Oil can rotating sound with wah and volume, 1976 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 Morley Rotating Wah-Volume (RWV).

A rotating speaker sound from a small oil can, after the Morley Rotating Wah-Volume. A drum turning in oil carries the guitar from one brush to another a few milliseconds later. The drum’s speed swings once a turn, so the pitch wavers, and mixed with the dry guitar it swirls. A treadle with a light and two photocells works the volume or the wah.

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. Q5 and Q4 drive the can. The 4.7 M pair and the 47 pF from Q4’s output back to its input set their gain at about 43, rising in the treble. The guitar goes onto the drum as electric charge at one brush and comes off at another, a few milliseconds on. The drum’s drive is uneven, so it speeds up and slows down once a turn, and the delay swings with it. A delay that swings bends the pitch up and down, as a turning horn does. Q3 reads the drum, and the rotating sound level trim sets how much of it joins the dry guitar from R22. The treadle moves a vane between a bulb and two photocells. With the wah off, LDR1 is the volume control. With the wah on, LDR2 tunes the bridged T around Q2 and Q1.

Download.

Parts and what they do

Stock values. Tap a part on the schematic to change it.

R5, 1.5 kΩ
From the doubler to the +55 V line, 1500 Ω. With C2 it filters the ripple.
C2, 50 µF electrolytic
Filter cap on the +55 V line, 50 µF.
R4, 10 kΩ
From the +55 V line to the +25 V line, 10 k.
C1, 50 µF electrolytic
Filter cap on the +25 V line, 50 µF.
R3, 10 kΩ
The +25 V line’s 10 k to ground. With R4 it halves the +55 V line.
Doubler middle, 34 V
The middle of the supply’s doubler, where C4 and C3 meet, modeled as a steady 34 V (half the doubler’s output, assumed). It lights the bulb.
R7, 150 Ω
In series with the bulb, 150 Ω.
PL1, #387 bulb, 28 V 40 mA
The #387 bulb, 28 V at 40 mA. It lights both photocells, and the treadle’s vane sets how much of its light reaches them.
R29, 220 kΩ
Input resistor, 220 k. Against the 9.4 M of feedback it sets the drive amp’s gain at about 43.
R27, 4.7 MΩ
Half of the drive amp’s feedback, 4.7 M.
C14, 47 pF
From the middle of the feedback to ground, 47 pF. Above about 1.4 kHz it shunts the feedback, so the drive rises in the treble, to make up for what the drum loses there.
R26, 4.7 MΩ
The other half of the drive amp’s feedback, 4.7 M, from the drive line.
C15, 10 nF
Coupling cap into Q5, .01 µF.
R12, 4.7 MΩ
Q5’s base from the +25 V line, 4.7 M.
R28, 2.2 MΩ
Q5’s base to ground, 2.2 M.
Q5a, MPSA12 (half)
First half of Q5, an MPSA12 Darlington.
Q5b, MPSA12 (half)
Second half of Q5, an emitter follower. It keeps the high-impedance input free of Q4’s load.
R25, 10 kΩ
Q5’s emitter resistor, 10 k.
C13, 1 µF electrolytic
From Q5 to Q4’s base, 1 µF.
R17, 560 kΩ
Q4’s collector to base, 560 k. With R24 it sets the bias.
R24, 27 kΩ
Q4’s base to ground, 27 k.
Q4, MPSA06
The drive amp’s gain stage, an MPSA06 on the +55 V line, so it can swing the drum hard.
R11, 10 kΩ
Q4’s collector resistor, 10 k, from the +55 V line.
R16, 22 Ω
Q4’s emitter resistor, 22 Ω.
C17, 10 µF electrolytic
Across Q4’s emitter resistor, 10 µF, for full gain.
C12, 10 nF
From Q4’s collector to the drive line, .01 µF.
Rotating sound modulator, Oil can (Rotating Wah disc, typical)
The rotating sound modulator: a small drum turning in oil, with two writing brushes and a reading brush. The drum turns about 6.7 times a second and the reading brush picks the guitar up about 3 ms after the first writing brush. The drive is uneven, so the drum’s speed swings about 10 % once a turn, and the delay with it. Every drum figure is assumed and can be changed.
C18, 330 pF
To the second writing brush, 330 pF. It passes mostly treble, so that brush adds a second, brighter copy an eighth of a turn earlier on the drum.
C9, 1 µF electrolytic
From the reading brush to Q3’s base, 1 µF.
R13, 4.7 MΩ
Q3’s base from the +25 V line, 4.7 M.
