Axis Face Debug - HELP!

[enigmur]

With a bit of playing around with Bias, I pretty much have the pedal usable. It is still quite tinny and whistly with the Fuzz cranked, but it isnt squealing.

I am going to try a 100pf cap on the C2 base and collector.

I've read about oscillation in fuzz circuits,  and saw a few people putting caps after the input and across the transistor.

One thing Im not sure about is why I cant get a gradual increase in gain from the Fuzz knob, it is still quite all of a sudden.

It is quite useable at this stage though - i'd just like to perfect it because having the gain on ful sounds awesome - minus the whistling.

I might also try measure what ohms the pot is running when it cuts out the oscillation, and try a resistor there.
Where would I actually place the resistor? Between the middle and end lug, or between the lug and the wire to the board?
I know tcobretti managed this with a  project in 2004 (I've been digging).

I guess I should try some shielded wire, I don't have any at this point which is a shame.

[tcobretti]

Is your fuzz pot a 1kA or 1kL?  If it is as audio taper (1kA), the gain will not increase linearly (1kL). 

In my experience, the 100pf cap is has a greater effect on oscillation if used on Q1.

BTW, the classic FF sound is with the fuzz pot all the way up, and you control the amount of fuzz with your guitar's volume knob.  Also, the value of the volume pot has a big effect on the sound of the pedal, so you might try different values if you have them.  Fuzz god Roger Mayer uses 47kA pots quite often, and you could also try a 250k or 500k.  Generally speaking, the higher the value, the darker the pedal will sound.

[enigmur]

All pots I am using are Linear. The volume pot works perfectly now after grounding it 

Could it be that the wrong voltage is being fed into the Fuzz pot, that's why it doesn't do anything at low levels?

The pedal sounds good though, instead of using the two BD139's, which are about 120-130Hfe, I have one BD139 in Q1, and a higher gain 2N5088 in Q2. I actually think I get slightly less squealing this way - plus the fuzz sound really raspy and sweet if I turn the Bias pot down, and really thick and grunty thick distortion with it up full (gonna name that pot the 'Grind/Balls' pot on the enclosure haha.

Is there a way to test if shielded wire would help?
I'll be sure to try that capacitor when I get home.
Where did you put that resistor on the Fuzz pot to stop it from maxing out and causing the squeal? This post http://www.diystompboxes.com/smfforum/index.php?topic=25722.0

[tcobretti]

I don't know what is going on with your fuzz pot's rotation problems.  However, I just built that Axis Face. If you build it as he specifies you should not get any oscillation, and I agree that it sounds great!  I built it a couple of years ago and somehow wasn't so impressed, but now I am.

What I just realized is that the layout you are using does not accurately reflect the newest Axis Face, and the difference is critical.

Here's the article by Phillip, who designed this iteration of the Fuzz Face.
http://fuzzcentral.ssguitar.com/axisface.php
Take a look at the schematic at the bottom.

The two key components that would stop oscillation are the .033 cap that parallels the 1.2k resistor (which is R5 - 330 ohms on your layout), and the 100pf cap the goes from collector to base of Q2.  In my post above, I said to put the 100pf cap on Q1, because that is what I have done in the past.  However, on the axis face the .033u cap takes the place of my cap on Q1.  Also on the Axis Face, Phillip puts that 100pf cap on Q2.  These caps are not there solely to cure oscillation; they help mimic the dark sound of the Germanium transistors in the original FF.  So, if I were you I would certainly put those caps in there if they are not there!  That will very likely cure your problem with oscillation.  If you don't have exact values, just get as close as you can; I didn't have a .033u so I used a .022u and it sounds great.  You know where the 100pf goes, and you can install the .033u cap right next to R5 in the exact same rows.

[enigmur]

You have been such an awesome help with this project, I found that diagram on fuzz central after I started using my one, I figured the one I'm using would work.

If you look on the diagram back one page, isnt C2 a 100pf cap which essentially is between C and B on Q2?

I think I have got a 0.033uf cap from buying a bunch to try in my wah, so that shouldnt be a problem!

Every day at work I find something new to test in this bloody circuit, then the day really seems to go slow until 5 

Cheers again for the help

[tcobretti]

I'm glad I could help; part of the fun of building is learning enough to help others.

From what you had said, I wasn't sure if you had put in that 100pf cap.  If you did, then just slap in the other one and I suspect it will help a lot with the squeal.

[enigmur]

Cool cant wait to try it.

