I’d highly recommend reading if you want to tweak any overdrive/distortion/fuzz circuit.
The one bit of experimenting that I held onto was taking out the second set of clipping diodes. I tried several combinations, but found that without that second clipping stage it sounded so much more open.
Everything that I tried was worth trying, it’s all trial and error, at this stage certainly, and as they say nothing ventured nothing succeeded, practice makes purple, etc. Knowing why things don’t work and when to try them is important.
Anyway, I originally gave this circuit the name Fat Gett because it was so boomy, but the real Fat Gett was born out of this Big Muff circuit and a happy accident, which I’ll discuss soon.
*gett = Geordie for git. just so you know.