In fact, you see this in many "Golden Age" fuzzes, which have Ge transistors with NO bias into the base it's not zero biased, the leakage is biasing it.
![high hfe silicon fuzz face high hfe silicon fuzz face](http://diy-fever.com/wordpress/wp-content/uploads/2012/09/fuzz_schem-1.png)
If this were the only problem, you could deal with it by tweaking the bias point. Leakiness is exactly the same effect as putting a large value resistor from the collector supply to the transistor base, it simply adds extra current into the base - and hence the collector - of the transistor. My last batch of AC128's was 75% unusable because of leakiness. The soft distortion is the result of the mushy saturation of the voltage feedback first stage, and the harder distortion is the result of the cutoff clipping of both first and second transistors. The first transistor's properties dominate the "soft" distortion sounds, the second one's dominate the "harder" distortion settings. In theory, the mismatch changes the amount of asymetry in the clipping. The first one can be as low as 70 and the second can be up to 140 (assuming that this is real gain, and not leakage as measured on a DMM gain test). The "two different gains" data was also posted by Mike Fuller about the FF some time back on the newsgroups. I've really been looking for that sweet spot, but from the beginning builders were reporting great ff's with silicons and extremely high hFE's, initally even Jack Orman thought low gain to be essential to FF sound - though all the time people were building great si fuzzes with enormous betas:Įvery MPSA18 I've ever measured has had an Hfe over 1000. ( Mike also prefers transistors with only certain colored epoxy sealant, which I can't see making any difference except coincidentally, but then, who knows?) He noted that he feels that he can affect the relative amount of symetrical versus asymetrical distortion by selecting for non-identical gains in the two positions. Mike Fuller, maker of the Fulltone "69" pedal, posted his preferences for Fuzz Face transistor gain to the usenet news groups, and they fall right in this range. Keeping in mind that preferences for distortion tone are definitely a matter of personal taste, the range of gains for unselected AC128's in this circuit would produce some really clunky-sounding devices. If you allow combinations of one high and one low gain device, the range widens out to 70 or so on the low end and perhaps 130 on the high end. There is a definite sweet spot for musical sounding clipping at transistor gains of about 80-110.
![high hfe silicon fuzz face high hfe silicon fuzz face](http://3.bp.blogspot.com/-wikMAbklni0/U4ZMQGJYVuI/AAAAAAAAHg4/PW1owJhLhoU/s1600/Eric+Johnson+Fuzz+Face.png)
![high hfe silicon fuzz face high hfe silicon fuzz face](http://1.bp.blogspot.com/-Dq3PZC6Hx20/VGJO3swCsaI/AAAAAAAAC-Y/rmDcr6TsS9k/w1200-h630-p-k-no-nu/Fuzz%2BFace.png)
Technology of the Fuzz Face wrote:I've done a lot of circuit simulation on the FF, twiddling the values of the transistor gains, and looking at the clipping waveforms and resulting harmonic spectra.