Biasing a FuzzFace

Frequently asked question on transistors: types, substitutions, how to test, use and misuse them.
Post Reply
User avatar
Frostbite Slim
Information
Posts: 10
Joined: 06 Nov 2011, 20:42

Post by Frostbite Slim »

When biasing a Fuzface that has a bias trim pot, do the controls (Vol, Fuzz) need to be set full ccw?

User avatar
DrNomis
Old Solderhand
Information
Posts: 6807
Joined: 16 Jul 2009, 04:56
my favorite amplifier: Self-Built Valve Amp Head :)
Completed builds: Dallas Arbiter Fuzz Face,Tone Bender Professional Mk 3,Tone Bender 3-Knob,Baja BK Butler Tube Driver,Baja Real Tube Overdrive,Roger Mayer Octavia,EH Soul Preacher,Tech 21 XXL Distortion,MFOS Weird Sound Generator.
Location: Darwin,Northern Territory Australia
Has thanked: 98 times
Been thanked: 278 times

Post by DrNomis »

Frostbite Slim wrote:When biasing a Fuzface that has a bias trim pot, do the controls (Vol, Fuzz) need to be set full ccw?

I've found that it actually doesn't really matter where you set the Fuzz and Volume controls when setting the bias.... :thumbsup
Genius is not all about 99% perspiration, and 1% inspiration - sometimes the solution is staring you right in the face.-Frequencycentral.

User avatar
Seiche
Old Solderhand
Information
Posts: 2526
Joined: 01 Nov 2010, 00:16
Location: Germany
Has thanked: 231 times
Been thanked: 189 times

Post by Seiche »

if you bias it by ear, i would set the controls where I like them best, or change them around to see which bias setting i like best throughout the range.

User avatar
guitarmongot
Breadboard Brother
Information
Posts: 106
Joined: 09 Jun 2009, 21:44
Location: Sweden
Has thanked: 15 times
Been thanked: 10 times

Post by guitarmongot »

Here´s some interesting reading :
http://www.physics.mcgill.ca/~grant/Stuff/fuzzface.txt
/cheers M

User avatar
Frostbite Slim
Information
Posts: 10
Joined: 06 Nov 2011, 20:42

Post by Frostbite Slim »

guitarmongot wrote:Here´s some interesting reading :
http://www.physics.mcgill.ca/~grant/Stuff/fuzzface.txt
/cheers M
VERY interesting read....Thanks

User avatar
soulsonic
Old Solderhand
Information
Posts: 3880
Joined: 27 Jun 2007, 03:38
my favorite amplifier: Traynor YVM-1
Completed builds: too many!
Location: Morgantown, WV
Has thanked: 183 times
Been thanked: 468 times
Contact:

Post by soulsonic »

Frostbite Slim wrote:
guitarmongot wrote:Here´s some interesting reading :
http://www.physics.mcgill.ca/~grant/Stuff/fuzzface.txt
/cheers M
VERY interesting read....Thanks
That's actually, a really poor way of doing it. The best way to bias a Fuzz Face (or Rangemaster) is to have a single trimmer for the Emitter resistor on the second stage. Instead of using the Fuzz control as the Emitter resistor, use a trimmer, and wire the Fuzz control as a variable resistor in series with the bypass cap (the one normally connected to its wiper). Or, use a slightly higher value for the Fuzz control (try 2k or 5k) and have it wired normally with a trimmer tacked on in parallel to it. 5k is usually the best value for the trimmer in any case. Adjust the trimmer so you get 4,5v at the Collector of Q2, and you're in business. The bias should end up being fine for the first stage as well, only time it isn't is when there is too much of a difference between the gains of the two transistors. But they don't have to be matched exactly for it to be right, just in the same general ballpark.
If you follow the advice of that article and replace everything with trimmers, you end up with them all "fighting" each other as you adjust it. Adjust one and it gets about right, start adjusting another, and then it's all off again... makes it way more complicated and messy than it needs to be. Also, the tone of the circuit can change dramatically when you change the value of the Collector load. Collector resistors should NEVER be used for bias trimming. Only time it's acceptable to have adjustable Collector loads is when you're intentionally wanting to have a thing that changes the tone of the circuit.
"Analog electronics in music is dead. Analog effects pedal design is a dead art." - Fran

User avatar
guitarmongot
Breadboard Brother
Information
Posts: 106
Joined: 09 Jun 2009, 21:44
Location: Sweden
Has thanked: 15 times
Been thanked: 10 times

Post by guitarmongot »

Thats interesting SoulSonic. For Rangemaster typ boost I find biasing with 5K trimpot on emitter is a great way (just as you describe).
For FF I broadbend classic stock values with trimpot on Q1 C (33K) and Q2 C (8.2K) and trim on 330 or 470 ohm (2K trim).
I go for 4.5V (or -5.5V for GE) on Q2C and 1-1.3V on Q2 C.Then I turn down guitar vol to 7 and fine tune trimmers for clean tone.
2K trim I set max output vol for FF. I usually have 2Kpot at 90% setting and 100 ohm in series for 22uf cap for Fuzzpot).Then I have a pair of FF trannies with fixed resistor values... :D
I know thats a lot of trimmers to adjust but usually your really close to stock values if you go for (80-120hfe GE ) and 250-300 hfe Si .
This weekend I will try your method of adjusting Q2 emiter on a FF : never tried that before should be fun...
/cheers m

Post Reply