Crybaby Q mod issue

[AndersonCouncil]

I have searched this forum for an answer to this problem and see that others have had the same issue but I'm not sure about a solution. I installed a B100K pot in series with a 20K resistor in place of the 33k resistor that's parallel to the inductor (the vocal mod). I tested both the pot and resistor before wiring and with the resistor connected to the wiper I was getting a range of 20K-120K as expected. However, once it is wired to the pcb in place of the resistor it does not seem to work. Turning the pot from low to a high setting sound the same.

And when I try to get a reading from the pot it now reads something like 16K—.489K. I have installed trim pots in place of the 68K input gain resistor and 1.5K mid resistor and when I take a reading from them they each read correctly. I initially had just the wiper and one of the outer lugs connected, then I tried bridging the unused 3rd lug to the wiper and neither made a difference. Any suggestions or explanations as to why this isn't working?

[MrStab]

hi AC, welcome aboard!

when i did this mod, it was quite subtle, but you could tell some difference.

check this out, it goes along with what you've read about other people experiencing this problem:

Note that while this action is clearly visible on a scope, the effect in subtle in practice. I initially replaced the 33k with a 22k resistor in series with a 50k pot. Wanting more effect, I swapped a 100k pot in, but it didn't seem to make any difference. The maximum Q is probably dependent on other circuit components, and continuing to increase the resistance probably does nothing beyond a certain point.

from http://www.jasonwolley.com/crybaby_revisited.html

have you changed the position of the gear & treadle at all? in some filter circuits, the Q varies with frequency, so maybe other people notice it more because they've changed the range of the wah to one where the change would be greater across the sweep. this is conjecture, admittedly, but it's one way of explaining it. it's possible other mods make this one more noticeable, though.

[drummer4gc]

There's another method for getting variable q that sounds a little different and is even simpler, if this way doesn't do it for ya.

Take a look at the 535q wah schematic - they add a 1k pot in series with the inductor. If I remember correctly, the standard cry baby circuit board has a jumper in this very spot, so you just remove the jumper and install your pot there. I'd also up the 33k q resistor to 100k, as the pot will basically just turn down the q now.

Enjoy!

[AndersonCouncil]

have you changed the position of the gear & treadle at all? in some filter circuits, the Q varies with frequency, so maybe other people notice it more because they've changed the range of the wah to one where the change would be greater across the sweep. this is conjecture, admittedly, but it's one way of explaining it. it's possible other mods make this one more noticeable, though.

I did back the treadle down about 2 or 3 clicks from the stock position, and you are right, it is more noticeable at lower frequencies. It was still had a little too much treble to notice a difference. I installed a switch to select between different sweep cap values and when I switched it to the cap with more bass, the variation in Q was much more pronounced.

Thanks for the reply!

[Transmogrifox]

Another trick to increase Q (if increase is what you want) is to replace the BJT with a Darlington JFET or MOSFET.  The resistance reflected from the BJT base comes to something near 50k, which shows up effectively in parallel with the 33k across the inductor.  You can see that increasing the value of the resistor in the 33k location reaches a point of diminishing returns.  The feed resistor shows up in parallel with the LC network and serves to reduce Q.

It's easier to get the bias to work with a MOSFET or darlington rather than a JFET.  Another way to increase Q is to make the input feed resistor bigger since this also reduces Q (especially at the higher end of the sweep range). 

In summary, try this:
Change input 68k feed resistor to something much larger, like, say, 220k.
Change input 10n feed capacitor to 3.3 nF
Change the BJT gain stage to a Darlington.
Change the 82k bias resistor to 150k
Change the 1.5k resistor to  4.7k

Then the 100k pot will make a little bit more audible difference.  At the top (high end) of the spectrum the Q changes from about 5 with the 33k up to about 11 when turned up to 100k.  Much more extreme at the low end.

You will notice volume drop from changing to 220k feed resistor.  If you want to compensate for that then also include these mods:
Emitter resistor change from 390 to 100 ohm
Feedback cap change from 10nF to 7nF  (or just leave it alone if you like the downward range shift).