Fuzz Face bias

[jlullo]

hey guys.  i built my fuzz face to spec from the fuzzcentral layout and schem.  I am boxing it up tonight and just have a question about the DC bias. 

I ordered my germs from small bear, and steve listed the correct resistors in order to put the collector of Q2 at 4.5 volts
  Here's his list:

R3 - 36k
R4 - 680
R5 - 8.2k
R6 - 110k

since FC doesn't show a schem for their layout, i'm assuming that R3 is 33k, R4 470 ohm, R5 is the trimmer, and R6 is 100k (according to what i have on my layout).  Now, do i need to change out my resistors to these values, or do i just worry about the trimpot being set at 8.2k?

thanks!

[jlullo]

also, why are there two pads for the -9v?

http://fuzzcentral.ssguitar.com/fuzzface/fuzzfacelayout.png

[jlullo]

one more, where the hell is "output" on the board?  i only have input, fuzz 2, grounds, -9vdc, vol 3, fuzz 3

nevermind.  i figured this one out from an old thread i started- vol lug 2 to output.

[joelap]

Output of the circuit comes from the middle lug (wiper) of the output pot.

I replaced the 8.2k resistor with a 10k trimpot to get the bias perfectly down to the -4.5V to -5V range, but if you ordered those from smallbear, those have to be specifically biased for the exact transistors you're using I'm assuming.  I think you;re right in your assumptions.  If you've got sockets for your transistors, try putting the transistors into the board with the resistors you already have in place and see if you get Q2's bias to be between -4.5 and -5V (if you're going PNP, 4.5 and 5 if you're going NPN).

I believe the second 9V pad is for the option of connecting an LED, I'm assuming.

Enjoy man, I just finished two!  One I put into a Dunlop reiussue housing, the other I build in a box from smallbear with some mods.  I'm planning on giving that to my guitar teacher, who covers a lot of hendrix in his sets.

Joe

[jlullo]

alright, so if i can get it down to 4.5 then i'm good?

thank you so much for the info!

[joelap]

Should be good I guess.  I'm no fuzz face guru though.  In mine, I used all the standard values from FC, but replaced the 8.2k with a 10k trimmer, got it set to around -4.7 volts and I left it.  Sounded good to me.

[jlullo]

sweet. thanks for the info!

[jlullo]

i'm really kind of going into a mind**** right now using fuzz central's wiring diagrams in their FAQ.  i'm pretty terrible with schematics, but i think i'm getting this (i'm used to tonepad's offboard wiring!).

for the DC jack (http://fuzzcentral.ssguitar.com/faq/powerjackwiring.gif), because the diagram is meant for NPN, i connect the battery - to where it says battery +, correct?  then, where it says "+9v to circuit board" is really where we're connecting to our -9v pad on our circuit board?

THEN, as shown in the diagram for wiring the switch (http://fuzzcentral.ssguitar.com/faq/switchwiringpnp.gif), on the upper third lug where it says "-9v DC", does this go to the -9v pad on the board or to the DC jack?

ahhh!

[Meanderthal]

 Maybe this one will make sense...
http://www.generalguitargadgets.com/diagrams/fuzzface_lo_pp.gif

Oh, and the LED thing was fine.

[jlullo]

thanks meanderthal.  i wired up the dc jack that way, and i'm pretty sure that all of my wiring is completely messed up.  i need to put this down for the night and come back to this tomorrow.  i don't know why i'm finding this so difficult understanding the way FC has it laid out, but i really am

[mac]

Despite the listed transistors sent by smallbear, I suggest you to use a trimpot instead of the 8.2k because germanium varies their gain with temparature and most likely you won't get 4.5v at Q2 collector. And if you use a a trimpot you can use the original values.
If you know the gain and the leakage download this helper app in win, mac or html:

http://geocities.com/guitarfxs/exes/biascalculator.zip

you can estimate the bias resistor with some accuracy.

mac

[Meanderthal]

 Sadly, the link is down...

[jlullo]

alright.  i just finished wiring up the pedal, i think correctly   i'm about to bias with a 10k trimmer.  all i do i set my DMM to 20k, put my black lead on a ground, red on the leg of the Q2 collector, and adjust the trimpot until my display says 4.5v, correct?

of course this is with the battery in the snap and a cable in the input jack...

[petemoore]

alright.  i just finished wiring up the pedal, i think correctly   i'm about to bias with a 10k trimmer.  all i do i set my DMM to 20k, put my black lead on a ground, red on the leg of the Q2 collector, and adjust the trimpot until my display says 4.5v, correct?[/b]
  Adjusting Q2CR value should move voltage at Q2C up/down, and around 1/2v, which would be 4.5v if the battery is 9v

of course this is with the battery in the snap and a cable in the input jack...

[jlullo]

i take that as a yes?

[mac]

Sadly, the link is down...

I tested it and it is working ok. Try copying it on the address bar of another window. Or send a PM.

mac