Rack flanger and doubler on two delay chips, 1979 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 MXR Flanger/Doubler (model 126).

A rack flanger with two delay chips on one clock. Flange uses a SAD1024 for 0.33 to 4.9 ms, and Double an R5101 on a quarter of the clock for about 15 to 70 ms. A triangle LFO sweeps both through a discrete VCO, Mix sets the dry against the wet, and Regen feeds the wet back in.

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. U6a is the input stage, gain 1 from LO IN. U7a lifts the treble about 20 dB from 2 kHz up, and U7b cuts it back down after the delay, so the chips’ hiss comes down with it. The signal feeds both delay chips at once, and S2 picks which one reaches the output. In Flange the SAD1024’s two halves run on opposite clock phases, a delay of 512 clock periods: 4.9 ms with Manual down, 0.33 ms where the clock stops at 1.55 MHz. In Double the R5101 runs on a quarter of the clock, about 70 to 15 ms, and S2’s second pole narrows the clock’s range. Mixed with the dry signal, a short delay cancels it at a row of frequencies: at 1.24 ms the notches fall near 400, 1,160 and 1,920 Hz, about 30 dB deep with Regen at 0. Regen feeds the wet back into U7a through two diodes that hold it under about 0.6 V. At full the peaks come up about 14 dB, and the loop doesn’t run away. Q6 and Q9 make an exponential converter, so each volt from Width moves the clock by a ratio. Speed runs the triangle from about 0.02 to 18 sweeps a second. CR8’s photocell is a fixed 1 kΩ here, lit, so the hiss filter that closes on quiet notes isn’t modeled. The R5101 has no data sheet, so its delay and gain are assumed. The VCO and the 4013 dividers are a stand-in clock. R94 is 15 kΩ in the model, not the drawing’s 16 kΩ, and U11 swings within 0.6 V of its rails. Otherwise the LFO stops at its top turn. The output is LO OUT, a quarter of HI OUT. The bypass switching isn’t drawn.

Download.

