Help me tweak this fuzz's bias

[squidsquad]

Built the Roger Mayer Axis Fuzz...& love it (and I'm hard to please). 
Was wondering which resistor I could swap w/a trimmer to tweak the bias?
http://fuzzcentral.ssguitar.com/schematics/axisfuzzschem.gif

[John Lyons]

I'd tweak the 22K...man that drawing is oddly drawn...
Did you use the 220R/.001?
How would you describe the sound compared to saya FF etc?

john

[squidsquad]

Thanks John. 
Yes...the schem IS strange...and I'm easily confused!  I'm a * different animal* when it comes to fuzzes.  I DON'T want
*over the top*...*folding over*...*amp gonna explode*.  And so...I HATE fuzz faces.  I've made around a dozen FFs...tweaked every part imaginable...tried cap selection...tone controls...etc...  But ALWAYS end up with something I can't use live.  Even at the lowest gains...a FF
puts some *layer* over my ax which makes MY tone become lost.  That being said...I LOVE this one.  Even though it is a bit bright, I can still hear my ax's tone thru it.  And it is easily *tamed* into quite a nice booster also.
Can't say this one cleans up perfectly (that's why I wanna play with the bias)...but it turns into something interesting.
I plan to do a big write up...once I'm done tweaking.  Maybe even a video.  I've made a simple mod...(without really changing the circuit)
and it ROCKS.
But right now...I'm having too much fun playing thru it!  For me...it's all about control...and this has that.  I ignored it for years...thinking (based on the name) it was another FF with maybe an octave thing going on.  But once I finally looked at the schem... I gave it a try...
and boy am I glad I did...highly recommended!

[squidsquad]

Forgot to mention:  the *optional r/c*...removes hiss...perhaps RF noise.  Did NOT make a large difference as stated...I tried it...but took it back out.  One more thing...I'm running it off a 12 volt wart...another reason I want to play w/the bias...just to see what happens.

[Nasse]

I dont know the answer but thought how would it react if you adjust supply voltage, perhaps a sweet spot can be found?

[Gus]

I would not change the 22k.

If you want to move the operating points adjust the 180K.

[DougH]

BFor me...it's all about control...and this has that.

I agree. That's what I love about the Axis Fuzz and it's one of my favorites.

[Dragonfly]

try this for a mod...

replace the 820k and the 180k with a 1 meg linear pot, lug 2 going to the 22uf... 


or ....replace the 820k with a 470k, the 120k with a 500k lin pot, lug 2 going to 22uf ....

[Gus]

I disagree I would just change the 180K.  Changing both the 820K and 180K to a 1meg pot will effect the input resistance.  Making the 820K adjustable will effect the input resistance.

Changing the 180K is better because it is AC coupled to ground at the 820K, 180K junction with a cap.  So changing the 180K will not effect the input resistance but will shift the bias of the first transistor and because the transistors are DC coupled the second transistor's operating point will be changed as well.

You can change other other parts to change the operation points but the 180k is easy and does not change the input resistance just the DC operating points

[John Lyons]

Ok, I can see that now Gus.
I was thinking of the second transistor's bias with the 22K...
Why is it that we usually adjust the bias with the collector resistor then in a FF type circuit?

Wow, 39K for the emitter in Q2! Does this make the bias very cold for Q2 and therefore very asymmetrical?

[DougH]

Why is it that we usually adjust the bias with the collector resistor then in a FF type circuit?

I guess I'm confused... There's really no need to adj the bias in a Fuzz Face unless you don't like the sound. It will work fine as drawn with adequately functional transistors with no need to adjust DC. As to why people keep screwing with variable collector resistors in BJT transistor circuits to achieve some magic bias point, I have no idea. It's a poor way of doing it as it changes the gain of the circuit as well as the bias.

Maybe I started some bad habits with the Meteor by suggesting you adj drain resistors to accomodate inconsistent JFETs. With JFETs there are inconsistency issues, but even so there are much better ways of biasing those circuits than adjusting the drain resistor. I've learned a lot since then... But BJTs are generally much more consistent and predictable than JFETs, aside from leaky Ge's which you wouldn't want to use anyway. Want to predict the bias point of your BJT circuit? Get out a pencil and piece of paper and work it out with Ohm's law. Art of Electronics does a good job of explaining it. It's simple arithmetic. I did that for my Rangemaster build and given typical parts tolerances, the mathematical prediction was spot on. Took me almost 5 minutes...

