Tried to resurrect a wah I was building last night (too many projects, and now that it is feasible to be in the garage for more than 30 seconds, I tend to go in there for a part, notice something and go "Oh yeah, THAT one" and bring it back inside to work on :oops:).  The circuit is one of the standard MPSA18-based units.  The inductor is one I bought locally on a nice round ferrite core (thinking of tinkering with magnets on it).  It apparently measures somewhere around 700mh which is high for most known circuits but not ridiculously so.  The pot is a standard Dunlop Hot Potz type I got for a song (under $6 for pot and inductor).

It wahs but here's the part that is baffling me.  The range of the wah is too wide.  Wide enough that 1/3 of the gear-portion of pot rotation (which, remember, is less than the total non-gear pot rotation) moves me from the lowest end of the range to total inaudibility.  I thought that maybe adding another cap in parallel with the stock .01uf cap would compensate for the inductor somehow and lower the overall range so that the highest point of the sweep would now be reasonable.  But doing this only seems to make the output lower and duller, negating the changes/benefits from playing with the Q resistor (changed it from 33k to 43k).

I suppose a good reread of RG's Tech paper is in order today, but this is baffling.  The pot is obviously produced in anticipation of the very circuit it is being used with so it can't be the pot or pot taper.  I suppose I can sub one of those Radio Shack interstage transformers for the inductor but I had so hoped that the inductor was similar to a Fasel, and they were so cheap ($1) I bought a half dozen.  I would really rather the existing inductor was suitable, but if need be I'll change.

I just don't understand what would make the sweep range so bloody wide.  I will only add that it is a perfboard build so...well...you know.  :roll:

Ideas?

My first ever build was my wah-wah. I started with a RS transformer which was way too mild-mannered(kind of like the wah portion of my shin-ei fuzz-wah). I then moved to a scavenged Dunlop Crybaby inductor(the black one) which is okay but still kind of mild. I finally splurged a few months ago for a red Fasel reissue which is the bomb! I can't explain exactly what the difference is. A smoother mid sound and sweep. It also now sticks out in the mix, whereas it used to get easily buried.

I also just used RS 2N3904 types for the transistors. I've been tempted to change those to a higher gain.

Regards

RDV

I think you have a gain problem - too much of it!

The trick in understanding inductor wahs is to understand that the rocker pot is acting like a gain control to a miller feedback capacitor. The rocker determines how much signal goes through the wah cap on the emitter of the second transistor. This makes the cap effectively variable because a miller feedback cap is effectively the voltage gain of the stage it's around times the voltage gain.

The inductor is fixed, it can't change value, so if you have a wah range that moves too far, then your capacitor is moving too far, and that can only be because the gain change is too big as you rock the pedal.

I would try reducing the gain of the first transistor by using a larger emitter resistor or smaller collector resistor.

If this doesn't change things much, I **really** want to figure out what's going on. 8-)

Ditto what R.G. said about reducing the gain, but I noticed you'd put another cap in parallel with the 0.01 - that would certainly make it duller.  Instead, try replacing the 0.01 with a 0.0068 or 0.0082.

Stuart

or somehow reduce the gain towards higher frequencies
(imagine: opposite of the wellknown 470p on the Vol-pot)

Geez you guys are good.  I have a 470R emitter resistor in there now, and last night when I tried to up the gain (after reading the GEOFEX article - a little knowledge is a dangerous thing, eh? :wink: ) by tacking on a 1k2 resistor in parallel with it to reduce the emitter-to-ground resistance, the wah all but disappeared.  So what you're saying makes perfect sense.

I will stick a 330R in series with a 1k trimpot tonight, tweak, and report.  I hope it works.  I haven't had a wah since about 1986.

Funny little story - off topic so I apologize ahead of time.  But here goes.  I haven't had a wah pedal in a few years either so I thought I would try and build one.  A friend had Crybaby lying around from the local music store that "they couldn't fix" so he gave it to me.  Yep a freebee.  Cool.  So before I commenced to gutting it I had to see if I could fix it.  Everything appeared OK at first glance but when I tried to engage the switch (not hooked up) my friend noticed there was no pop.  So, I checked the switch, raised it, hooked it up and "wah la" (pun intended).  Free, 100% functional wah pedal.  So much for the "expertise" at the local shop. I can't wait to tear it apart.

Again, Mark I apologize for heisting your thread.  But I found this pretty amusing and after that incident I am now convinced that I am more than quailified to open my own music store and pretend to be able to repair things.  :shock:

AL

Quoteor somehow reduce the gain towards higher frequencies
(imagine: opposite of the wellknown 470p on the Vol-pot)

This got me thinking....
Has anybody ever experimented with caps across the pot? How would this influence the sound?
The Dunlop JH wah uses a 0.022uF cap here. Anybody ever played with one?

http://www.geocities.com/j4_student/jh1wah.gif

again - heisting the thread, but...

AL - I got a Rocktron 'purple haze' pedal for $15 from a shop that thought it was broken (I was just gonna reuse the cool enclosure)..

it was a dead battery.
"-)

Nobody tinkered with a cap across the pot?  

I tried the trimpot and it helped a bit, but given that this was a perfboard build, I didn't think I could trust myself, so I yanked it apart, rebuilt, and finally had it up and running again late last night with a 390R fixed resistor and 1k trimpot in series onthe emitter side of Q1.  The sheer size of the inductor (easily an inch across) makes perfboard planning awkward (especially for impatient types  :roll:  ) so I stuck it off in the corner of the board and then planned everything around that.   Awkward, but whaddya want for $1 inductors?

I am pleased to say, though, that this thing sounds fabulous.  Every bit as vocal and warm as I'd hoped for.  Although part of the problem did seem to be the gain of Q1 (adjustments of it do shift the range and emphasis in addition to the level/gain), I think my initial build may have had an error or two in there as well, since I can replicate some of the flaws of the first build but not all.  I also mistakenly thought I was using the Hotpotz to start with but when I uninstalled it, I realized it was an audio-taper Alpha pot from Radio Shack.  The Hotpotz was still out in the garage on the other bench because it wasn't able to fit the space provided in the wah shell.  Once the Dunlop pot was wired into the circuit (sitting outside the shell), the sweep was just right.  I'll be dremeling away this weekend, making a little more room so I can fit that big pot in there.

Bottom line: Yes, an inductor in the high 700mh's can work well in a wah circuit.  Adjusting the gain of Q1 via a trimpot on the emitter side can be helpful in bringing out the best.