Author Topic: What is a good gain for Q1andQ2 BC 108 Si  (Read 1622 times)

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tjmicsak

What is a good gain for Q1andQ2 BC 108 Si
« on: July 23, 2009, 08:34:58 AM »
I read about the typical ranges for Ge trannys, but what about using Si?
Is there an optimum gain range for Si as well whcih would be somewhat higher numbers?
I have some AC 127s and BC108s and BC 108b for both NPN Ge and Si Fuzz Face and would like to know a good starting point as by the time I get them swapped out they either are warmed up from handling or I simply loose audio memory of the sounds.

petemoore

Re: What is a good gain for Q1andQ2 BC 108 Si
« Reply #1 on: July 23, 2009, 09:46:44 AM »
I read about the typical ranges for Ge trannys, but what about using Si?
  Si's ~don't leak. Ge's gain means: Gain = Gain - leakage. And leakage tends to = more noise.
Is there an optimum gain range for Si as well whcih would be somewhat higher numbers?
  Oh yes, try lower or higher gain transistors, the FF cares most about this. It has the ability to be very high gain, making a reliable oscillator. Adjust bias after swaps.
  Lower gain FF's tend to be less harsh.
I have some AC 127s and BC108s and BC 108b for both NPN Ge and Si Fuzz Face and would like to know a good starting point as by the time I get them swapped out they either are warmed up from handling or I simply loose audio memory of the sounds.
  Considering the operational temperature may help keep bias in range during use.
  Watch the orientation/diode polarity of the transistors...adjust bias...
  I found it's difficult to tell what it's going to do until it does it. and it does it different to an extrodinary amount when I swap amp or try to adjust volume. I built two and started to race-tweek them, really took a while to find out what I wanted from what it can do. Pretty much an Axis Face or a Ge model, after lotsa swapsa [especially on that GE circuit, the Si...I think I could have just popped in the Q's [measured for gains based on JD's and RG's pages], biased, and adjusted the gain knob.
Convention creates following, following creates convention.

jrod

Re: What is a good gain for Q1andQ2 BC 108 Si
« Reply #2 on: July 23, 2009, 09:52:15 AM »
Man I really like about 60-70 for a Si Fuzz Face!

BAARON

Re: What is a good gain for Q1andQ2 BC 108 Si
« Reply #3 on: July 23, 2009, 09:52:57 AM »
BC10x transistors aren't heat-sensitive like the AC127s are because they're Si, so you needn't worry about that aspect when you're swapping them out (as long as you test the Ge ones first).

If you're trying to get the classic old-timey Fuzz Face sound, the gain range is the same for Si as it is Ge.  RG's article suggests about 70-130 hFE with the lower gain transistor being Q1.  If you want a higher gain sound, Steve Daniels of SmallBear suggests 90-120 for Q1 and 150-190 for Q2.  You'll probably find that your BC10x transistors are higher than that though.
B. Aaron Ennis
If somebody makes a mistake, help them understand what went wrong.  Show them how to do it right.  Be helpful.  Don't just say "you're wrong, moron."

slacker

Re: What is a good gain for Q1andQ2 BC 108 Si
« Reply #4 on: July 23, 2009, 12:47:19 PM »
I like BC108s or BC109s with gains of around 350. This won't sound like a nice germanium fuzz, but it will get that Dave Gilmour, Live at Pompeii or Dark side of the moon sound, or as close as I'm ever going to get to it.

mac

Re: What is a good gain for Q1andQ2 BC 108 Si
« Reply #5 on: July 24, 2009, 03:10:31 PM »
There is a trick to decrease q1 gain using a 100 resitor at the emiter. Or a trimmer until you get the right gain.
q2 gain is controlled by the drive pot. set it a little before max.

mac