Vox Grey Wah Build Report / Inductor Revelation

[Paul Marossy]

I used my Vox V846 PCB Layout to whip up a Vox Grey Wah circuit a few days ago. I used tropical fish caps in it, BC109 transistors and wired it for true bypass. For an inductor, I used a 370mH inductor from an old RadioShack EQ which has a DC resistance of about 105 ohms. I stuck it in a wah shell last night and finished up the wiring. It actually sounds pretty good as is, maybe a little on the trebly side, but it responds really well to rolling back the tone control a little bit. It sounds really good with a Marshall amp kind of distortion, too.

In the next few days, I am going to experiment with resistors in parallel with the inductor for fun. I know that the "ideal" inductor for the standard wah circuit is 500mH with a DC resistance of around 60-85 ohms, but there seems to be room for inductors with a lesser Q than all the reading I have done has led me to believe. According to the original wah patent text, the circuit has a very high Q, so I think it can stand an inductor that's less than perfect - at least to my ears. I think in this case, I just got lucky. But, I am still looking for a 250mH inductor, though. Anyone know where to get one?!

EDIT: I'll post some pictures of it later on today.

[Paul Marossy]

Here's a view of the PCB as installed:

[Paul Marossy]

I tried a 50K pot in parallel with the inductor yesterday. There is a very small range before before you lose the wah effect. Then I noticed one interesting thing while looking at the wah patent last night - it doesn't show a resistor in parallel with the 500mH inductor. That was apparently added at a later date, after the patent was submitted.

Armed with this information, I can only assume that the 33K resistor was added because the Q of the circuit was still too high, and this was their solution to bring it down some. This little experiment has shown me that the resistor in parallel with the inductor is pretty important to how vocal the pedal sounds. Make it too small, and it'll become mute! 

I think in my case, since the inductor is a 370mH with a DC resistance of 125 ohms, the Q is low enough already. Taking this a little futher, it may be possible to use the RadioShack transformers as suggested at GEO to good effect by adjusting or omitting the resistor in parallel with the inductor. YMMV.

Here is the grey wah schematic for anyone interested.

[no one ever]

good job, paul! vintage to the point of antiquity 

[formerMember1]

hey Paul!!

That looks cool to me! 
Is that a 4.0uf cap on the left from Mouser?

there are some inductors in the mouser catalog that are either 220 or 270mH, they have a resistance of like 30 or 50 or something and are some kind of ferrite,...they also say they have a minimum Q of 20 30 50 or something,..i read these once in the mouser catalog but forget what number, if you want i could search for it,..i am pretty sure the above was correct 

ya think the trebly sided tone is from the inductor in that wah?

Paul, what camera did you use to take the pic?

[Paul Marossy]

Is that a 4.0uf cap on the left from Mouser?

No, I'm just indicating a 4uF cap because that is what was in those early circuits.

there are some inductors in the mouser catalog that are either 220 or 270mH, they have a resistance of like 30 or 50 or something and are some kind of ferrite,...they also say they have a minimum Q of 20 30 50 or something,..i read these once in the mouser catalog but forget what number, if you want i could search for it,..i am pretty sure the above was correct

I haven't really tried searching for an actual 250mH inductor, yet. I kind of like it with the inductor that I have in it now, though. If you want to point me in the right direction, be my guest. 

ya think the trebly sided tone is from the inductor in that wah?

I believe so. But, if you like to use your neck pickup a lot and it's a humbucker, it actually works pretty well. Especially when pushing a distortion circuit.

Paul, what camera did you use to take the pic?

Belive it or not, it's a digital camera that's built into my JVC VHS-C video camera. 

[formerMember1]

No, I'm just indicating a 4uF cap because that is what was in those early circuits.

oh, i meant that the cap looked silver and long for an electro so i was wondering what it was,..i just assumed it was the 4.0uf caps from Mouser cuz they are silver,..

It is too late for me now, but first thing tomorrow i will point out some inductors from Mouser 

Belive it or not, it's a digital camera that's built into my JVC VHS-C video camera. 

yeah that is funny, i just bought a  $479.00 digital camera, and the old digital camera in my 6-7 year old vhs video camera looks just as good,...the reason i asked was your pic was clearer then my new camera 

[Paul Marossy]

oh, i meant that the cap looked silver and long for an electro so i was wondering what it was,..i just assumed it was the 4.0uf caps from Mouser cuz they are silver,..

