Feedback Loop cap

[dosmun]

I just built a TS with the "Landgraff" mods.  It would oscillate when maxed out.  I increased the 51pF  to 100pF.  Will this have much of an effect on the tone?  I haven't gotten the diode switch reinstalled yet so I can't tell if it made much of a difference or not.  I am just curious as to what this change will make.

The pedal actually worked fine before I boxed it up. After I got it in the enclosure I started having the oscillation.  I couldn't find the cause so I tried increasing the cap until it quit.  I most likely am going to rewire it again and try the 51pF but if the 100pF won't change much I may just keep it that way.  I did change the wires on the Volume pot before boxing it up but moving the wires around on the pot didn't seem to affect it in any way.

What do you guys think?

[LP Hovercraft]

When you had it outside of the box, was there a switch attached?  Sometimes oscillations will occur when the internal contacts within the switch have the input and output too close together causing oscillations.  If this is the case, try wiring to a 3PDT switch using the center row for the LED instead.  I've made squealers shut up that way before.  Increasing said cap value will unfortunately dull some of your high end.  If you can find a happy pF value that stops the oscillations and retains the tone you want, then go for it.  Another thing you could try is to use shielded cable for your input and output wiring.  Try moving wires around within the box.  Sometimes the wrong two can be right next to each other and it's a matter of moving them apart.  Oscillations are a very frustrating problem, just back off from what you're doing for a while if you feel like grabbing the hammer.  Good luck!

[dosmun]

I didn't have the 3pdt hooked up before putting it in the enclosure, but I do have the LED using the middle lugs of the switch already.   Tonight I will probably pull it back out of the box and start working backwards to find out what is causing the problem.

I do think I will be able to increase the cap size without hurting the tone too much.  The TS has quit a bit of high end available anyway and I usually run it on the lower side.

[WGTP]

If I did the formula correctly, with a 1 meg resistance in parallel, 100pf will start rolling off the highs a 1600Hz.  At 500K it will be 3200Hz at 250K it will be 6400Hz.  As you increase the resistance you get more high frequency reduction.  If you half the cap to 50pf, it doubles the frequency.