FuzzFace squals when high volume on amp

[carrejans]

Hi guys!!

I built a FuzzFace, with pcb and specs of Tonepad. (neg ground) Used two AC128's.
It sounds not bad; but when my amp (Bassman 5F6A) is set at a high volume, the fuzz is at no control. It starts to squal, when not playing.
I know it's probably not easy to make a fuzz pedal quiet when you're not playing. But this is really annoying loud squals.

How can I reduce this?

Thank you !

[Papa_lazerous]

Try switching it to positive GND for a start

[carrejans]

Try switching it to positive GND for a start

Can you explain a bit why this would help?

[carrejans]

bump...

[anchovie]

Can you explain a bit why this would help?

Because you are using PNP transistors in a neg ground circuit. It's not guaranteed to not squeal, in fact in the Fuzz Factory it's done on purpose to induce squeal!

[Paul Marossy]

That can/will happen with practically any distortion pedal. Doesn't sound like anything unusual to me.

[carrejans]

But it is unusual. It's really annoying. When you are playing something really rythmic and you damp your strings, it starts squaling.
So if I make the fuzzface positive ground, the problem will be gone?
Or maybe I should use a Noise Gate? (never used one before)

[John Lyons]

PNP transistors use a positive ground (usually)
NPN use a negative ground.

Right now you have them mixed up.
Switch to a positive ground setup and it should work better.

[carrejans]

Or can I just plug in NPN transistors in my circuit?

[John Lyons]

Yes, of course. If you can find some decent NPN germaniums.

[nick d]

This may well not be a fault in the pedal. If I crank up the gain on my little 10W amp , and stand wthin a yard or so , it squeals like a stuck pig! Try backing off the volume on the guitar or pedal ( or even the amp!) This phenomenon is called feedback , and is aggravated by high gain , which is a basic property of distortion pedals. Some guitars are more prone to this than others. If you can , borrow a different pedal and guitar , and see if it still happens in various combinations - this might nail the cause of the problem.

[carrejans]

Nick, it definately isn't feedback.

I will try to find NPN, or to change to positive ground.

Thanks guys!!

[wavley]

I would think if it were oscillation due to the negative ground that it would happen at low volume also.  Sounds to me like it might be a microphonic pickup that with the combination of distortion and sound pressure level makes a squeal.  Of course I'm not there to hear it so I'm just guessing.

[carrejans]

So, I'm using a pcb from Tonepad. I wired it like in the negative ground layout: http://www.tonepad.com/getFile.asp?id=82

Should I change anything else when using NPN transistors?
Or do I just have to map base to base etc.?

thanks...

[carrejans]

So, I'm using a pcb from Tonepad. I wired it like in the negative ground layout (with PNP's): http://www.tonepad.com/project.asp?id=38

Should I change anything else when using NPN transistors?
Or do I just have to map base to base etc.?

thanks...

[carrejans]

I did it this way:

I used the positive ground PNP layout. Put in NPN's and change the polarisation of the battery.
But didn't work. 

So can you guys help me out, how to correctly do this?

Thank you

[Quackzed]

you also have to reverse any polarized caps. caps that had their + side going to ground need to be flipped and have their - side going to ground as well ,thats for npn. if you used a pnp circuit and want to put npn's in it, you need to reverse the battery, reverse the polarized caps(for  - to ground) and use npns.

[R.G.]

Did anyone but me ever notice that piezoelectric pickups and ceramic caps are made out of similar materials?

Ceramic caps are occasionally microphonic.

Unless the amp and the fuzz face share a common power supply and power ground, or there is some other means for the signal to be getting back into the FF, I think that checking for microphonic something or other in the FF would be a good idea.

[petemoore]

  The easy way to explain to get polarities correct in a FF is to point out the schematic as showing all relevant polarity.
  The polarized components needed for working FF:
Battery
Capacitor
Transistor
The battery shows polarity, red/small button is +.
Electrolytic capacitors usually have a - marking or polarity indication. There are bipolar electrolytics [non-polarized].
Film caps [which you may be able to find values for the input and output DC blocking capacitors for] have no polarity, either way around is fine.
  Transistors have a little arrow pointing in, or out, these are shown on the schematic, and the data sheet, they have to match.
  If using a diode to protect against battery touching backwards [or other forms of reverse polarization], it too must be oriented correctly, they have a line, and the schematics exactly illustrate the orientation of the RP diode.
  Choose the transistors and 1 schematic, make the build match the schematic in every way.
  Resistors are of course non-polarized.
  Skip what I typed  below until after reading at GEO: Secret life of pots.
  Pots are 2 resistances that are divided in the middle, if the right value is used it'll 'work'. Reversed outside lugs on log pots 'works' if the right value but the taper will be 'funny'.
  Also, Smallbear sells the FF gainpot with a value that makes the adjustment spread nicely over the whole range, other pots'll make a ff work, but with most/all the gain control at the last 1/8th of the knob turn.
 

[amptramp]

I have looked at the schematic for the tonepad board and it appears they have no electrolytic or film bypass capacitor for the 9 VDC power.  The internal resistance of the battery is in series with both transistors and you can get feedback between both amplifier stages through the supply via the common impedance of the battery.  Wall wart supplies may not be any better - they frequently have a resistance in series with the internal filter cap, so this may not help and it will be on the wrong end of a long cable.

And as R. G. says, never use ceramic caps in audio applications - part of the voltage goes to flexing the ceramic because it is piezoelectric and the output of the cap is significantly different from the input.  They can act as microphones as well, so you can expect microphonics.  There is an explanation with oscilloscope traces shown here:

http://greygum.net/sbench/sbench102/caps.html

Yup, this is the old Steve Bench site, one of the most interesting in audio, gone but not forgotten in its new spot in the wayback machine.