Lamp and photocell phase shifter, 1969 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 Shin-ei Uni-Vibe.

Four transistor phase-shift stages, each with a photocell, all lit by one bulb that a transistor oscillator swings. In Chorus the dry signal mixes back in. Vibrato is the shifted signal by itself.

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. Hold a note and watch the output scope. The oscillator, Q11 and Q12 with a three-cap ladder, swings slowly at its emitter, and Q13 turns that swing into current through the bulb. The bulb brightens and dims, and the four photocells around it drop and rise in resistance together, from about 20 kΩ to 370 kΩ at the default settings. Each stage passes the signal at full level but shifts its phase, more as the cell resistance rises. In Chorus the shifted signal mixes with the dry one, so notches open where the two cancel and sweep up and down the strings. In Vibrato only the shifted signal comes out, and the moving phase shift is heard as pitch wobble. The stages have different caps, 15 nF, 220 nF, 10 nF and 6 nF, so they shift at different frequencies. The filament needs time to warm and cool, which rounds the sweep at high Speed. The lamp and cells are typical figures, and real ones vary from unit to unit.

Download.

Parts and what they do

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

C24, 1 mF electrolytic
Reservoir cap on the 22 V rail, after the transformer’s half-wave rectifier (not drawn).
R51, 470 Ω
First supply dropping resistor.
C25, 220 µF electrolytic
Filter cap between the two drops.
R52, 470 Ω
Second dropping resistor. The signal stages pull about 5 mA through both, so their rail sits near 17.4 V.
C26, 220 µF electrolytic
Filter cap on the 18 V rail that feeds the signal stages.
R3, 47 kΩ
Input pulldown. With R1 (22 kΩ, ahead of the jack, not drawn) it sets the load the guitar sees.
C1, 1 µF electrolytic
Input coupling cap.
R4, 1 MΩ
Q1’s base bias, taken from Q2’s emitter, so the pair holds its own operating point.
C2, 1 µF electrolytic
Bypasses Q2’s emitter, so R4 carries DC only.
R8, 6.8 kΩ
Q2’s emitter resistor.
Q1, 2SC539
Input transistor, the first of a direct-coupled pair with Q2. Its emitter connects to Q3’s emitter divider, which closes the feedback loop around all three.
R5, 1 MΩ
Q1’s collector load.
R6, 100 kΩ
Feeds Q1’s load from the 18 V rail, filtered by C3.
C3, 1 µF electrolytic
Filter cap for Q1’s supply.
Q2, 2SC828
Second gain transistor, direct from Q1’s collector.
R7, 47 kΩ
Q2’s collector load.
C4, 330 pF
Rolls off the extreme highs at Q2’s collector.
Q3, 2SC828
The first phase splitter. Its collector and emitter carry the signal in opposite phase at about the same level.
R11, 4.7 kΩ
Q3’s collector load.
R9, 3.3 kΩ
Upper part of Q3’s emitter resistor.
R10, 1.2 kΩ
Lower part of Q3’s emitter resistor. Q1’s emitter connects above it.
C5, 1 µF electrolytic
Dry signal coupling cap, from Q3’s emitter.
R12, 100 kΩ
Dry signal into the Chorus mix.
C7, 15 nF
Stage 1’s phase cap, 15 nF, from the splitter’s collector. With the cell it sets where the stage shifts the phase by 90 degrees.
C6, 1 µF electrolytic
Couples stage 1’s emitter to its cell.
PR1, Uni-Vibe cell
Stage 1’s photocell, lit by LP1. Its resistance falls as the bulb brightens, and that moves the stage’s phase shift.
R13, 4.7 kΩ
In series with PR1, so the stage’s resistance never falls under 4.7 kΩ.
R16, 100 kΩ
Q4’s bias, top.
R14, 100 kΩ
Q4’s bias, into its base.
R15, 47 kΩ
Q4’s bias, bottom.
Q4, 2SC828
Emitter follower into Q5, so the phase network ahead of it isn’t loaded.
Q5, 2SC828
Stage 2’s phase splitter: collector and emitter in opposite phase at about the same level.
R18, 4.7 kΩ
Q5’s collector load.
R17, 4.7 kΩ
Q5’s emitter resistor, equal to the collector load.
C8, 1 µF electrolytic
Bootstraps Q4’s bias from Q5’s emitter, so R14 barely loads the signal at the base.
C9, 220 nF
Stage 2’s phase cap, 220 nF, from the splitter’s collector. With the cell it sets where the stage shifts the phase by 90 degrees.
C10, 1 µF electrolytic
Couples stage 2’s emitter to its cell.
PR2, Uni-Vibe cell
Stage 2’s photocell, lit by LP1. Its resistance falls as the bulb brightens, and that moves the stage’s phase shift.
R19, 4.7 kΩ
In series with PR2, so the stage’s resistance never falls under 4.7 kΩ.
R22, 100 kΩ
Q6’s bias, top.
R20, 100 kΩ
Q6’s bias, into its base.
R21, 47 kΩ
Q6’s bias, bottom.
Q6, 2SC828
Emitter follower into Q7, so the phase network ahead of it isn’t loaded.
Q7, 2SC828
Stage 3’s phase splitter: collector and emitter in opposite phase at about the same level.
R24, 4.7 kΩ
Q7’s collector load.
R23, 4.7 kΩ
Q7’s emitter resistor, equal to the collector load.
