The famous Mike Matthews Electro-Harmonix Big Muff guitar pedal. Pink Floyd used it. So did the Smashing Pumpkins. Jimmy Hedndrix gave it immortality. The list of artists is endless.
There are countless variations too. You can buy kits on the net with PCB & parts supplied to save you the hassle of understanding the circuit. But I like to know how things work. I'll build this on perf board.
So here is the basic circuit.
The signal comes in through R2 and C1 before hitting the first stage, a basic transistor signal booster (Q4). Once the signal is boosted, it goes through C4 and enters R24 (the sustain potentiometer). This controls the gain.
The signal goes through C5 and R19 before hitting the next stage, which is doing 2 things. Boosting and clipping the signal. The clipping is achieved via the diodes (D1 and D2) & C6/C12.
The signal then goes through C13 and R12 before hitting the next stage : tone control.
It's identical to the previous diode clipping stage apart from the tone pot (R25) which pans between a high-pass and a low-pass. This explains why when you turn the tone control down the sound is very bassy, while turning it up cuts the bass, and it gets very bright.
After leaving the tone pot, the signal goes through C3 and enters into the final signal boosting stage before exiting through C2 and going out of the volume control into the pedal’s output.
Many thanks to Kit Rae for a wonderful website.
If you haven't visited it yet here is the link:
This is the site to go to for everything you could ever need on this topic.
The first pedals came out in the late 1960s and over the last nearly 60 years the designs have barely changed. It says a lot about how good the basic design is. Of course there has been lots of tweaking over the decades. At one point the four transistor design was changed to op amps. And for another one of those decades Electro-harmonics transferred production to Russia.
Today the company is strong and still building Big Muffs. I highly recommend the modern versions (I own a few and love them) but I think it is fun and educational to explore the older variations. These go for astronomical prices on Ebay & Reverb today. Vintage "Triangle" and "Foxey Lady" pedals regularly change hands for over $1500.
If you can afford the real vintage thing go for it as I think nothing really beats vintage components. They have a persona which only the years can give. But probably the only chance I will have to experience something like the vintage sound will be to build my own. So below is a brief comparison between the different variations. It's a great way to explore the evolution.
Starting with the early "Triangle" pedal
|Circa 1971||Circa 1971/2||Circa 1972-73|
|R11||15K||15K (or 12K)||22k|
|R13||15K||10K (or 15K)||22k|
|C2||1uF (or .1uF)||0.1uF||0.1uF|
The values seem to have changed slightly over those 3 years.
There is a lot of scope to do some tweaking to find that sound that's right for your needs.
Maybe play around with carbon vs metal resistors? In the 1970s those carbon resistors would have been 10% tolerance or more ???
R24, 25 & R26 are the pots.
Vintage USA Big Muffs (V1, V2, V3) used single-gang, linear taper, 24mm,100k potentiomers.
A possible future mod could be to use B100K for R25 & R24 & A100k for the Tone (R25).
Log pots (A-100K type) have a longer usuable mid position and less at the ends. ... better fine tuning.
Could be useful for the tone section.
Leakage current for the 1N4148 as 5 µA .... I think the same as the old 1N914
What transistors to use???
They need to be NPN bipolar
BC549C, BC550, BC239, SE4010, and 2N5210 work according to Kit Rae.
2N5088, 2N5089, MPSA18, 2N3904, 2N4401 are possible candidates.
It might be best to use sockets to allow experimentation.
Were the transistors matched in the early pedals?
Kit Rae's valuable diagnosis identifies three sets of components which alter the BMPs the most.
1. The clipping blocking caps. (C6, C7)
2. The feedback / filter caps. (C10, C12, C11)
3. The Tone stack filter. (C9, C8, R8, R5).
The clipping blocking caps
Feedback Filter caps;
The pics of vintage units indicate ceramics.
The ToneStack filter.
These 4 components surround the tone pot (R25).
"Higher values have less mids scooped out and lower values have more mids scooped. This resistor also works in conjunction with the high pass tone capacitor (.0004uF in the example above) to affect the treble at low tone settings. Reducing this cap to .003uF alters the range and scoops slightly more mids. Increasing to .004uF reduces the scoop. Combined, a larger high pass cap and smaller resistor decreases the treble at low tone settings." (Kit Rae)
The pics of vintage V1 boards suggest C8 is a polyester & C9 be ceramic.
Muffs Guts (Kit Rae)
Electro-Harmonic's Time Line
To be continued......