controlling the modulation of a pt2399 echo circuit

[thatdavejordan]

hey guys, i've lurked here for a long time and have gotten a ton of useful information that's helped me in my building journey, but this time i couldn't quite find what i was looking for.  I've started messing with a basic pt2399 echo circuit on my breadboard based on the following schematic.


how i start learning and designing is usually starting with a basic circuit schematic like this and just starting to mess with things and see what does what, so i've swapped a few capacitor values but haven't done too much else.  i'm not sure if i maybe did something slightly off but there's quite a bit of vibrato modulation on the repeats, which is very cool but i'm mostly curious what good methods there are for controlling said modulation.  i've dug a little into the electrosmash page about the chip but it's a little over my head at this point, so i guess i'm mostly looking for some kind of explanation like "mess with the value of resistor off pin 12" or a resource that breaks down what the different pins of the chip are doing more functionally, thanks!

[Ripthorn]

Modulation occurs when the delay time wobbles around. I don't see anything that will do it on that circuit unless you really changed values a lot. A very common technique for modulating the repeats is to connect an LFO to pin 6 similar to how it is done in the Keeley magnetic echo. There is a schematic at PedalPCB.com. Look for the Magnetron delay.

[anotherjim]

Depending on the breadboard layout, pin6 may be picking up some AC hum at high delay settings that might cause the delay time to wobble.

[merlinb]

Maybe you've accidentally discovered the self-modulation effect that deadastronaut reported?
https://www.diystompboxes.com/smfforum/index.php?topic=128518.msg1238065#msg1238065
https://www.diystompboxes.com/smfforum/index.php?topic=129208.msg1247519#msg1247519

[anotherjim]

^ That a possibility if there's something going on with the cap to pin7, bad contact or actually of lower value than you think.

[thatdavejordan]

Modulation occurs when the delay time wobbles around. I don't see anything that will do it on that circuit unless you really changed values a lot.

that's what i thought, but most of my changes were on the 9-16 side so more eq filtering and feedback

Depending on the breadboard layout, pin6 may be picking up some AC hum at high delay settings that might cause the delay time to wobble.

this is certainly possible, it's a breadboard so there's all sorts of extra noise, i guess it'll be interesting to see if it does it when i build it up.

Maybe you've accidentally discovered the self-modulation effect that deadastronaut reported?
https://www.diystompboxes.com/smfforum/index.php?topic=128518.msg1238065#msg1238065
https://www.diystompboxes.com/smfforum/index.php?topic=129208.msg1247519#msg1247519

there's no bridge between those pins, YET

^ That a possibility if there's something going on with the cap to pin7, bad contact or actually of lower value than you think.

tested them, they read at 95nF, so wouldn't think that'd far enough out of spec to really make a huge difference.