fuzz factory differences..wireing.

[toolguy]

Ok..so my latest build (compact f.factory) is going "ok".
The oscillation is much wilder than on the videos..and the pedal is very hard to control. I either get Velcro fuzz (ie."walking on the sun" fuzz)...or hard screeching oscillation. I was able to tame it down a tad by adding a 8.2k resistor across the gain pot (just from experimenting) but all it really did was give me a tad more range on that pot. The noise seems to come more from the gain pot than the stab. In fact all the stab.pot does is control the frequency of the oscillation not stabilize it at all.
 Figuring I did something wrong...I had a look at the tag layout. the only big difference I see aside from it not having a few resistors (like the 1m input) is that the way the comp. and drive pots are wired. On the tag comp 1 goes direct to q3(E) and shared  with comp 2 and drive 3. Drive 3 then goes through the 47k to Q2 (B).  Where on the compact layout they all head to Q3 (E) 1st...then via a  jumper to Q2 (B). Is this accomplish the same thing...or does the circuit react differently in the tag version?

http://tagboardeffects.blogspot.com/2010/02/zvex-fuzz-factory.html

vs
http://tagboardeffects.blogspot.com/2012/10/zvex-fuzz-factory-compact-layout.html#comment-form
 
Also I thought (if possible) lowering the gain on Q (2n3904) might help...but I am not sure if adjusting the 220K on Q1 (b) would do that..and how it will effect things down the line.


 Don't know too much theory behind how the npn's work in a Fuzz (up for a good read though if anyone has a useful link or two).

 Thanks again for the help this week guys.

JD

[induction]

Regarding the differences between layouts, for the most part, a jumper is a jumper. Consider all points connected by a jumper to be one point, and you'll get your answer. This amounts to assuming that the resistivity of a jumper is 0 ohms. This isn't entirely true, but it's pretty close when you compare it to the resistor values in the circuit. The times it makes a difference are when you want to avoid contaminating your signal with voltage surges (eg. ticking from tremolo circuits) and when you want to avoid oscillation in high gain circuits from capacitative coupling of input and output wires. It sounds like the two layouts are the same to me, but I'm basing that on your description, I haven't compared the layouts.

Don't put too much faith in the names of the controls in this (or any) pedal. The Stab pot just reduces voltage to the circuit. It's called "Sag" in many other pedals. I wouldn't expect it to stabilize anything. The Gate pot controls the gain and bias of Q2, The Comp pot controls the DC gain and bias of Q3, and the Drive pot controls the AC gain of Q3. The Volume control does actually control the output volume, but it's not wired like a standard volume control because there's DC voltage across it. You can expect most of the controls to be pretty interactive, especially because of the feedback between Q2 and Q3.

Finally, the 2n3904 is a grounded emitter gain stage. That means that its gain is controlled by the hfe of the transistor and the output impedance of whatever comes before it. The 220k is for controlling bias, not gain. If you want to reduce the gain, use a lower hfe transistor, turn down the volume knob on your guitar, or put a resistor inline with the input. You can copy the Smooth pot at the input of Fuller's version of the fuzz face (look here, near the bottom of the page) if you want to make it variable.

This circuit is fairly sensitive to the hfe of the transistors you use, and to the guitar you use it with, and it will respond very differently if there is a buffer or another pedal between your guitar and the input. Most people would want it first in the chain, but feel free to experiment.

[tjdracz]

Quickly looking at the two layouts, they are pretty much the same. 1M is there as pulldown resistor, anti-pop measure when switching it on and off

What are your germanium transistors hfe and leakages? Also, what's your 2N3904 hfe? I found that ones I got from Tayda had much higher HFE that ones from other sources (400 against something like 200-240).
Can you post your voltages for all the transistors? With the stab fully CW so supplying full 9V or so.

Not sure about recommended Q1 hfe but Q2, Q3 should normally follow your usual Fuzz Face set. So something like 70 for Q2 and 90  for Q3 in terms of hfe should be just about right.

[Stinger25]

I've built few Fuzz Factories in the past, and it's a tricky circuit as it seems to be very sensitive to parts tolerance, IMO it's almost impossible to build 2 pedals that have identical sound on the same knobs settings. As mentioned before, the transistor selection is important in this circuit although I haven't noticed any problem using germs with high leakage (unlike the fuzz face). Also, if I remember correctly, you can't use millennium bypass with circuit since it messes up the impedance (I think).

[Muadzin]

I agree. I have built about 4 to 5 of them and they all behave differently. With a set of measured and matched trannies for a FF not necessarily sounding better then a set of random thrown together trannies.

[toolguy]

Ok...thanks for the help.
So the pots act strange as stab. RAISES voltage ccw and lowers it cw..but.
Comp at full ccw lowers voltage on q3 while slightly raising it on q2 amd vice versa...

So with Stab at full ccw
and Comp at full clockwise I get.
Q1
e-0
b-.63
c-1.07

q2
c-6.92
b-7.10
e-7.39

q3
c-7.32
b-6.85
c-7.33

with comp at full ccw
q2
c-2.85
b-7.16
e-8.32

q3
c-2.91
b-2.88
e-3.15

No idea if this is within range or not.

Will check hfe on the bag of 3904s I have.
I did socket the 128's but then soldered them to the socket when I thought it was working right...so I can't test them.

[toolguy]

So far (on my cheap Centech hfe tester) I have different manufacture 2n3904 ranging from 185 to 352 hfe...hmmm.
Lowest rated ones are marked cen2n3904
and have a e b c stamp on them.
Think they were from the shack.