Wah pedal build distorting...

[Projectile]

I built a wah pedal using the tonepad schematic and layout. All the parts values are stock expect for the input cap, which I increased to .05uf becasue I intend to use the pedal for bass as well as guitar. This is the same input cap value used on the maestro boomerang, so  it should be fine. I used NOS BC 109 transistors and a red fasel inductor. The pedal sounds absolutely fantastic except for one problem. On some notes, when played hard, it clips and distorts pretty badly. It's only certain notes on the neck, around 'D', at multiple octaves, and it only distorts at certain sweep positions, but it's enough to cause a problem. I can minimize the distortion by reducing the gain (increasing the value of the emitter resistor on Q1), and removing the input buffer, but it doesn't completely get rid of the problem unless I lower the gain so much that the pedal gets too quiet. Using the stock resistor values for Q1 (standard crybaby values), the problem is pretty obvious. It's almost like the pedal exhibits a fixed resonant peak at a certain frequency, and distorts when it gets too much signal at that frequency.

After much tinkering, I tracked the problem down to Q2 (not what I expected). It seems that when I replace the BC109 there with a 2n5088, the problem goes away. No more fart noises. I would just leave it at that, but I REALLY like the sound of the BC109. I'm shocked at how much changing this transistor affects the sound of the pedal. The BC109 adds a nice "sharpness" to the sound and changes the feel of the sweep quite a bit. It feels much more sterile without it, and I miss the sound.

So, what can I do? Could it just be a bad transistor? Unfortunately, I don't have any more B109s to try out. Maybe I just need a BC109 with a higher gain? The one I used had an hfe of about 185, which isn't bad, but I know there are better. Has anyone else experience this problem? Any ideas?

[joegagan]

even a stock boomerang has a gain hump in the low mids or high lows, this is part of the beauty of the sound of it. if it especially noticeable on certain notes, try using lower gain transistors and or raising the  E to gd reisistor a little.

your sitiuation sounds about right, just play around a little more with the gain.

[Projectile]

even a stock boomerang has a gain hump in the low mids or high lows, this is part of the beauty of the sound of it. if it especially noticeable on certain notes, try using lower gain transistors and or raising the  E to gd reisistor a little.

your sitiuation sounds about right, just play around a little more with the gain.

Sounds about right? I don't think a wah pedal is supposed to harshly clip. This is not a pleasant overdrive sound we are talking about. It flat out clips and makes an obnoxious farting noise. Like i said, I did try lowering the gain, but in order to get rid of the fart noise I had to lower the gain below what I would consider a reasonable level, to the point where it was significantly quieter than my other stock crybaby.

[joegagan]

oh, sorry i misread the post and also made an assumption.  is this a boomerang based or vox based wah?

you are correct, it should  not make nasty farty distortion. it could be a weird solder joint, or something weird around the inductor.

i am still suspicious of the gain in q1. try a 1k pot to ground in that location so you can play around with it.

[Ronan]

I bought a 2nd hand Morley bad horsie that does a similar thing, breaking up really bad on some peaks. I have high output humbuckers, but have not experienced this symptom with other wah's. This pedal uses an inductor, exact copy of the "Technology of the Wah" schematic, very similar to the tonepad wah. I was going to run a signal in and probe with an oscilloscope to find the source of the problem, but its not very high on my list. I'm surprised the 2nd transistor has an effect on tone, since its wired in as an emitter follower, therefore it doesn't add gain, well not voltage gain, just current gain. Maybe check the voltages on the collector, base & emitter of Q2 and see if they make sense, with no signal input.

[Projectile]

oh, sorry i misread the post and also made an assumption.  is this a boomerang based or vox based wah?

you are correct, it should  not make nasty farty distortion. it could be a weird solder joint, or something weird around the inductor.

i am still suspicious of the gain in q1. try a 1k pot to ground in that location so you can play around with it.

No problem, I appreciate the help. I just re-flowed the solder joints, so I'll try sticking the bc109 back in. Maybe that solved the problem. I already have a trimmer on the emitter of q1 to play with the gain.

I bought a 2nd hand Morley bad horsie that does a similar thing, breaking up really bad on some peaks. I have high output humbuckers, but have not experienced this symptom with other wah's. This pedal uses an inductor, exact copy of the "Technology of the Wah" schematic, very similar to the tonepad wah. I was going to run a signal in and probe with an oscilloscope to find the source of the problem, but its not very high on my list. I'm surprised the 2nd transistor has an effect on tone, since its wired in as an emitter follower, therefore it doesn't add gain, well not voltage gain, just current gain. Maybe check the voltages on the collector, base & emitter of Q2 and see if they make sense, with no signal input.

Yeah, I'll poke around a bit more to see what's going on with the voltages around q2. I was really surprised that q2 would alter the tone so much and that it would be the source of the distortion, since, as you said, it's simply an emitter follower. But when I replaced the BC109 in q2 with a 2n5088, the problem went away. I don't have a scope at home, but I'm thinking about buying one. This is the second time this month I've really wished I had a decent oscilloscope. Thanks for the suggestion.

[Paul Marossy]

Yeah, I'll poke around a bit more to see what's going on with the voltages around q2. I was really surprised that q2 would alter the tone so much and that it would be the source of the distortion, since, as you said, it's simply an emitter follower. But when I replaced the BC109 in q2 with a 2n5088, the problem went away.

Must have something to do with the input of Q2 being overloaded from the previous circuitry?