I like Mayer's way of filtering power and providing a way to adj DC without changing Zin or gain.

[squidsquad]

Glad to see some respected people joining the thread...thanks guys!  I've been called to work & won't be able to tinker for a few days.  But thought I'd share my simple mod.  I found there were 2 sounds I wanted: #1 = Almost full (I used a resistor - carefully measured for my strat - (between 30 & 70 ohms)...on the Q1 emitter so it just tips into the *fuzz department*.  This helps bring noise down & adds control. #2: I wanted the drive lower than the pot could take it.  I replaced the drive pot w/a 10k trimmer...set to the point where there is NO distortion...but a small boost.  This makes that bypass cap around it more sensitive.  A value of .01uf will leave your tone the same.  Using a .02 *removes the blanket*.  A .047 give quite a treble lift.  And using a .6 = an ice pick treble boost...(which would be great followed by a low pass filter & another gain stage).  Anyway, I added a second 3 pole stomp switch...which also switches between 2 volume controls...since there is a large difference between full fuzz and light boost.  Now the 2 sounds can be balanced.  The boost sounds GREAT.  Not super loud...but like a happy marrige of a mosfet/LPB.
And...if I hit the pedal with my Blackstone Appliances pedal...I get 2 MORE sounds = a *fuzz light* which cleans up well + a long sustaining fuzz.  VERY pleased!

[John Lyons]

Doug
I was thinking of germanium devices but carried it over to this circuit since the thread was about changing the bias.
Yes, with silicon devices the bias should be fine. Nothing wrong with fooling around with it to get different sounds though right?
I know about adjusting the bias like in Gus's FF with the 5K trim off the emitter...
As far as Jfets what is a better way to bias them than the collector resistor?

Thanks for the tweaks and overview of your build Squidsquad.

[Gus]

That 5K bias pot is to allow different sounds gated to compressed a fixed 1k works fine with that variation of the two transistor fuzz.
  Look at the early two transistor vox 820ohm under the 1K gain.  Square face has a 2K gain pot.

[DougH]

John,

There's no reason to worry about it with Ge devices either.

I have no issue at all with using variable resistors to try different sounds. I get the feeling though that people are using them as some sort of band-aid because they don't know how to bias a circuit. The bottom line is that there is no reason for them in these bjt circuits unless you are using them to get different sounds. If not, just figure out your desired bias point (which you can predict on a piece of paper), stick in a fixed resistor and go.

[Dragonfly]

I disagree I would just change the 180K.  Changing both the 820K and 180K to a 1meg pot will effect the input resistance.  Making the 820K adjustable will effect the input resistance.

Changing the 180K is better because it is AC coupled to ground at the 820K, 180K junction with a cap.  So changing the 180K will not effect the input resistance but will shift the bias of the first transistor and because the transistors are DC coupled the second transistor's operating point will be changed as well.

You can change other other parts to change the operation points but the 180k is easy and does not change the input resistance just the DC operating points

except for one thing...

notice that I NEVER said "this is a bias mod"....

Its a mod that isnt designed to bias the transistors, though it effects the bias somewhat.....

Try it and see what happens

[John Lyons]

What is a good way to bias a FET other than using a trim pot off the collector?
I assume by adjusting the source resistance and drain resistance. Possibly by biasing the gate with pull up/down resistors as well?
Can you pointy me to an example or reading on this?

Thanks

John

[Dragonfly]

What is a good way to bias a FET other than using a trim pot off the collector?
I assume by adjusting the source resistance and drain resistance. Possibly by biasing the gate with pull up/down resistors as well?
Can you pointy me to an example or reading on this?

Thanks

John

http://www.colorado.edu/physics/phys3330/phys3330_fa06/pdfdocs/AN102FETbiasing.pdf

[John Lyons]

Thanks Andy
I'll look that over.

John

[sevenisthenumber]

I would not change the 22k.

If you want to move the operating points adjust the 180K.

What value pot do you use?