I see. Actually, it's an NOS 4.7uF Sprague axial electro cap.

yeah that is funny, i just bought a  $479.00 digital camera, and the old digital camera in my 6-7 year old vhs video camera looks just as good,...the reason i asked was your pic was clearer then my new camera

And that's before I used a program called Xarax to shrink the size of the file down and stuff... 

[formerMember1]

PAUL:

here is an indutor that looks like a resistor,

1641-224K 220 mH   55 Q min

1641-274K 270mH   55 Q min(don't know what Q means,..)

both $1.32 each,...


1025-76k  220mH    min 30 Q      .83cents each
1025-78k   270mH    min 30 Q      $1.04 each

they look like resistor shaped, but are definitly inductors,  i am looking at them in the Mouser paper catalog,...
I don't know if they will work/sound good or not,...personally i am gonna stick with an arielfx halo,..

But, TOM from arielfx is VERY nice,...one of the nicest people i ever dealt with,..always helpful,..

He will probably wind you a halo to your specs if you want,...currently he winds 500-520mh and around 30ohms,..

he will probably make you one at 250mH, but it won't be 30ohms most likely,..probably higher around 80-100ohms,...

give him a email and ask him,..if you know the specs for the series resistance tell him that  number then,..it will probably be a little more than the normal price though,...probably 1/3rd more,...

I plan on asking him to make a halo for 500mH but with 89ohms instead of around 30ohms,...

[Paul Marossy]

they look like resistor shaped, but are definitly inductors,  i am looking at them in the Mouser paper catalog,...
I don't know if they will work/sound good or not,...personally i am gonna stick with an arielfx halo,..

Hmm... those are RF inductors. I have no idea how they would work in a wah circuit. You're probably better off sticking with what we know works, unless you are feeling adventurous.

To answer your question about what "Q" means: the Q factor (or quality factor) is a measure of the "quality" of a resonant system. Resonant systems respond to frequencies close to their natural frequency much more strongly than they respond to other frequencies. The Q factor indicates the amount of resistance to resonance in a system. Systems with a high Q factor resonate with a greater amplitude (at the resonant frequency) than systems with a low Q factor. Damping decreases the Q factor. (from http://en.wikipedia.org/wiki/Q_factor)

[formerMember1]

Hmm... those are RF inductors

whoops sorry,..i didn't know, i just saw inductor and thought it was usable,.. 

thanks for the info on "Q" 

[petemoore]

  Could an inductor be built from inductors?
  Parallel, series, with or without resistors/caps..
  Say starting with inductors that are close to 1/2 or 2x the value the wah wants, then *series or *paralleling them...
  *Obvious questions I have are about using more than one inductor...
  So that target henry or mhz values could be aimed for...like say if you had a source of fairly inexpensive inductors you could choose from many combinations to look for the ones that when combined 'match' to a wah circuit?
  Just rambling, I haven't sourced inductors of any kind except wah replacements.
  Could be a way to source cheep wah build parts, find the exact value needed etc.
  Is ~500mhz 'it', or was that simply made a chosen #, so that circuits could be built consistantly around 'a' value inductor...perhaps the wah circuit could be redesigned to operate as well, but starting with a higher or lower value inductor?
 

[Paul Marossy]

Could an inductor be built from inductors?
  Parallel, series, with or without resistors/caps..
  Say starting with inductors that are close to 1/2 or 2x the value the wah wants, then *series or *paralleling them...

Yes. Inductors are kind of like resistors in this respect - parallel them to divide the total inductance into a certain value, or in series, they're additive.

*Obvious questions I have are about using more than one inductor...
  So that target henry or mhz values could be aimed for...like say if you had a source of fairly inexpensive inductors you could choose from many combinations to look for the ones that when combined 'match' to a wah circuit?

You mean mH for milli-Henries, not megaHertz, right? Anyway, yeah you could do as you suggest, but it would be something of a trial and error process...

[Arn C.]

I built a selectable tone circuit into my guitar and it has 5each  100 millihenry inductors in series and works great!!!!

Peace!
Arn C.

[Paul Marossy]

I built a selectable tone circuit into my guitar and it has 5each  100 millihenry inductors in series and works great!!!!

Got a schematic for that? I happen to have a few 100mH inductors that I would like to put to use for something...