Parts and what they do

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

R22, 1 MΩ
LO IN’s load to ground.
C7, 100 nF
LO IN’s coupling cap.
R24, 300 kΩ
LO IN’s input resistor. Against R28 the gain is 1, turned over.
R26, 300 kΩ
From HI IN’s node. With nothing in HI IN, it and R27 hang from the summing node and change nothing.
R27, 100 kΩ
HI IN’s divider to ground. HI IN (C8, R25, R23) is not drawn.
U6a, RC4558
Input stage, inverting, gain 1 from LO IN. Its output is the dry signal and feeds the delay side. A 4558.
R28, 300 kΩ
Feedback resistor of the input stage.
C9, 10 pF
Rolls the input stage off above about 53 kHz.
R29, 5.6 kΩ
With C10 the treble path into U7a. It sets where the lift levels off, near 19 kHz.
C10, 1.5 nF
Passes the treble around R30, so the lift starts near 2 kHz.
R30, 51 kΩ
Input resistor for the lows. Against R31 the gain is about 2.
U7a, RC4558
Pre-emphasis and the sum of the guitar and Regen, inverting. The treble comes up about 20 dB before the delay chips, so their hiss comes down after. A 4558.
R31, 100 kΩ
Feedback resistor.
C11, 30 pF
Rolls the top off above about 53 kHz.
Regen, 10 kΩ pot, linear taper
Regen, 10 kΩ linear, from the wet line to ground. It sets how much of the wet goes back into the delay.
R77, 10 kΩ
From the Regen wiper to the limiter diodes.
CR2, CR3, 1N914 / 1N4148, one each way
Two diodes back to back to ground. They hold the Regen signal under about 0.6 V peak, so full Regen can’t run away into the rails.
C39, 1.5 nF
With R77 it cuts the Regen signal above about 10 kHz.
C38, 15 nF
With R78 it keeps the lows out of the Regen path, below about 250 Hz.
R78, 43 kΩ
Regen mixing resistor into U7a.
R32, 2.4 kΩ
With C12 a low-pass near 20 kHz.
C12, 3.3 nF
Low-pass cap to ground.
R33, 75 kΩ
First resistor of U9a’s low-pass.
C13, 1 nF
Feedback cap of U9a’s low-pass.
R34, 13 kΩ
Second resistor of U9a’s low-pass.
C14, 62 pF
Cap to ground on U9a’s + input.
U9a, MC1458
Follower of a two-pole low-pass ahead of the delay chips. A 1458.
R35, 12 kΩ
First resistor of U10a’s low-pass.
C15, 1 nF
Feedback cap of U10a’s low-pass.
R36, 6.8 kΩ
Second resistor of U10a’s low-pass.
C16, 470 pF
Cap to ground on U10a’s + input.
U10a, MC1458
Follower of the second low-pass. It drives both delay chips at once. A 1458.
C17, 2.2 µF electrolytic
Couples the signal toward the R5101.
R37, 10 kΩ
With R38 it divides the lows by about 1.8 into the chip.
C18, 2.2 nF
Passes the treble around R37, above about 7 kHz.
R38, 12 kΩ
Sets the chip input’s DC from the bias trim.
R39, 13 kΩ
Top of the bias trim’s divider.
R40, 25 kΩ pot, linear taper
CCD BIAS trim, 25 kΩ. At half, assumed: 3.95 V on the chip’s input.
C19, 4.7 µF electrolytic
Holds the bias trim’s wiper still.
U4, R5101 (stand-in)
Reticon R5101, the Doubler’s delay. No data sheet is known. A stand-in delays by 5120 periods of the quarter clock the unit feeds it, which puts the Doubler at about 15 to 70 ms. Its two outputs are one pin here, and its gain, 0.53, is assumed so the Doubler’s wet matches the Flanger’s.
R41, 2 kΩ
From the chip’s first output toward the balance trim.
R42, 2 kΩ
From the chip’s second output toward the balance trim.
R43, 2.5 kΩ pot, linear taper
CCD BAL trim, 2.5 kΩ. It balances the two outputs to cancel the clock. Both outputs are one pin here, so it does nothing in the model. At half.
R45, 1 kΩ
Across one side of the balance trim.
R44, 1 kΩ
Across the other side of the balance trim.
Q1, 2N3904
A transistor wired as a diode, the input of a current mirror. The chip’s output current runs through it. 2N3904 assumed.
R49, 270 Ω
Q1’s emitter resistor.
Q2, 2N3904
The mirror’s output. It pulls the same current through R50, so the chip’s output becomes a voltage there, turned over. 2N3904 assumed.
R51, 270 Ω
Q2’s emitter resistor.
R50, 2.7 kΩ
The mirror’s load from +12 V.
C23, 1 nF
With R50 it cuts the clock’s steps above about 59 kHz.
Q3, 2N3904
Emitter follower out of the mirror. 2N3904 assumed.
R52, 10 kΩ
Q3’s emitter resistor.
C25, 1 µF electrolytic
Couples the R5101’s side to S2. U3’s gate ahead of it, closed with the effect in, is drawn as a wire.
R55, 470 kΩ
Holds the Doubler contact at 0 V.
C26, 2.2 µF electrolytic
Couples the signal toward the SAD1024.
R56, 15 kΩ
With R57 it divides the signal by 2.5 into the chip.
R57, 10 kΩ
Sets the chip input’s DC from the bias trim.
R58, 25 kΩ pot, linear taper
BBD BIAS trim, 25 kΩ across +12 V. At half, assumed: 6.0 V on both halves’ inputs.
C27, 4.7 µF electrolytic
Holds the bias trim’s wiper still.
U5, SAD1024 (both halves in turn)
SAD1024, the Flanger’s delay. Both 512-stage halves take the same signal on opposite clock phases, and their outputs meet at the BBD BAL trim, so the chip is one 1024-stage line: a delay of 512 VCO periods. The trim, R59, is left out and the chip’s one out pin goes to R60.
