Inductor wah, 1967 style

Pedal and tube amp circuits, solved part by part the way SPICE (the circuit simulator engineers use) does it, while you listen and watch. Turn the knobs, change any part, or tap a wire to put a probe on it.

How to use the bench

Every part is simulated. Nothing you hear is a recording.

  1. 1Pick a circuit

    Choose one from the menu, or type a name, maker or year in Find a circuit. The buttons under the menu switch between versions.

  2. 2Set the knobs and the input

    Turn the circuit’s knobs on the front panel. In the Input box, choose Guitar or Signal generator, and the note or chord.

  3. 3Press Play

    Press Play through the circuit to hear a picked note or chord. Play the string is the guitar by itself.

  4. 4Tap a part

    Tap any part on the schematic to change its value, or swap the transistor, tube or diode. The red numbers are DC voltages, and they update as you go.

  5. 5Tap a wire

    Tap a wire to put the probe there. The Probe graph and the spectrum show the signal there.

  6. 6Try a change

    Press Try it on a card under Try this to hear one change, before and after. Modify this circuit, below the schematic, adds or removes parts.

Good to know

Modeled after the Vox Clyde McCoy and Dunlop Cry Baby wah pedals.

A common-emitter stage and an emitter follower around a 500 mH inductor. The treadle pot sets how much of the output is fed back through a 0.01 µF cap, moving a resonant peak from about 400 Hz to 2 kHz.

Input

Add

Peak voltage from the pickup for each note. Strum hard and a single coil puts out a few hundred millivolts. When you play a chord, its strings share this level.

How it’s played
Tap a part to change its value, or a wire to probe it. To add or take out parts, open Modify this circuit, below. Red numbers are DC voltages to ground, not warnings. Scroll or drag sideways to see the whole circuit.
Modify this circuit Add parts, take them out, or put one in series with another, then hear what it does. Tap to open.

Pick what to do, then tap the schematic. Your changes show in purple. Any change can go on a switch beside the knobs, from the menu in its row under Your changes.

Add a part

Tap where each leg goes. Tap both ends of a part to put the new one across that part.

Change a part

With Remove a part on, tap any part of the pedal or amp to take it out, and tap it again to put it back. Tapping a purple part you added deletes it.

Turn on Add in series with a leg and tap a part. In that part’s box, below the schematic, pick which leg to lift and what goes in the gap, such as a resistor between a cap and ground or a stopper in front of a grid.

Change a wire

With Cut into a wire on, tap any wire. Pick a part from the menu and it goes in the gap, wired in. Volume pot puts a pot there with the input on one end, the output on the wiper and the other end to ground.

With Run a wire on, tap a leg, then where the wire goes. Drag any piece of a wire you ran, or of an added part’s lead, to square it up or move it.

Keep and share

Your changes stay in this browser, on this device, for your next visit. Nothing is sent to us. Anyone using this browser can see them. They’re lost if you clear your browsing data or use a private window, so save a file to keep a copy. A shared link carries the circuit after the #, a part browsers never send to a server.

Test tone for the graphs
The graphs use a steady tone. Play sounds a plucked string that dies away.

Input

pickup, unloadedat the pedal input

Output

at the amp input

Probe

Spectrum

Frequency response

What the curves mean

The ink curve is the response to a very quiet signal, from the generator to the probe. The blue curve is measured with pink noise at the level you set, like an audio analyzer. When stages clip, the blue curve pulls away from the ink one and the gray coherence line (right-hand scale) drops below 1. Then no single curve tells the whole story. Both use the signal generator, whichever Input tab is picked.

Output

Bias

every voltage and current at rest

What to listen for. Rock the Treadle knob and watch the frequency response: the peak moves and grows as it rises. Press "Sweep the treadle" to see it at six positions at once. With a linear pot most of the movement comes in the last quarter toward the toe, since the fed-back share only falls steeply once the wiper nears ground. The Q resistor across the inductor sets how sharp the peak is: 33 k on the Cry Baby, 100 k on the Clyde McCoy, and larger is the "vocal mod". The feedback cap sets where the sweep sits, larger for bass. The first stage’s input impedance is under 100 kΩ, so the wah loads the pickup, and with the guitar volume down the top end drops more than it would into an amp. On the bench the treadle is held still for each note. A real wah is usually moving.

