My Rat whistles???

[tech9_79]

Hey guys,    Im have problems with my BYOC rat.   I have a thread going over there but no one seems to be able to help,

The effect works fine,  but if I turn the gain and tone knob all the way up the pedal starts to squeal!!   now if I turn either the gain or tone controls down(about 1/5) the squeal fades out.  I also found that if I unplug the output touch the pedal jack and wave my hand over the cord I can hear the noise!   
I tried pulling the wires to my diode switches with no luck.   moved wires around ect

Now this is the second time I have built a rat with the same problems???


anyway I used a tL070 and a 71 IC  2N5458 tranny   the 33pf comp cap is in place but not connected.   

[RDV]

the 33pf comp cap is in place but not connected.   

Hook it up.

[tech9_79]

well I had it connected on the first rat and it still squealed, then i disconnected it with no change,  even tried a few other caps on up to 330pf,   nothin..

[Shepherd]

http://www.diystompboxes.com/smfforum/index.php?topic=52556.0

[tech9_79]

well my first rat had all film caps,  the amp im using is a roland cube 30,  could that have something to do with it?

[Papa_lazerous]

When you say whistle could this be feedback from having gain and volume right up and having amp up or is it an actual whistle?

[tech9_79]

nope its not feed back,  I have red Leds clippers mounted to the box so when I play they flash, turn up the controls the led lights up,playin or not,  with the amp on or not.     

To best describe the sound I would say it sounds just like a theremin. 

Now I think the pedal is going microphonic, alot of static with gain or filter all the way up.

I have been looking at the 100pf cap that goes across the gain pot, does this control high frequency rolloff? I used a film cap on the first rat, maybe a higher value?

Also electrolytic caps, what part is the metal tops? the reason i ask is I touched the tops of the 4.7uf(next to IC) and 2.2uf with my pilers and the squeal stopped! with little change in tone.

Any ideas?

[Meanderthal]

 I'm pretty sure the metal top of the electrolytics is negative. Are they oriented correctly? Also, that 33pf compensation cap isn't necessary for a TL070, that's  for in case you want to use a LM308. I'd either get the(well personally I'd definitely get it) 308, or remove the cap altogether. Also, that tranny's an fet, right? Maybe something lower gain like a 102 might tame the oscillation.

[Mark Hammer]

The "whistle" IS the sound of a dimed Rat from what I understand, except that the whistle should be a sort of background rather than foreground sound.  It normally comes frmo the use of an LM308, ridiculously high gain for the treble part of the signal, and the compensating capacitor that keeps the op-amp from being too unstable.  Normally, or traditionally, it should be a "whooooo" and not a "wheeeeeeee".

Although a TL071 is internally compensated and requires no external cap, a TL070 DOES require such a cap.