Bias control on Fuzz Face

[gaz ariff]

I've just started to build pedals (2 Fuzz Faces so far). I've been reading up on tweaking them. I've heard of bias controls, but as a beginner i'm not too sure how to add on a variable bias control. The schematic i've been using is a NPN BC108 transistor circuit. I've tried to put a link on here, but i can't seem to get it to work. So i've just put the web address in.
Thanks, Gaz

http://www.muzique.com/schem/fuzzface.gif

[vendettav]

well a bias would go in place of R1 i guess, you'd want a 100k trimpot in there. write it the following way: lug 3 is connected to 9+ and lug 2, lug 1 goes to the Collector of Q1 (again, it replaces R1). you want to get 4.5v in at Collector of Q1. then you can measure the resistance of the trimpot and put a fixed resistor of the same value in there. i hope im right cause i don't have much of tweaking experience with Fuzz Faces

[Scruffie]

well a bias would go in place of R1 i guess, you'd want a 100k trimpot in there. write it the following way: lug 3 is connected to 9+ and lug 2, lug 1 goes to the Collector of Q1 (again, it replaces R1). you want to get 4.5v in at Collector of Q1. then you can measure the resistance of the trimpot and put a fixed resistor of the same value in there. i hope im right cause i don't have much of tweaking experience with Fuzz Faces

It's standard practice to put it on the Collector of Q2, i.e. a 10k Replacing the 8k2, otherwise you're correct 

[vendettav]

^ oops thanx for that... i've never done it with FF like schems so i couldnt tell 

[LucifersTrip]

well a bias would go in place of R1 i guess, you'd want a 100k trimpot in there. write it the following way: lug 3 is connected to 9+ and lug 2, lug 1 goes to the Collector of Q1 (again, it replaces R1). you want to get 4.5v in at Collector of Q1. then you can measure the resistance of the trimpot and put a fixed resistor of the same value in there. i hope im right cause i don't have much of tweaking experience with Fuzz Faces

It's standard practice to put it on the Collector of Q2, i.e. a 10k Replacing the 8k2, otherwise you're correct 

yes, with a 2k-5k resistor cutoff in series with the 10k pot. Adjust the 10K pot to get Q2's C voltage to 4.5.

[Scruffie]

well a bias would go in place of R1 i guess, you'd want a 100k trimpot in there. write it the following way: lug 3 is connected to 9+ and lug 2, lug 1 goes to the Collector of Q1 (again, it replaces R1). you want to get 4.5v in at Collector of Q1. then you can measure the resistance of the trimpot and put a fixed resistor of the same value in there. i hope im right cause i don't have much of tweaking experience with Fuzz Faces

It's standard practice to put it on the Collector of Q2, i.e. a 10k Replacing the 8k2, otherwise you're correct 

yes, with a 2k-5k resistor cutoff in series with the 10k pot. Adjust the 10K pot to get Q2's C voltage to 4.5.

You don't have to have the cut off, just makes it a bit easier to dial in, probably more useful with germanium than silicons for extended range.

Also you don't have to closely follow 4.5V, you might prefer it at anywhere from 4-6V, I like mine around 5.2V.

[LucifersTrip]

well a bias would go in place of R1 i guess, you'd want a 100k trimpot in there. write it the following way: lug 3 is connected to 9+ and lug 2, lug 1 goes to the Collector of Q1 (again, it replaces R1). you want to get 4.5v in at Collector of Q1. then you can measure the resistance of the trimpot and put a fixed resistor of the same value in there. i hope im right cause i don't have much of tweaking experience with Fuzz Faces

It's standard practice to put it on the Collector of Q2, i.e. a 10k Replacing the 8k2, otherwise you're correct 

yes, with a 2k-5k resistor cutoff in series with the 10k pot. Adjust the 10K pot to get Q2's C voltage to 4.5.

You don't have to have the cut off, just makes it a bit easier to dial in, probably more useful with germanium than silicons for extended range.

It's not really for extended range as for the fact that most FF's sound like sh*t when that pot is at 0K. The cutoff insures that you actually
have a usable sweep for the full range of the pot.

[Scruffie]

well a bias would go in place of R1 i guess, you'd want a 100k trimpot in there. write it the following way: lug 3 is connected to 9+ and lug 2, lug 1 goes to the Collector of Q1 (again, it replaces R1). you want to get 4.5v in at Collector of Q1. then you can measure the resistance of the trimpot and put a fixed resistor of the same value in there. i hope im right cause i don't have much of tweaking experience with Fuzz Faces

It's standard practice to put it on the Collector of Q2, i.e. a 10k Replacing the 8k2, otherwise you're correct  

yes, with a 2k-5k resistor cutoff in series with the 10k pot. Adjust the 10K pot to get Q2's C voltage to 4.5.

You don't have to have the cut off, just makes it a bit easier to dial in, probably more useful with germanium than silicons for extended range.

It's not really for extended range as for the fact that most FF's sound like sh*t when that pot is at 0K. The cutoff insures that you actually
have a usable sweep for the full range of the pot.

Yes but with Silicons, he shouldn't hit 0k at all and if it did have to hit 0k, having resistors in series or not wouldn't help, just transistors that aren't going to bias properly.

If it hits 0k with something in series and biases then it stands to reason that without that resistor in series, it wouldn't hit 0k anyway, it would be whatever value the resistor you have in series with it is.

You're effectively making the pot/trimmer 12-15k but it doesn't make it go negative, it just adds more resistance in the up range, thus, extending it for transistors that may require that higher bias.

