Fuzzrite help please

[fixr1984]

Here is my layout for the fuzzrite I used this schematic  http://www.montagar.com/~patj/mfuzrite.gif
Im having problems with it not having much fuzz. With the gain all the way down there is fuzz but as i increase it the sound gets
muddier and more of a overdrive.Thinking I had a bad pot I tried another 100k as well as 500k pots and get the same result.
Some of my voltages are Q1, c=2.4  b=.65  e=0  Q2, c=8.97  b=.67  e=0 Batt= 9.01
When checking volts on track D5 I have 2.4v and with Q1 removed it goes up to batt voltage. I made Dragonflys version and it sounds like it should,
http://aronnelson.com/gallery/album17/MOSRITE_FUZZRITE
so im assuming that its something in my layout thats wrong. Any help or pointers is appreciated

[Sir H C]

I assume you have to cut the trace under R5 or you are shorting the input to ground.

[fixr1984]

yea its hard to see but there is a cut there

[Torchy]

You have C4 going between the two collectors - it should go from Q1 collector to Q2 base/r4 junction.

Easy to fix, take the jumper from C4 right hand side and extend it by one row onto the base.

[slacker]

Your connection from C4 to the second transistor is wrong. You've got it going to the collector it should be going to the base.
Like this


Hope that sorts it out 

[fixr1984]

Silly me, I will give it a try. Thanks fellas

[fixr1984]

That took care of my problem, now its a nasty nasty sounding fuzz just like it should be.
The pic on the top is updated now and its verified if anyone wants to use it. I will upload it to Banika's layout site.
(thanks for that and the program by the way)

[petemoore]

  so./.I'm surmizing you found that schematic works...
  I remember having a failing circuit from that schematic and later reading that the 470k resistors [on the collectors] worked much better as 47k...

[fixr1984]

I actually had 470k in there and it wasnt bad but after messing around getting the tone taken care of i ended up changing
the layout to 10k. I actually now have 100k in there and I think it turned out really well.

[brett]

Hi
those 470k resistors were intended to get maximum oomph out of early silicon transistors (probably hFE around 150).  These days you can get the same gain with 10k to 100k.  This is because gain is approximately equal to the ratio of the collector resistor to the internal emitter resistance (Now lower than in old devices).

One option is to use power transistors.  BD139s are perfect for this circuit or Q2 in a silicon fuzzface - hFE from 120 to 200 (mine are all about 160).  For other power devices, get medium-power ones that are good for 1 to 10W.  High power devices (>50W) often have loo low a hFE (e.g. MJE3055/2N3055 is under 100).  I believe that Gus suggested power devices for fuzz circuits way back when the forum was born.   

Adding a small emitter resistor (about 22 ohms ?) would also work, but that raises the emitter, base and collector voltages, which might affect tone.  (e.g. Most people prefer low hFE transistors for Q1 in a fuzzface than to add an emmiter resistor.)

cheers

[brett]

OOPS
I meant to post this in the other fuzzrite thread.

Hi.
The collector resistor is important in this circuit for at least two reasons.
1.  Asuming that we're working at less than the transistor's maximum gain, the gain is approximately equal to the collector resistance divided by the emitter resistance.  Ah! But you can clearly see that there's no emitter resistor.  True, but the emitter has a small internal resistance (depends on the base to emitter current - usually a few ohms).  So making the 470k collector resistor smaller would reduce the gain.  Halve it, and the gain goes down by half.
2.  Because of the biasing arrangement in this circuit, the collector resistor also affects the base (Ib) and collector (Ic) currents.  That is because the transistor is turned "on" by a small amount of current flowing through those 2 x 470k resistors.  If we assume the base is at 1V and the battery + is 9V, the base current will be about 8uA (8V/940k).  From this we can also calculate Ic, because the transistor amplifies Ib by hFE (say 500 for some modern transistors).  8uA x 500 gives 4mA, which is a fairly large collector current.  If we reduce the collector resistor from 470k to 10k, the base current goes up to 16uA, and the collector current up to 8mA.  Because the original circuit had 2 x 470k resistors and low hFE transistors (150?), the original circuit probably had Ic about 1.2mA.  Having Ic of 4 to 8 mA may affect gain and tone, and will affect battery life, so I'm now thinking that I'll use  low gain transistors, like BD139s, and keep the 2 x 470k resistors on each transistor.

That may not be too clear.  If not, ask again.

cheers

[fixr1984]

I used 2n3904's and my DMM read them at 150-170 hfe.
Going by what you said should I still be using the 470k's on the collectors? I currently have 100k.
I dont know if i would want any more gain on this. With the fuzz knob all the way down and the volume knob
at about 3 o'clock (12 is off) There is a huge amount of gain, im talking major squeel if you dont cover the strings.
But man is it an awesome sound, it can get down right nasty depending on your settings.