Silicon Fuzz Face build report

[mordechai]

So I just put together your friendly neighborhood Silicon Fuzz Face clone, with the following features and tweaks:

Q1 - BC107A Hfe 190
Q2 - BC109B Hfe 297

I put a 39pF cap across the BC of Q1 and a 68pF cap across the BC of Q2.  With input/output caps of 1uF/.01uF respectively, the tone was surprisingly smooth, with JUST enough bite to distinguish it as a higher gain beast than the standard germanium FF.  The small pF caps across the BC of each transistor made a really, really big difference.  I also incorporated a bit of power filtering with a small series resistor and a 100uF cap coming off the battery, and this helped as well.  Sustain was excellent, and very little background hiss.

Really a major improvement over the BC108 Dunlop FF reissue -- that one didn't sound bad, but this one really is in another league, on par with my BC183 Sun Face. 

[deadastronaut]

the pf caps across BC on both Q1/2 definately take the noise and harshness out......ive experimented with those too...

[mordechai]

I think the key to retaining the familiar silicon tone, though, is to keep those pF caps really small when the transistors aren't super high gain.  I tried building this with Q1=320 and Q2=550 (both BC108, IIRC), and I put pF caps on those as well (82pF on Q1, 120pF on Q2).  It helped, but there was still quite a bit of hiss, and the tone was just too harsh and ragged.  I experimented with input cap replacement down to a .1uF, but the end result was too "choked" a sound.  I also used the Axis Face trick of putting a cap (.033uF) across the power rail resistor (1K) between the emitters of Q1/Q2 to curb high end, but this didn't make too much of a difference. 

On this version, since Q1/Q2 are of a bit of a lower gain, I think it automatically cut out most of the nasty hiss and ragged edges.  Then the much smaller pF caps just rounded it out a bit more without sacrificing the hoarse growl I wanted.  I put in higher value pF caps on these and it didn't sound right -- actually, it sounded a little too muddy and wimpy.  With the lower value pF caps, it still sings and gets aggressive but doesn't bite too much.  I also still have the cap (.022uF) across the 1K power rail resistor which adds just a little bit more warmth, but it's in a socket and I'm thinking of leaving it out as the tone without it is still very nice, and "rings" a little more when it's not in place.

[joegagan]

that is a cool report. thanks for the detailed info.

one of the things i liked about Si fuzzfaces that have higher gain transitors is the ragged ' ripping' quality and the hint of upper octave. does your lower gain trans version still do this?

[mordechai]

Actually Joe, it does indeed.  I think that the 400-600 Hfe transistors probably do this mroe readily but of course, they bring their own tonal problems to the table.  I do think, though, that a "medium" level Q2 Si transistor of about 300 Hfe can still do this in a slightly more subtle way...the rippin' tone and octave overtones are still in there, just not quite as prominently.  When I open up the volume control to about the 2-3 o'clock mark, they become a little more pronounced. 

[Kesh]

Never gone for high hfe silicon. But made a few nice Si FFs with hfe close to the Germanium originals. Also have placed a pot in series with the Q2 cap mentioned above.

[Gus]

something from a sim with a grounded emitter at Q1.  try 2n5088s 2n5089s etc.  R7 is a 0 to 5K rheostat bias control, R8 and R9 are one 100K volume control, R5 and R6 are one 1K gain control.