DIY Wah No sound. Transistor?

[billywilly92]

Hi I am new to the forum and to building guitar pedals too. This is my first pedal build and I decided to build a JH-1 wah pedal but with a yellow fasel inductor. Here is the schematic I am using

I have everything hooked up to my breadboard but there is no sound. I have a guitar going into the input and then the output goes to my solid state amp.
I removed the diode since I am just using a battery.
I have everything grounded together (input ground, battery ground) since I don't have it wired up yet to allow the input jack to ground the battery.
I also replaced the NPN MPSA13 with a NPN 2n3904 because I didn't have the first one. I thought that it would be ok because another crybaby schematic I found calls for a 2n3904 and because the terminals are in the same order.

Here is the values on the transistors.

2n3904 (replacing the MPSA13)
C 8.50
B 1.94
E 2.47

MPSA18 (This is the one in the middle of the schematic)
C 7.69
B 1.08
E 0.69

MPSA18
C 8.40
B 1.29
E .83

I think my problem starts with the first transistor due to the voltages being weird. Anyone know what could be wrong? Could this cause there to be no sound at all?

[Talon5051]

Welcome to the forum.  Try bypassing the 1st transistor and have the signal start at the 68K resistor.  The very 1st stage is just a buffer.  Also, the MPSA13 is a darlington pair and the 2N3904 is not.  Not really sure if this will make a difference in whether or not you will get sound.  Hopefully someone else will chime in on that.

[billywilly92]

I tried bypassing the transistor but it didn't change anything, there is still no sound. Could I try moving where the output jack starts on the breadboard to see where there is and isn't sound? Wouldn't that be able to tell me where the problem is?

I also measured the 1.8m resistor but it only measured out to 1.2 Could this be a problem?

[Talon5051]

Could I try moving where the output jack starts on the breadboard to see where there is and isn't sound?

Yes that is a good idea.  Use an audio probe like the one in this link: http://www.diystompboxes.com/pedals/debug.html

[R.G.]

Thank you for providing the information we need.
I also replaced the NPN MPSA13 with a NPN 2n3904 because I didn't have the first one. I thought that it would be ok because another crybaby schematic I found calls for a 2n3904 and because the terminals are in the same order.

2n3904 (replacing the MPSA13)
C 8.50
B 1.94
E 2.47
[/quote]
That replacement may be a problem. The 2N3094 has a vastly lower DC gain than the MPSA13. So the base voltage on the MPSA13 should be about 4.5-5.0V. The 2N3904 sucks more base current and may not be able to be biased correctly by the large value 1.8M and 2.2M resistors. If the MPSA13 was in circuit, I'd expect the emitter voltage to be about 3.5V. But 2.47 may not be all that bad.

Measuring the voltage at the base will be fraught with errors because the resistances are so high there that your meter will change the loading and the results.

MPSA18 (This is the one in the middle of the schematic)
C 7.69
B 1.08
E 0.69

Not too bad. I'd like to see the base and emitter be higher and the collector lower.

MPSA18
C 8.40
B 1.29
E .83

Not horribly bad. I'd like to see the emitter be higher, up at a couple of volts.

... no sound at all?

This one is not clear-cut. The numbers are off a bit, but not so much that it does no sound at all.

The previous advice was probably best - audio probe. Look for sound at the emitter of the first transistor, collector of the second, and emitter of the third.

[billywilly92]

There is sound at the first transistor emitter. The second transistor collector.  But no sound at the third transistor emitter

[billywilly92]

I decided to try building a more simple wah pedal to see if I could do it. I made this one http://www.aronnelson.com/gallery/main.php/v/WAH-ARCHIVE_/WAH-SCHEMATICS/wah1.gif.html and I get sound but only on my amps distortion channel. Also there isn't much of a wah sound. The 100k pot does seem to affect the sound but it doesn't really make that wah sound that most wah pedals do.

Anyone have any ideas on what could be wrong with either of these pedals?

[runmikeyrun]

concentrate on one pedal at a time.  Change the 2N3904 to the MPSA13 and see what happens.  repost voltages.

[billywilly92]

concentrate on one pedal at a time.  Change the 2N3904 to the MPSA13 and see what happens.  repost voltages.

I was trying to make that other pedal to make sure that I was reading schematics correctly. I figured if the new one didn't work either then I must not be reading schematics correctly. Since it does work I am back to working on the first one.

I don't have an MPSA13 but I am going to check around at local stores because it would cost me $6 to ship a $.20 part. Ill repost voltages once I get the new transistor.

[Talon5051]

The 100k pot does seem to affect the sound but it doesn't really make that wah sound that most wah pedals do.

