Fuzz Face bias.......again

[ZtaRDuZT]

A new take on the fuzz face bias question.


I'm just about to finish up my first "real build", Dallas Arbiter Positive ground. Done a lot of test stuff with trimpots that I use for
biasing and som modifications before, but not a complete stompbox.
I'm currently working with a set of matched germanium transistors that unfortunatley didn't match the supplied
resistor values. Ended up with 2.3V on Q2 collector. Easy fixed when you have made labcircuits for both negative and positive
ground fuzz face. I fix the problem by tweaking either resistor (33k) on Q1 collector or resistor (8.2k) on Q2 collector.
A quick twist on either trimpot sets me right in fuzz face paradise (4.5v)

Question: Does it matter wich resistor I tweak or is 4.5v on Q2 collector the answer to everything?

[stm]

Certainly if you are allowed to vary both collector resistors you will be able to find many different combinations that produce 4.5V or so at Q2's collector.

On some article I read (I think at www.pisotones.com--unfortunately in spanish only) one of the guys there posted that you could find the sweet spot for virtually any pair of transistors (not only limited to the magic gains of 70-90 and 110-130).

The trick there was that when both collectors currents are the same and C2 is near 4.5V you are in the sweet spot, which is what he claimed you get with the standard resistor values and said magic gain values--I haven't verified this though.

In summary, you could place a 100 ohm resistor in series with each collector so you can compare collector currents easily with a DVM (digital voltmeter), and replace the 33k resistor with a 100k pot at C1, and use a 25k pot at C2. The rest is just tweaking. Of course you can try a trial and error approach with many listening test to find the sound that suits better what you want.

Let us know if you come to something here!

[lenwood]

Check this out this helped me:

http://www.physics.mcgill.ca/~grant/Stuff/fuzzface.txt

[formerMember1]

Try this,

Set the 33kpot to 33k and set the 8.2k pot to 8.2k

Now turn only the 33k pot until you get 4.5volts on Q2's collector, and play.

Now do the opposite, now try setting everything at 33k and 8.2k and try setting the collector at 4.25 or 4.00 or 3.50, by turning the 8.2k pot only, then turn the 33k pot, until the collector is at 4.50volts.

You will discover many tones, FretWire taught me this.

Also what i did, was added a 1k resistor in series with the cold lug,(ground) lug of the 1k fuzz pot, and then set the 8.2k pot for 8.2k and set the 33k pot until you get 4.50volts on collector,  (this will get you very very close to jimi hendrix's tone.) It will make the fuzz have a great gatey sound, but not too gatey like an unbiased sound does, think hendrix's purple haze, and foxey lady, especially Come on "let the good times roll", it nails that tone!!

It matters what the voltages on the other leads are, don't just go by the Q2 collector!!!!

this all worked for me great with my Axis Face silicon rev 3, from fuzzcentral, really close to Ge, if not right on,...(i didn't use phillip's values though, for more info read my axis face thread...)

I am gonna build a Ge arbiter fuzzface like yours anyway though, just too see the difference in my axis face sili rev 3.


let me know if you try this,.. 

PS:I thought it was the best when only changing the 33k resistor, this works way better than changing the 8.2k,  and actually, if i build another fuzz, i am gonna swap the 33k for a trim pot, not 8.2k.

You will have to remove the trim pots, and test with a DMM to get it right on the 33k and 8.2k values,..you actually could only remove the 8.2k trim pot and measure it, and put it back, and just turn the 33k pot.
unless you marked the trimpots for 33k and 8.2k before you installed them, then just turn them to the stock mark, and then turn the 33k pot until it biases at 4.50 on collector.

I used a 8.2k resistor and a 28k instead of 33k, to get my transistors to bias right with the Q2 c on 4.50 volts,(Q1 hfe around 64, and Q2 around 124 IIRC)

i heard for the best tone ever, get transistors that bias naturally, like ones that have the right gain, and using the stock 33k and 8.2k  to bias it.  but i never tried this though, and doubt there was any magic to it.  but then again, i don't know, cuz i never was able to get the magic gain transistor pair, so i can't comment my opinion. 

[Fret Wire]

Try this,
Set the 33kpot to 33k and set the 8.2k pot to 8.2k
Now turn only the 33k pot until you get 4.5volts on Q2's collector, and play.
Now do the opposite, now try setting everything at 33k and 8.2k and try setting the collector at 4.25 or 4.00 or 3.50, by turning the 8.2k pot only, then turn the 33k pot, until the collector is at 4.50volts.
You will discover many tones, FretWire taught me this.

Actually, all I did was give you some specific values to try for your particular trannys and fuzz pot. But you got the idea, biasing by Q2 alone doesn't usually get the best sound. Optimum biasing values for Q1 & Q2 will depend on their gain, as well as the fuzz pot value.

[MartyMart]

Or .... just use your ears  !!
I have 50k and 20k trimpots on both my FF's now and can get a
range of great tones, without a DMM in sight 
I have no idea what voltages are going on ......

MM.

[ZtaRDuZT]

Thx!



A lot of ideas to try out!
I've already made the circuit with regular resistors so trimpot's is out of the question.
I'll have to go back to the breadboard to do this. Anyone know if the Transistor voltage values at GGG is good to follow?
I mean if you can twidlle about with the resistors and find yourself really close to those values on ALL legs, would
that be close to the same current on both collectors?
I know that in the end it's your own ears and preference that will do the trick but it would be nice to have some
general rule to follow besides just Q2 4.5v because that can be achived in so many ways, it seems.

Values for my germanium transistors are as follows: Q1 82, Q2 134 - 2n1305
regular pot values: 1k fuzz, 500k volume.
I use the vishay 20uF capacitor, protection diod 1n4007 and filter capacitor 100 uF as well.
Otherwise I've followed specs at GGG

I think I will try to measure collector currents as well as voltages on ALL the legs and see where that takes me.
If I don't find the sweet spot maybe I'll find myself close enough to smell it.


Really appreciate the answers!!!