Fuzz Face debug driving me nuts! Oscillation issue.

[PurpleStrat]

So I am build ing a FF on a BYOC board and it oscillats like crazy. I know that means that something is crossing but I have taked eveything off the board peice by peice and redid it and still it's their! How can I find the issue?

[petemoore]

  Have you turned the gain down?
  The circuit has the potential to self-oscillate if gain is above a certain level.
  Good first step [after checking for no PS shorts] is to verify it is a no-bug FF circuit, take bias voltage measurements.
 

[PurpleStrat]

  Have you turned the gain down?
  The circuit has the potential to self-oscillate if gain is above a certain level.
  Good first step [after checking for no PS shorts] is to verify it is a no-bug FF circuit, take bias voltage measurements.
 

It's at 4.5v 

[John Lyons]

Try a small cap from base to ground off Q1
Even a moderate sized cap like .01 will help and still sound good.
Sounds too big but it does work.

john

[R.G.]

Just out of curiousity, did you make this with PNP transistors and a negative ground?

[PurpleStrat]

No it's Si NPN but I have come to the conclusion I am using transistors with to much gain. Thing is I hear a great tone under the oscillation.

Is there a way to make a high gain FF work with out oscillation? Maybe I should take another look at the JH series from Dulnlop.

[John Lyons]

Put a resistor between ground and the emitter of Q1, 100 or more ohms.
Which transistors are you using by the way?

john

[raycroft]

he asked you a question,  boy!!!! !!

just kidding 

[PurpleStrat]

Put a resistor between ground and the emitter of Q1, 100 or more ohms.
Which transistors are you using by the way?

john

I have tried BC108B's (newer version)  and 2n2222's. Even with the 2n2222's  that have an hfe around 100 it oscillates. I still need to try the cap suggestion you made as well.

[DougH]

I suspect this is useless to ask at this point but have you tried contacting BYOC about it?

[petemoore]

Is there a way to make a high gain FF work with out oscillation?
  Fine out ! Freq./gain correspondence.
  Try taming the HF's. This can be done with the gain, and kirkpacitors. Putting a cap here or there 'not in the signal path' is goods way to try. See 'Axis Face'.
  B/C capacitor or capicitors. Capacitor to ground. Input loading or input gain control].
  gain is a term used for 'wide frequency'. High frequency gain is of course part of gain. High frequency gain taming is just that. You might want to exp. with a third transistor [like add boost to FF = Tonebender] for a higher gain result.
  I suspect this is useless to ask at this point but have you tried contacting BYOC about it?
  What did Dorothy say after clicking her glass heels ?

[PurpleStrat]

I suspect this is useless to ask at this point but have you tried contacting BYOC about it?

I'm only using the BYOC board. Everything else is my stuff. Plus you guys seem alot better then they do over their! I am going to try some of the suggestions in this thread and some other transistors and I'll see what happens.

[tcobretti]

Hmm, it seems like it shouldn't oscillate with the 2222s.  You should post the exact schem you are using.  Also, are the input and output wires close to each other?  Signal can bleed thru the wires and oscillate on high gain circuits.

[Pushtone]

I got a bunch of 2N2219A's and they sound great in an Axis Face.
All the data sheets I've see have the spec hfe around 75.



Good luck, Dave

[petemoore]

  Because signal is present ~all over in the FF...that said, perhaps audio probing might give a clue.
  Check all ground points, transistor pin voltages.
  Widgety little circuit sometimes, but it should work with 2n2222's..
 

[PurpleStrat]

OK I got it to work with BC107A's as well as the 2n2222's if I lower the gain just a hair. Also it did work with the .1uf to ground. It's just so dark and bassy! I put in an AC187 for Q2 and a 2n2222 for Q1 ad it got much better. MNow I have a cool little hybrid! Still to much bass! I'm gonna have to try lowering the 2.2uf next. Thanks for all the help!! 

[tcobretti]

Another trick you might try is to take out the .1u cap to ground, and put a smaller resistor in the feedback loop maybe 80k to 90k to start with.  This lessens the overall fuzz of the circuit.  A trimpot might be the best idea.  You can also use higher gain transistors if you play with this value.