R31, 2.2 MΩ
Q3’s base to ground, 2.2 M. The reading brush sees this high impedance, so it reads the drum’s voltage.
Q3a, MPSA12 (half)
First half of Q3, an MPSA12 Darlington.
Q3b, MPSA12 (half)
Second half of Q3, the read follower.
R30, 10 kΩ
Q3’s emitter resistor, 10 k.
C16, 47 nF
Out of the read follower, .047 µF.
P1, 100 kΩ pot, linear taper
The rotating sound level control, 100 k, a trim inside. It sets how much of the drum joins the dry guitar. Set at half (assumed).
R21, 1.5 kΩ
From the level trim’s wiper to ground, 1.5 k. It loads the wiper, so the trim works more like an audio taper.
R23, 10 kΩ
The rotating sound into the mix point, 10 k.
R22, 1 MΩ
The dry guitar into the mix point, 1 M, from the drive line. The drive amp’s gain makes up for most of what this loses.
R32, 1 MΩ
From the mix point into the wah, 1 M.
C11, 10 nF
Coupling cap into Q2, .01 µF.
R1, 4.7 MΩ
Q2’s base from the +25 V line, 4.7 M.
R2, 2.2 MΩ
Q2’s base to ground, 2.2 M.
Q2a, MPSA12 (half)
First half of Q2, an MPSA12 Darlington.
Q2b, MPSA12 (half)
Second half of Q2, the wah’s input follower.
R9, 10 kΩ
Q2’s emitter resistor, 10 k.
C5, 1 µF electrolytic
From Q2 to Q1’s base, 1 µF.
R6, 560 kΩ
Q1’s collector to base, 560 k.
R10, 27 kΩ
Q1’s base to ground, 27 k.
Q1, MPSA06
The wah’s gain stage, an MPSA06. It inverts, so the bridged T from its output back to Q2’s input is negative feedback.
R8, 10 kΩ
Q1’s collector resistor, 10 k, from the +55 V line.
R14, 22 Ω
Q1’s emitter resistor, 22 Ω.
C7, 5 µF electrolytic
Across Q1’s emitter resistor, 5 µF.
C6, 1 µF electrolytic
From Q1’s collector to the wah’s feedback point, 1 µF.
R15, 1.8 MΩ
Across the top of the bridged T, 1.8 M.
C10, 2.2 nF
One of the bridged T’s two caps, .0022 µF.
C8, 2.2 nF
The bridged T’s other cap, .0022 µF.
LDR2, Morley treadle cell
The wah’s photocell, from the middle of the bridged T to ground (pin 4). The treadle’s vane sets how much light it gets. The bridged T passes least near 1 / (2π × .0022 µF × √(1.8 M × the cell)), so with it in the feedback the wah peaks there: toe down, more light, a lower resistance and a higher peak.
R20, 220 kΩ
From the bridged T back to the wah’s input, 220 k.
C19, 10 nF
Out of the wah, .01 µF.
R19, 220 kΩ
Out of the wah, 220 k.
R18, 27 kΩ
From the wah’s output to pin 7, 27 k. Rot Sound grounds pin 7 while it is on, so with the rotating sound in, the wah comes out about 19 dB lower.
LDR1, Morley treadle cell
The volume photocell, in series with the signal. The treadle’s vane sets how much light it gets: toe down, more light, a lower resistance and more volume. Into the 68 k at the jack, it fades out near heel down.
R33, 68 kΩ
Across the output jack, 68 k.

About this version

As drawn, 1976
Tel-Ray’s drawing of 27 January 1976. The can’s figures are assumed.

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. Caps are ideal, and the battery or HT is a voltage behind a resistance.

Spot an error, or want a circuit added? Email info@swellamps.com. A schematic or a clear photo of one helps. Anything you send may be used to improve these models and our desktop app.

© 2026 Swell Amplification LLC. All rights reserved. The simulator, its circuit models, drawings, presets and text are the property of Swell Amplification LLC and may not be copied, reproduced, redistributed or used to build another product without written permission. The original circuits are historical designs documented in public sources, and no claim is made to them. Everything here is provided as is, without warranty of any kind, for study and personal use. Results are simulations of typical parts, not measurements of any real unit. Don’t rely on them for the safety of any build or repair. Tube amplifiers carry lethal voltages.

Product and company names are trademarks of their owners. They are used here only to identify the circuits these models are based on; this site is not affiliated with or endorsed by any of them.

From Swell Amplification, swellamps.com.