Double check the .01uF cap that goes to the emitter of the 3rd transistor.  I placed in too small of caps (in the nano fared range) in that position once and found I got sound but very little change with the 100K pot.

[Talon5051]

I want to correct my previous statement.  It was not nano but pico fareds.

[billywilly92]

I replaced the 2n3904 with an MPSA13 like the schematic calls for, but I still don't have sound besides buzzing. Here are the voltages.

MPSA13
C 8.38
B 2.30
E 3.68

MPSA18 (This is the one in the middle of the schematic)
C 8.25
B 1.17
E 0.77

MPSA18
C 8.79
B 5.22
E 6.81

What causes lots of buzzing, a bad transistor? Because maybe that is why the middle one has such low voltages. Or maybe I have it installed incorrectly, I'll have to check that.

[R.G.]

MPSA13
C 8.38
B 2.30
E 3.68

The MPSA13 is a darlington. That means its base to emitter is actually two junctions one after the other. The base must be 1.0 to 1.4V higher than the emitter for it to work at all. You have shown the emitter higher than the base.
- maybe you mixed up which was emitter and which was base
- if you are correctly measuring the voltages and identifying the pins, this one issue would keep any audio from going through.

I suggest removing either the 68K or 0.01uF from the emitter of the MPSA13 to the base of the second transistor, then remeasuring, being very, very, very certain of which is base and which is emitter. If the voltages remain this way, there is something badly wrong with the parts in the first stage or how they are hooked up.

MPSA18 (This is the one in the middle of the schematic)
C 8.25
B 1.17
E 0.77

You do not say what your battery voltage is. That makes it impossible to tell whether the collector is at the battery voltage and pulling no current or a bit below the battery voltage and pulling too little. There is a reason for every bit of information asked for in "what to do when it doesn't work". It's not just make work. Note that the base-emitter are about the right distance apart on this transistor for a non darlington BJT. It's probably OK except for that collector voltage thing.

MPSA18
C 8.79
B 5.22
E 6.81

This has the same issue as the first transistor: emitter higher than base. This will not work correctly, if the information about voltages and pins is correctly measured.

What causes lots of buzzing, a bad transistor? /quote]
Generally, incorrect hookup. Transistors which are damaged don't as a rule buzz.

[familyortiz]

Would it be possible to get more 1st stage bias by reducing the 10K emitter resistor value?
Vbe shoud increase a bit for more Ic.

[billywilly92]

On a schematic the transistor symbol goes
Collector on top
Base in the middle
Emitter on the bottom

Correct?

[familyortiz]

yes, and the emitter has the arrow which will indicate npn or pnp type.

[billywilly92]

I have checked all of the transistors to make sure I had the pins correct according to their datasheets. I also checked the wiring before the mpsa13 and it looks correct compared to the schematic. The voltages on the two transistors base and emitters are still incorrect so I haven't fixed it yet. I think I might rebuild the pedal using 2 breadboards, that way I have more room for all the parts. Hopefully my problem will eb fixed once I rebuild the pedal.

You do not say what your battery voltage is. That makes it impossible to tell whether the collector is at the battery voltage and pulling no current or a bit below the battery voltage and pulling too little. There is a reason for every bit of information asked for in "what to do when it doesn't work". It's not just make work. Note that the base-emitter are about the right distance apart on this transistor for a non darlington BJT. It's probably OK except for that collector voltage thing.

The battery voltage was around 8.90 to 9.00 volts.

[Gus]

If you are doing a bread board build here is something to try

Build the three transistor stages one at a time

Build the EF input buffer and get that working correctly with correct transistor voltages

Add the gain stage and get that working correctly with correct transistor voltage

Add the EF stage after the gain stage get that working again correct transistor voltages

Finish up the build and test if you still have problems it is mostly likely not the transistors but something else.

[billywilly92]

If you are doing a bread board build here is something to try

Build the three transistor stages one at a time

Build the EF input buffer and get that working correctly with correct transistor voltages

Add the gain stage and get that working correctly with correct transistor voltage

Add the EF stage after the gain stage get that working again correct transistor voltages

Finish up the build and test if you still have problems it is mostly likely not the transistors but something else.

I'll have to try this idea, hopefully it works. Is there anyway I can set it up so I can test audio at each stage as well?

[billywilly92]

I built the MPSA13 by itself on a different bread board and it does not connect to the other transistors circuit. The voltages are

Battery ~ 9.07
C 9.07
B 2.48
E 4.07

I checked the transistor data sheet multiple times and it goes top to bottom CBE if the circle side is facing left. So basically it looks just like the schematic symbol for a transistor.

Also with the battery off the 2.2M resistor doesn't read 2.2M on the breadboard but it does off the board. Shouldn't it read 2.2M on the board also?