Vox V846 clone problem!

[jcsus]

I built a Vox V846 board and dropped it into an old Crybaby shell. Works well but when I put the heel all the way down it makes a crazy feedback sound!

I put in an extra ground off the output jack and I changed to wah sweep on the pot by turning it a little, but to no avail.

Has anyone every had this happen?

This wah sounds amazing, except for when I put the heel all the way down!

I used a whipple inductor and tropical fish caps. PCB was from PigeonFX.  All other parts were from Mouser.

[Joe Hart]

Is it a strange "hollow" type of sound? Like some kind of resonance? I've had that happen and I believe it is a little too much gain in the bass frequencies. Or a microphonic part/solder joint? Sorry I don't have a definitive answer for you, but this is what seemed to be the problem with mine. I'm sure someone else will chime in.
-Joe Hart

[jcsus]

That's exactly what it sounds like! Just overpowering!

Where would I even start for a problem like this? How would I shield a microphonic part. I thought it may be the inductor, because of its close proximity to the output jack.

And to the smart a**es out there... I have read it! (http://www.diystompboxes.com/smfforum/index.php?topic=29816.0) 

And thanks for the start Joe!

[jcsus]

I tried putting foam under the circuit board and messing with the play on pot. But neither worked. It definitely sounds like a case of microphonics, but how do I fix it?

I want to try to isolate the inductor but I'm not sure where to start! 

Any help would be greatly appreciated!

[jcsus]

Possibly could the tubes from my amp interfere and cause microphonics?

[jcsus]

So....No takers? I still need help!!!!

[paulyy]

So....No takers? I still need help!!!!

Do you have any pic's of the board. Maybe you have a wrong resister value or maybe a wrong cap value. You'd be suprised how easy it is to miss something like that. It happened to me a few weeks ago working on a wah layout. I mistaken the .01 caps for .001.

[jcsus]

I'll post pics soon! I triple checked the parts! 

[paulyy]

I just thought about this. Are you using BC109 or BC108 trannies. I've tryed both and they had that sound that you described. I put in 2n3904 silicon trannies and it sounded fine. You can use most NPN silicon trannies to see if this helps. Those BC109's and BC108's tend to be noisy or maybe I got bad batch. Hope that helps.

[joegagan]

wahs have a resonance hump sometimes. sounds like your biasing is making the gain really hot at that specific frequency. some of my favorite wha sounds have a lot character because they are hanging on the ragged edge of self feeding back. commercial wahs try to design this out so that they don't get complaints.

i would say if you just raise the emitter to ground R a little bit, you would have a great sounding wah!

you could also play around with lower gain trans, but i have found it easier to adjust biasing first to see if i like that. both accomplish generally the same thing. there are some finer points to comparing the two methods, but i won't clutter the thread with them here.

if simply lowering the output of Q1 doesn't help, there are a few cap alterations we can try as well. using a slightly (10%?) smaller sweep cap, the .01 at q2 emitter/inductor junction, this may move the resonant hump upward enough to solve the prob as well.

in very rare cases, there is a mechanical resonance with the wah and the tensioning spring, to check this out, spraying a little lubricant on the axle makes that sound go away. but i doubt this it.

[joegagan]

if you install a 1k trimpot in place of the q1 emitter to gd R, you can tune the gain. i would recommend this method , as you can tune the wah to just barely get rid of the feedback, this is where the coolest tone would be IMO.