Experimenting with Great Cheddar!

[Slade]

Hello,
I built a Big Cheese clone from Geofex and I'm pretty much appreciated to R.G. and the developers of this pedal/project.

Well, I've been experiencing a little with this effect, mainly with the transistors, and after hours experiencing with different ones I finally get my sound with a BC108C in Q1, a 2N5088 in Q2 and a 2N3904 in Q3. Also, I installed a trimmer for the collector of Q2 (R8) to have more control on the sustain and the "semi-octave down" effect.

Anyway, I can't figure out the function of that third transistor in the circuit and I would like to know what is this transistor and the diode beside it for? I've noted that when I take this transistor out the sound gets louder and "less compressed"..
For the diode, I would like to experiment with different kind of diodes, but don't know if this would make any difference.

I didn't found much information about this project, and I think it's already one of the beast fuzzes I've built.

Greetings,
Fernando.

[Slade]

Well, thinking in that transistor with my friend Labaris, we conclude that the transistor is simply acting like 2 clipping diodes, and that would be the cause of the sudden loss of signal. So, there's an asymetrical clipping there. I'm gonna try some germanium diodes to see what happens.

PS: By the way, this what I'm talking about http://www.geofex.com/Article_Folders/grtchedrschm6.PDF

[Processaurus]

This is my favorite fuzz pedal, because it gets that nice, squishy shutdown when play harder, because it narrows the pulse width of the fuzz as the input signal gets louder.  Yes, Q3 is just used as a clipping diode (I think it only conducts one way), you could get similar asymmetrical clipping with an LED and a silicon diode, back to back, or Ge and Si, etc.

Here are some mods if you're interested:
http://www.diystompboxes.com/smfforum/index.php?topic=54568.0

Some good notes from the publisher of the project:

The basis of the circuit looks to be a dual-feedback shunt-series pair. This has a casual resemblence to a FF, but includes another feedback path from the collector of the second transistor to the emitter of the first; the FF has only one feedback path, emitter of the second transistor to the base of the first.

The chief characteristic of the shunt feedback input stage of both the FF and the shunt-series pair is that they are very low impedance inputs, and can easily be thought of as current amplifiers. The gain depends on the impedance of the driving source. This is one reason the FF is so sensitive to what drives it; guitar pickups have impedances in the tens of K's, and that's what sounds good. Driving it with an opamp output overdrives it pretty solidly.

The BC uses the same input, but puts a weenie 0.047uF cap in front of the input, between the base of Q1 and the driving opamp. This gives a steadily rising driving impedance with frequency, and is small enough that the bass frequencies don't overdrive it (See - The Technology of the Tube Screamer at GEO for some discussion of bass rolloff keeping distortion from being flabby).

Near as I can tell, from both scope tracing and simulation, the shunt-series pair produces a slope-sided square wave for any reasonable input. The opamp just blasts the input *hard* and the pair saturates both ways. Thankfully, the SS pair is simple enough that it doesn't do some of the ugly things that can happen from overdriving things like diffamps.

What it *does* do is produce a rectangular wave at Q2's collector that varies in duty cycle if not shape or size with the magnitude of the signal driving the input. It's touch sensitive over the entire range of reasonable guitar inputs. The actual unit has a variation from smaller to 50/50 and back smaller as input drive goes up. The simulation only shows the small to 50/50, so there's another reason to only trust simulators a little bit.

The BC is perhaps more interesting for the response of the SS pair under multiple feedback. The linear response (if you could put in a signal that small) has a response peak at about 200Hz, then flattens off out to about 10KHz. The SS pair shows some promise for adaptation to some other things than distortion pedals...

R.G.

[Slade]

Thanks for that!
I see your post last night and I find it really interesting.. That's experimentation!

[beatnik]

my great cheddar build in the fourth position of the rotary switch doesn't pass the sound. with fuzz pot maxed the sound is VERy gated, almost no sound.

on the other positions the pedal is working ok, so i am asking if this is normal or i have some kind of problem.

thanks in advance