[LucifersTrip]

well a bias would go in place of R1 i guess, you'd want a 100k trimpot in there. write it the following way: lug 3 is connected to 9+ and lug 2, lug 1 goes to the Collector of Q1 (again, it replaces R1). you want to get 4.5v in at Collector of Q1. then you can measure the resistance of the trimpot and put a fixed resistor of the same value in there. i hope im right cause i don't have much of tweaking experience with Fuzz Faces

It's standard practice to put it on the Collector of Q2, i.e. a 10k Replacing the 8k2, otherwise you're correct  

yes, with a 2k-5k resistor cutoff in series with the 10k pot. Adjust the 10K pot to get Q2's C voltage to 4.5.

You don't have to have the cut off, just makes it a bit easier to dial in, probably more useful with germanium than silicons for extended range.

It's not really for extended range as for the fact that most FF's sound like sh*t when that pot is at 0K. The cutoff insures that you actually
have a usable sweep for the full range of the pot.

Yes but with Silicons, he shouldn't hit 0k at all and if it did have to hit 0k, having resistors in series or not wouldn't help, just transistors that aren't going to bias properly.

Of course he shouldn't...then why have a pot that can go to 0K without a cutoff? Do you want to be able to have a sweep that starts off sounding like sh*t?

You're effectively making the pot/trimmer 12-15k but it doesn't make it go negative, it just adds more resistance in the up range, thus, extending it for transistors that may require that higher bias.

If the transistors need much higher than 15K to bias, then you have probably chosen the wrong transistors to get a good FF sound and should choose again

[Scruffie]

well a bias would go in place of R1 i guess, you'd want a 100k trimpot in there. write it the following way: lug 3 is connected to 9+ and lug 2, lug 1 goes to the Collector of Q1 (again, it replaces R1). you want to get 4.5v in at Collector of Q1. then you can measure the resistance of the trimpot and put a fixed resistor of the same value in there. i hope im right cause i don't have much of tweaking experience with Fuzz Faces

It's standard practice to put it on the Collector of Q2, i.e. a 10k Replacing the 8k2, otherwise you're correct  

yes, with a 2k-5k resistor cutoff in series with the 10k pot. Adjust the 10K pot to get Q2's C voltage to 4.5.

You don't have to have the cut off, just makes it a bit easier to dial in, probably more useful with germanium than silicons for extended range.

It's not really for extended range as for the fact that most FF's sound like sh*t when that pot is at 0K. The cutoff insures that you actually
have a usable sweep for the full range of the pot.

Yes but with Silicons, he shouldn't hit 0k at all and if it did have to hit 0k, having resistors in series or not wouldn't help, just transistors that aren't going to bias properly.

Of course he shouldn't...then why have a pot that can go to 0K without a cutoff? Do you want to be able to have a sweep that starts off sounding like sh*t?

You're effectively making the pot/trimmer 12-15k but it doesn't make it go negative, it just adds more resistance in the up range, thus, extending it for transistors that may require that higher bias.

If the transistors need much higher than 15K to bias, then you have probably chosen the wrong transistors to get a good FF sound and should choose again

I see what's going on here... you're talking about an external Bias Knob, where as i'm referring to a Fixed internal Bias (Due to the talk of bias voltage, you don't really bias an external knob to a particular value) now your idea makes more sense 

[gaz ariff]

Cheers guys, very informative. I didn't realise just how much work is involved in such a small circuit. I'd rather have an external Bias control (just saves opening up if it sounds a bit pish) for easy use. Now i just need to learn how to test and match germanium transistors. I do have a very good document on that subject. Any recommendations for other good starting projects?

Cheers, Gaz

[LucifersTrip]

Cheers guys, very informative. I didn't realise just how much work is involved in such a small circuit. I'd rather have an external Bias control (just saves opening up if it sounds a bit pish) for easy use. Now i just need to learn how to test and match germanium transistors. I do have a very good document on that subject. Any recommendations for other good starting projects?

Cheers, Gaz

hahaha...it's not that involved...we just went on a little long on that one. Ge Fuzz Faces are not as tough as many make it seem. You don't actually have to match the transistors. As long as you pick a low leak Q1 around 60-90 hfe and a low leak Q2 around  100-120 hfe, you prob won't have many problems (I've done best with Q1 ~ 75-85 and Q2 ~ 110-120).

If you get Q2's collector voltage to 4.5 when the resistor on Q2's collector is close to 8.2K, you'll most likely get a good sound. When you use leaky transistors, you'll start winding up with hiss or hum.

If the resistor on Q2's collector is lower (say 3-5k) when you get 4.5, the tone will be less fuzzy and closer to a distortion. If the resistor on Q2's collector is higher (say 11-15k) when you get 4.5, you will have a softer more mushy fuzz and less output volume.

And of course...if you haven't already read it:
http://www.geofex.com/Article_Folders/ffselect.htm
---------

Another good GE project I can recommend is the Fuzz Rite derivative, The Orpheum. It is not nearly as picky as the FF for transistors you have to use and the addition of a pot on Q2's collector gives you a full sweep of tone, rather than just being used for a bias.
http://www.luciferstrip.com/fuzz/orpheum.jpg

[rmachado]

Hello friends, sorry if I posted in the wrong place, but here it goes:
I recently bought a Boss PH-1r, and it not looks a lot like another PH-1r I had a few years ago.
What does the bias trimpot?, as shown in the schematic at this link:

http://www.geofex.com/FX_images/Boss%20PH-1r.pdf

Sorry my english.
Thank you all.