I might actually raise that 330R resistor to around the 800R - 1k mark, just so I have more volume to work with if I want it.
There really doesnt seem to be too much difference in these circuits - except that one has a Smooth pot instead of the Bias pot - I might add a Smooth pot in after if I like what it does. And it has polarity protection. Is the input capacitor the 2u2f elect cap on the diagram I went from?
I like setting the bias on the trim pots so on the knob I can get a raspy grinding fuzz like Hendrix with wah engaged - but if I turn it on full I get a totally saturated beasty distortion - Very cool!
I wonder why they got rid of the external bias pot on that newer revision?

[enigmur]

Its going! That capacitor did it!

I'll do a Build report

[enigmur]

Looking at the layout I used - first page... Can you see anything which would stop the Fuzz control from working properly? It doesnt really effect the sound until the last turn of the pot.

Is anything laid out wrong on that schem which could do that?

[tcobretti]

Congrats on getting it to work right! 

I don't see anything in the layout that would make the fuzz pot not work right.  But like I said, some people don't even install the fuzz pot; they just use the guitar's volume pot to control the amount of fuzz.

[enigmur]

Yeah it is still usable, there is just like 80% of doing nothing, and then the last 10% does the most.

It has Germaniums in there now, pretty slick 

Just building the box for it at the moment, ill be sure to post pics

[enigmur]

Here are a couple of pics. I've done about 4-5 coats of primer, and will be applying the orange before I go to bed.

And a shot of my circuit before it goes home!







Will post Orange pics tomorrow.

My friend who is a screen printer is going to make me some vinyl stick on lettering for the box aswell - handy!

[enigmur]

Heres a couple of shots of the box looking a bit more orange.

The bottom has one coat of enamel, it isnt perfect but I dont care about the bottom 

However, the top has one hair and three specks of dust in it (Am I being ANAL!?)

Anywho, heres a pic of the font and to be decals (If I can convince my fantastic screenprinting friend...)
And the boxes in my cupboard.







I am going to very lightly sand the top tomorrow and give it one more coat, and if my friend will do the decals I will wait before coating it

[tcobretti]

Nice job.  I wish my enclosures turned out that nice. 

You might want to post these in the Pictures thread, where they will be seen by more people.

[enigmur]

Thanks 
Yeah I will post them in the pics thread, I still need to do a bit of work on it but its looking okay! My first attempt was horrible.

I suggest that you actually practise spray painting before expecting something to turn out well  My crash courses in stompboxery don't seem to be going too badly!

[petemoore]

  Small Bear sourced and sells [might want to check current availability..] the 1k FF gain pot.
  Other available tapers will provide the same approximate resistance range [this can be altered by adding fixed resistors], but most of the gain control you use is bunched up at the last 10% or so of the gain control, if you have trouble dialing it in get the SB pot.

[enigmur]

Cheers for the info Pete, How do I add the resistor to the pot to get the different taper effect?

[petemoore]

  You get a couple pots say 500 ohm, and one at 5k.
  To increase gain, an increased resistance 'in' the 1k gainpot...there's a 100 ways to wire the pot, I started and ended by splicing a resistance between the gain pot and ground, something that adds to a total of <2k, which is too high gain for the gain on the gain of transistors I'm using ie experiment with the 1k - 1k5 range a 1k pot + 500ohm fixed resistor does.
  Then if you want less max gain, reduce the resistance 'of' the 1k pot, using a parallel resistance across it [parallel resistors divide], the 5k pot will bring the 1k to smaller values.
  Two testclipped wires off lugs 2 and 3 of the pot makes it pretty easy to try resistance mods, increasing it is pretty easy too, just lift the ground connection at the gain pot to splice in added resistance...which of course bumps the whole range up by it's value.
  If the math seems confusing start with just the DMM, the pot and the resistors to see what values are ranged by adding caps. a 1k pot and 1k resistor ranges between 1k and 2k. 1k4 pot and a 500 ohm ranges between 500ohm and 1k9.

[enigmur]

So if I were to wire a 500 ohm resistor in line to ground, and use a 1k pot, this would make the range 500 ohm to 1k5? This seems it would work beter, as I get no action really from 0 to about 800 ohms. I'd end up with being able to set in way too much gain, but that's okay/

[enigmur]

I have finally got this going sweet and in it's box.

I have a couple of problems -

It sounds like crap with my wah - if I tilt my wah back it squeals with that ocillation. A fuzz that sounds bad with wah just sucks. The wah doesnt really have much effect on the fuzz either.

2nd it is really really loud, it sounds best with volume maxed but it is waaaay too loud compared to clean. Its like there is something boosting the signal way too much in there and I'm not sure what...