How to limit the volume of runaway feedback/oscillation in PT2399 delay pedals?

[sdb guitars]

I'm tinkering around with various incarnations on PT2399-based delay pedals, and I love the ability to set off runaway feedback/self-oscillation, but the exponential volume increase is killing me.  Is there any way to limit that so you get the great sound of the oscillation without blowing up your amp (or manually turning down the mix on the pedal)?

Thanks,

Shawn.

[jimilee]

I think there's a thread somewhere around here about that same issue,you put a resistor on lugs 2 and 3 of  mix pot I believe. I can't recall the value though.

[RobA]

On the Zero Point Micro, he has the diode clamp mod. It works well. I've been playing with the self oscillation on it a bit and I like it. It doesn't seem to add much noticeable distortion because you are saturating from the noise of the lo-fi-ness of the PT2399 infinite repeat anyway.

It looks like it just takes back-to-back 1N914's from the wiper of the feedback control to ground. You could adjust the level and character of the clamp by using different diode configurations here.

[danwelsh]

So could you use zener diodes on the zero point micro with the clamp mod?

[RobA]

Yeah, I suppose you could use a Zener. But, I don't think it is going to get you anything beyond a normal Si diode. Since they are back-to-back, you aren't going to get near the reverse voltage. So, it's going to act like a normal Si diode.

Trying different Schottky and Ge diodes will get you different voltage levels and perhaps a different tone to the distortion in the oscillation.

[sdb guitars]

So, on something like a Sea urchin build, would you just disconnect the wiper of the regen pot from the board and solder a pair of back-to-back diodes between the wiper and it's pad on the board?

Thanks,

Shawn.

[RobA]

My guess as to what's going on with the Zero Point Micro is that you have two diodes, one up, one down from the wiper of the feedback pot to ground (lug 2 to ground). This is essentially the clipping configuration for the Distortion + type circuit. Anything over the forward voltage of the diode gets shunted to ground. One diode handles the positive swing and the other handles the negative swing of the signal.

So, you want the diodes to be perpendicular to the signal flow, not inline with it. I haven't built a Sea Urchin, but it looks to me like the easiest way to do this mod would be to wire the diodes directly between lug 2 and lug 1 on the FDBK pot. The original connections on the pot still need to be made the same way they are on the unmodded circuit. Remember that you want the diodes to be reversed from each other and both have one lead to lug 2 and the other leg to lug 1.

[sdb guitars]

Hmmmm  I will have to give that a try this weekend.  Thanks!

[RobA]

You're welcome. Let us know how it works.

[sdb guitars]

An update - success!  It works pretty well. It still gets louder than the standard signal, but it hits a point where it doesn't get any louder, it just distorts, which isn't unpleasant, as it turns into a sort of wash/crashing waves sound that is a nice ambient noise.

[RobA]

Cool. I think you could get different output levels by using different diodes (Schottky or Ge) with lower forward voltages. But, they might get too quiet and would change the character of the distortion.

I like the it ends up in the washy noise thing too. It's fun to play with and tune using the feedback level and delay knobs. On the ZP micro, the modulation enhances the washiness too.