Fuzz Face biasing Q

[Johnny Guitar]

Hello,

I'm wondering if there is a general equation for biasing the transistors of a Fuzz Face involving the hfe of the transistors used in the particular circuit, or do people just plug resistors in trial and error?

I'm working on another Fuzz Face project. I have an etched board and two germanium transistors 2n388a. I got both the board and the transistors from a VERY reliable source. When I check the hfe on a meter I get 82 for one and 115 for the other which definitely puts them in the ball park for Fuzz Face usage.

I filled the board (a few years ago) using the original resistor values of a Fuzz Face. The circuit passed a very attenuated and undistorted version of the original signal. I've come back to the circuit now having come closer to success on my second FF with transistors I got from Small Bear. I'm hoping I can get it to work well without too much trouble.

Thanks!
J

[b_rogers]

i would measure q2's collector and adjust that resistor until you get from 4.5-5v.  maybe a 20k trim pot wired as a variable resistor.

hope this helps

Brent

[brett]

AFAIK, it is certainly possible to calculate the resistance required for Q2's collector.  But there are at least 2 factors which work against this approach; knowledge of the parameters for the equation (hFE, leakage, etc for Q1 and Q2, supply voltage, etc...) and the sensitivity of the sound and voltage on Q2 to the resistance.  The latter effect means that a trimpot works better than fixed resistors, because you can get *just* the right sound.  And once you put a trimpot in, you don't need the equation - you can just "twiddle".   Anyway, that's why I've never bothered with the maths...

cheers

[cd]

You HAVE read everything on the FF at www.geofex.com, right?

[GKinTN]

Seems many put a trimmer in place of the 8.2k, however I have had better luck putting a trimmer in place of the 33k and using stock values everywhere else.   Seems most trannies I've had bias out at 5.5v on Q2 with the 33k value trimmed back down to around 27-29k.

[Johnny Guitar]

You HAVE read everything on the FF at www.geofex.com, right?

Yes, maybe, but not necessarily understood it all  :?.

Thanks to the "BR"s who mention replacing an R with a trimmer. I have thought of replacing the 8.2K R of the "classic" circuit with a 10K trimmer (easy enough). Maybe that's all I need to do!

I did want to check that though since the germaniums I got from Small Bear indicated three value changes of resistors. With the particular transistors I got, a Q1 hfe of 88 and Q2 hfe of 132, Steve @ Small Bear recommends:
1) 39K in place of the 33K
2) 140K in place of the 100K
3) 3.9K in place of the 8.2K

I'm not sure how much the first two effect the operation.  I am hoping not to replace all three with trimmers. I was hoping to see the rationale behind his selections.

J

[Steben]

Seems many put a trimmer in place of the 8.2k, however I have had better luck putting a trimmer in place of the 33k and using stock values everywhere else.   Seems most trannies I've had bias out at 5.5v on Q2 with the 33k value trimmed back down to around 27-29k.

I agree, this seems to work well.
After all, this the way the RM's CLASSIC flying saucers are biased.

[petemoore]

I'm suggesting the Multi Face from ROG.
 This one is very flexible, and I like them...both of them...I have two, and have them compete...one has a Mosfet, the other has a BCxxx...basically I forget because I just twiiddle 'till I like it and put the bottom plate back on the box...
 Trimpotting Q2 is what I like to do. Otherwise, I tried trimming Q1, but found the 33k-100k works just fine, the trimpot there has the effect mostly from what I can tell of just turning that transistor off with very low resistance...didn't notice much [if any] difference between 33k and 100k for that Q1 collector resistor.