Cave Dweller Dub Ed Oscillation Problem

[nieradka]

Hi all, I built the cave dweller dub ed, and it sounds great except when I have the dwell pot past half it starts to go into runaway oscillation increasing in volume. Any ideas on the possible cause?

I checked for solder bridges, remelt,  and checked values against the BOM. I built two and they have identical problems. Its not boxed.

I did not implement the mix mod. And left the pads open.  I had to substitute a 27pf cap for C6 (22pf) cause I forgot to order them and it was as close as I could find. I didnt think it was a big enough difference to matter.

(Also, Is there a good resource for the PT2399 based circuits, I understand half  of whats going on from the datasheet, but is there anything more comprehensive? )

[nieradka]

Oh, and R12 is 750k, R20 is 200k as recommended in the build doc.

[madbean]

That's more or less normal behavior. The dwell pot is pretty sensitive after the halfway mark. You can change this pretty easily, though. Either increase R12 to 1M or the Dwell pot to 100k or 250k. I'd probably go with increasing R12.

[midwayfair]

Also, Is there a good resource for the PT2399 based circuits, I understand half  of whats going on from the datasheet, but is there anything more comprehensive?

http://www.valvewizard.co.uk/PT2399_Data_Notes.pdf

[nieradka]

Thank you, Increasing R12 to 1M improved matters significantly.

And thanks, that was the kind of resource I was looking for.

[jimilee]

That's more or less normal behavior. The dwell pot is pretty sensitive after the halfway mark. You can change this pretty easily, though. Either increase R12 to 1M or the Dwell pot to 100k or 250k. I'd probably go with increasing R12.

This is what I like about the cave dweller, works great for when we play industrial music, make great sounds.

[wolfingsworth]

Not to change the subject, but I didn't want to start a new thread about the same project. Can I use a mpsa18 instead of bc550, or would it have too much gain?

[madbean]

The BC550 is just a current mirror, so gain is not a factor. You should be able to use any BJT. The pinout for the MPSA18 will be 180 degrees around.

[wolfingsworth]

The BC550 is just a current mirror, so gain is not a factor. You should be able to use any BJT. The pinout for the MPSA18 will be 180 degrees around.

Awesome thanks!