C11, 1 µF electrolytic
Bootstraps Q6’s bias from Q7’s emitter, so R20 barely loads the signal at the base.
C13, 10 nF
Stage 3’s phase cap, 10 nF, from the splitter’s collector. With the cell it sets where the stage shifts the phase by 90 degrees.
C12, 1 µF electrolytic
Couples stage 3’s emitter to its cell.
PR3, Uni-Vibe cell
Stage 3’s photocell, lit by LP1. Its resistance falls as the bulb brightens, and that moves the stage’s phase shift.
R25, 4.7 kΩ
In series with PR3, so the stage’s resistance never falls under 4.7 kΩ.
R28, 100 kΩ
Q8’s bias, top.
R26, 100 kΩ
Q8’s bias, into its base.
R27, 47 kΩ
Q8’s bias, bottom.
Q8, 2SC828
Emitter follower into Q9, so the phase network ahead of it isn’t loaded.
Q9, 2SC828
Stage 4’s phase splitter: collector and emitter in opposite phase at about the same level.
R30, 4.7 kΩ
Q9’s collector load.
R29, 4.7 kΩ
Q9’s emitter resistor, equal to the collector load.
C14, 1 µF electrolytic
Bootstraps Q8’s bias from Q9’s emitter, so R26 barely loads the signal at the base.
C16, 6 nF
Stage 4’s phase cap, 6 nF, from the splitter’s collector. With the cell it sets where the stage shifts the phase by 90 degrees.
C15, 1 µF electrolytic
Couples stage 4’s emitter to its cell.
PR4, Uni-Vibe cell
Stage 4’s photocell, lit by LP1. Its resistance falls as the bulb brightens, and that moves the stage’s phase shift.
R31, 4.7 kΩ
In series with PR4, so the stage’s resistance never falls under 4.7 kΩ.
R34, 68 kΩ
Q10’s bias, top.
R32, 100 kΩ
Q10’s bias, into its base.
R33, 47 kΩ
Q10’s bias, bottom.
Q10, 2SC828
Output emitter follower.
R35, 22 kΩ
Q10’s emitter resistor.
C17, 1 µF electrolytic
Bootstraps Q10’s bias from Q10’s emitter, so R32 barely loads the signal at the base.
C18, 1 µF electrolytic
Coupling cap from Q10’s emitter.
R36, 100 kΩ
Shifted signal into the Chorus mix, equal to R12, so the two mix half and half.
R37, 100 kΩ
Vibrato divider, top.
R38, 220 kΩ
Vibrato divider, bottom. It brings the Vibrato level near the Chorus level.
Volume, 100 kΩ pot, audio taper
Volume, 100 kΩ, taper assumed audio.
R41, 3.3 kΩ
Oscillator bias divider, top.
R40, 4.7 kΩ
Oscillator bias divider, bottom.
R39, 2.2 MΩ
Feeds the divider’s voltage to the oscillator’s base. Large, so it barely loads the ladder.
Q11, 2SC828
Oscillator, the first half of a Darlington follower that drives the ladder.
Q12, 2SC828
Oscillator, the second half of the Darlington.
R44, 4.7 kΩ
The oscillator’s emitter load.
C19, 1 µF electrolytic
Ladder cap from the oscillator’s base.
C20, 1 µF electrolytic
Middle ladder cap. CR1 and CR2 across it set the oscillator’s swing.
C21, 1 µF electrolytic
Ladder cap to ground.
R42, 220 kΩ
Feedback from the oscillator’s output to the ladder’s upper join.
R43, 220 kΩ
Feedback from the oscillator’s output to the ladder’s lower join.
CR1, CR2, 1N456, one each way
CR1 and CR2, SM-150s, back to back across C20. They limit how far the ladder swings and so the oscillator’s level. The 1N456 stands in: its forward drop at a few microamps is closer to a rectifier’s than the 1N4001 model.
R45, 1.8 kΩ
In series with Speed, so the resistance never reaches zero.
R46, 1.8 kΩ
In series with Speed’s second gang.
Speed, 100 kΩ pot, reverse audio taper
Speed, one gang of the treadle’s 100 kΩ dual pot, as a variable resistor in parallel with R42. Less resistance is faster. The drawing marks it audio. It is set as reverse audio, the same pot turned the other way, so turning it up is faster along the whole travel.
Speed (2), 100 kΩ pot, reverse audio taper
Speed’s second gang, in parallel with R43, turned by the same treadle.
C22, 10 µF electrolytic
Couples the oscillator to Intensity.
Intensity, 50 kΩ pot, linear taper
Intensity, 50 kΩ linear. It sets how much of the oscillator’s swing reaches the lamp driver.
C23, 10 µF electrolytic
Couples Intensity to the lamp driver.
R47, 4.7 kΩ
Lamp driver’s base resistor.
R48, 47 kΩ
Lamp driver bias, to ground.
R50, 100 kΩ
Lamp driver bias from its collector, which steadies its resting current.
Q13, 2SC828
The lamp driver. Its emitter resistors set how much current the swing at its base moves through the bulb.
LP1, 12 V bulb, 40 mA
The bulb, 12 V at 40 mA, as marked on the drawing. Its light falls steeply as it cools, so the cells sweep most near the top of its swing.
TR1, 500 Ω pot, linear taper
Lamp bias trimmer, 500 Ω, in Q13’s emitter. Turned down it raises the bulb’s resting current. Set at 0 here, where the cells sweep most.
R49, 150 Ω
Q13’s fixed emitter resistor, the least the emitter ever has.

About this version

Uni-Vibe, 2SC828
From a redrawn schematic.

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.

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