This is a bit of a Frankenstein consisting of the Octave UP circuit borrowed from the FOXX Tone Machine, the Octave down from Joe Davisson’s Shocktave. Both are fed into a Roger Mayer Mongoose. The name came from listening to a lot of early Genesis stuff while breadboarding this…
The artwork was made with a nice drawing from my lovely wife 🙂
I stumbled upon the Mongoose by coincidence and decided giving it a try. Turns out it’s a GREAT distortion. It has some similarities with a RAT but has a gain stage before the LM308 which gives it a lot more oomph. It has great sustain and can cover quite a lot of distortion territory. Somewhere between a Fuzz, a distortion and even overdrive on milder settings.
The original Mongoose has the LM308 directly DC coupled to the output of the transistor gain stage that provides a bias of 5.5V. Since I had to separate the transistor gain stage from the LM308 to insert the mixing stage in between, I experimented with different bias settings. Turns out that this slight off center bias gave the distortion its particular character. It makes the op amp clip asymmetrically. With the usual 4.5V half supply bias it sounded a lot more behaved, not quite the same character. I found that I could create lots of different sounds by just changing the bias voltage so I added a “Stability” pot that shifts the bias point for the LM308. CCW it literally starves the op amp and allows to dial in some gritty, gated and full CW you have 4.5V.
I’m not much of a parts snob usually (especially when it comes to op amps) but in this case the LM308 really does seem to make a difference (as has been said for the RAT). I tried several options and none was really doing the same thing. The worst was a 5534… ugh … The closest I could find was a TL070.
For the tone control, I tried lots of different incarnations of the BMP tone control but decided to go with the WSTC in the end. Simple and effective.
I changed the gain control a bit. Reduced the gain pot from 10k to 5k. On the original circuit, when turning down the gain, the cutoff frequency also changed and gave a muffled sound. To avoid this I added a fixed gain path (a 4k7 resistor and a 100n) cap that adds some clarity on cleaner settings and is not noticeable on heavier settings.
The input signal is first buffered by the first op amp stage and split to the input of the FOXX and the Mongoose input transistor stage.
The output of the 2 octaves and the Mongoose transistor stage are then mixed together with an inverting op amp stage that goes to the LM308.
The octave on/off switch disconnects the 2 octaves from the mixing stage.
Not much to say about the 2 octavers. I did not change anything on the stock FOXX circuit except for using Shotkys instead of Germanium diodes.
The Shocktave takes its signal from the collector of the second FOXX transistor. The signal there is already boosted and filtered (some treble is removed) and goes into a SHO-like MOSFET booster (replacing the original gain stage) and another low pass filter before hitting the Shocktave.
All in all I’m pretty happy with how it turned out. I just wish I could have done the signal routing and mixing a bit simpler and also would have preferred not to separate the Mongoose’s input gain stage from the LM308. There are plenty of sounds in there. Along with a good solid fuzz/distortion, it can do jackwhiteblackkeys stuff, neilyoungbrokenfendertweed stuff, even synth-like stuff.