Download.

Parts and what they do

Stock values for the Cry Baby, 2N3904 version. Tap a part on the schematic to change it.

C1, 10 nF
Input cap. With the 68 k and the stage behind it, it cuts below about 200 Hz.
R1, 68 kΩ
Input resistor, most of the pedal’s input impedance, under 100 kΩ, so the pedal loads the pickup. The feedback signal arrives at the base and works against R1.
Q1, 2N3904
Common-emitter gain stage, about 35 dB open loop (22 k over the emitter resistor), pulled down to about 19 dB by the bias network from collector to base.
R3, 22 kΩ
Q1 collector load.
R4, 470 Ω
Q1 emitter resistor. Smaller means more gain and slightly more bass (390 Ω on the GCB-95, 510 Ω on the V847).
R6, 470 kΩ
Bias feedback from the collector to the inductor’s top. With R8 it sets the base voltage through the inductor and R2, so the stage biases itself.
R8, 82 kΩ
Bottom of the bias divider (100 k on the Vox). It changes the bias slightly and the sound hardly at all.
C3, 4.7 µF electrolytic
Grounds the inductor’s top for audio, so the inductor runs from signal ground to the feedback node.
L1, 660 mH
The wah inductor, 500 mH on most units. With the feedback cap it sets the frequency range. Anything from about 200 mH to 1 H has been used.
L1 winding, 60 Ω
The inductor’s winding resistance, a typical figure (wah inductors measure about 10 to 200 Ω). At these frequencies it matters much less than R7.
R7, 33 kΩ
Q resistor across the inductor, damping the resonance. A larger value makes a sharper, taller peak: the "vocal mod" is 39 k to 100 k, and the Clyde McCoy uses 100 k.
R2, 1.5 kΩ
Joins Q1’s base to the bottom of the inductor, where the feedback arrives. Larger (2 to 2.7 k) keeps more midrange at heel down.
C4, 220 nF
Output coupling cap. The output comes from this point, before the treadle pot, so the pedal’s level doesn’t follow the treadle.
Treadle, 100 kΩ pot, linear taper
The treadle pot, linear as on most wahs, heel down 0, toe down 10. It divides the output down before the feedback buffer. Heel down feeds back the most, which makes the feedback cap look larger to the inductor and puts the peak lowest. Toe down feeds back almost nothing, and the peak sits near 1 / (2π√(L C2)). A real treadle doesn’t reach either end of the track, and the rack or lever that turns it shapes the sweep. A lever can make a linear pot act close to reverse audio.
C5, 220 nF
Couples the treadle wiper to the buffer.
R5, 470 kΩ
Biases Q2 from Q1’s collector.
Q2, 2N3904
Emitter follower that drives the feedback cap from a low impedance.
R9, 1 kΩ
Q2 collector resistor, there to stop the follower oscillating.
R10, 10 kΩ
Q2 emitter load.
C2, 10 nF
Feedback cap, the capacitor of the resonance. Larger moves the whole sweep down (0.022 to 0.068 µF for bass). Smaller moves it up.

Versions

Clyde McCoy, BC109, modeled after the Vox Clyde McCoy wah
Vox, 1967. 100 k across the inductor instead of 33 k, so the peak is sharper and taller, and 100 k from its top to ground.
Cry Baby, 2N3904, modeled after the Dunlop Cry Baby wah
From a redrawn Cry Baby schematic with a 660 mH inductor and 2N3904s. No input buffer.
GCB-95, MPSA18, modeled after the Dunlop Cry Baby GCB-95 wah
Dunlop GCB-95 before its 1991 input buffer, with a 390 Ω emitter resistor for slightly more gain, a 500 mH inductor and MPSA18s.

Sources

How this tool works. Every part is an equation, and the whole circuit is solved several times per audio sample: 192,000 samples a second while you drag a knob, 384,000 once you let go. Transistors use Ebers-Moll with gain, leakage, Early effect, base resistance, junction capacitance and temperature. Their charge storage is set at the bias point, close for small signals and rough while they switch hard. Tubes use Koren’s equations fitted to each datasheet, plus grid current. Diodes are exponential junctions with series resistance. Presets are typical of each type, not measured parts. Caps are ideal, and the battery or HT is a voltage behind a resistance.

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