R60, 30 kΩ
The load on the chip’s outputs, from the balance trim’s wiper.
C30, 1 µF electrolytic
Couples the SAD1024’s side to S2. U3’s gate ahead of it, closed with the effect in, is drawn as a wire.
R63, 470 kΩ
Holds the Flange contact at 0 V.
R64, 62 kΩ
A load on S2’s common.
R65, 30 kΩ
First resistor of U10b’s low-pass.
C31, 470 pF
Feedback cap of U10b’s low-pass.
R66, 30 kΩ
Second resistor of U10b’s low-pass.
C32, 300 pF
Cap to ground. With C31 the corner is near 14 kHz.
U10b, MC1458
Follower of a two-pole low-pass that smooths the delay chips’ steps. A 1458.
R67, 15 kΩ
First resistor of U9b’s low-pass.
C33, 2.2 nF
Feedback cap of U9b’s low-pass.
R68, 15 kΩ
Second resistor of U9b’s low-pass.
C34, 150 pF
Cap to ground. With C33 the corner is near 18 kHz.
U9b, MC1458
Follower of the second smoothing low-pass. A 1458.
C35, 4.7 µF electrolytic
Coupling cap into the invert stage.
R69, 10 kΩ
Holds the invert stage’s input at 0 V.
R71, 10 kΩ
Input resistor to U8b’s − input.
R70, 10 kΩ
Into U8b’s + input. With S3 open the stage passes the wet at +1, with S3 closed at −1.
U8b, MC1458
The invert stage, gain +1 or −1 by S3. A 1458.
R72, 10 kΩ
Feedback resistor of the invert stage.
R127, 10 kΩ
With C53 a low-pass near 1 kHz when CR8’s cell is dark.
CR8, 1 kΩ
Stand-in for CR8’s photocell, across R127. On the unit U12 lights CR8’s LED by the wet’s level, so the top end opens on loud notes and closes on quiet ones to hide hiss. Fixed at 1 kΩ here, lit, assumed: the corner sits near 12 kHz. U12 and the LED’s bridge are not drawn.
C53, 15 nF
The hiss filter’s cap to ground.
R73, 25 kΩ pot, linear taper
DELAY GAIN trim, 25 kΩ, used as a variable resistor. It sets the wet level. At 0.28, assumed, so the wet meets the dry at the mixer for the deepest notches.
R74, 33 kΩ
Input resistor of the de-emphasis stage.
U7b, RC4558
De-emphasis, inverting. It cuts the treble back down from about 2 kHz. A 4558.
R75, 51 kΩ
Feedback resistor of the de-emphasis stage.
C36, 1.5 nF
Across R75, the de-emphasis: the treble falls from about 2.1 kHz.
C37, 4.7 µF electrolytic
Couples the wet onto the wet line, to Mix and to Regen.
C40, 1 µF electrolytic
Couples the dry signal into Mix.
R80, 10 kΩ
From the dry signal to Mix’s dry end.
R82, 10 kΩ
Dry mixing resistor into U6b.
R79, 10 kΩ
From the wet line to Mix’s wet end.
R81, 10 kΩ
Wet mixing resistor into U6b.
Mix, 10 kΩ pot, linear taper
Mix, 10 kΩ linear, its wiper to ground. Turned down it grounds the wet, turned up the dry. At half both come through equally.
U6b, RC4558
The mixer, inverting. A 4558.
R83, 120 kΩ
Feedback resistor of the mixer.
C41, 30 pF
Rolls the mixer off above about 44 kHz.
C42, 15 µF electrolytic
Output coupling cap.
R85, 3 kΩ
Top of the LO OUT divider. LO OUT, the output drawn here, is a quarter of HI OUT. HI OUT, through R84 100 Ω from the cap, is not drawn.
R86, 1 kΩ
Bottom of the LO OUT divider.
Manual, 10 kΩ pot, linear taper
Manual, 10 kΩ. It sets the delay the sweep moves around. Turned up, the clock runs faster and the delay gets shorter.
R89, 150 kΩ
From Manual’s wiper to the follower.
R91, 560 kΩ
From +12 V. With R89 and R90 it holds Manual’s voltage between about 2.3 and 10.7 V.
R90, 1 MΩ
To ground, the bottom of Manual’s range.
U8a, MC1458
Follower for Manual’s voltage. A 1458.
Width, 10 kΩ pot, linear taper
Width, 10 kΩ, between Manual’s voltage and the triangle. Its wiper picks a mix of the two, so it sets how far the sweep reaches. At 0 the delay holds where Manual sets it. The wiper also goes to the CV OUT jack, normalled through CV IN, which is not drawn.
R103, 470 kΩ
Turns Width’s voltage into a current toward Q6.
R106, 430 kΩ
The rest of the control current’s path into Q6’s base.
R104, 680 kΩ
Top of the Doubler’s pull, from +12 V.
R105, 220 kΩ
Bottom of the Doubler’s pull. In Double, S2 ties R103 and R106’s middle to 2.9 V through about 166 kΩ, which narrows the sweep and moves it faster.
C45, 3.3 µF electrolytic
Smooths Q6’s base. At the fast Speed settings it rounds the triangle and narrows the sweep.
R109, 10 kΩ
Between Q6’s base and Q9. The control current through it lifts Q6’s base above Q9, and each 2.6 µA multiplies Q6’s current by about 2.7.
Q9, 2N3904
A transistor wired as a diode, the start of Q6’s exponential curve. 2N3904 assumed.
R108, 430 kΩ
Feeds Q9 a standing current from the clock trim.
R107, 25 kΩ pot, linear taper
CLOCK TRIM, 25 kΩ. It sets Q9’s standing current, so the slowest clock. Set for the listed 4.9 ms at Manual 0 in Flange, assumed.
Q6, 2N3904
The VCO’s current source. Its collector current empties the 20 pF timing cap, C47, each cycle, so it sets the clock. 2N3904 assumed. Q7 above it, a cascode, is not drawn.
VCO, VCO and divider (stand-in)
Stand-in for the VCO (Q7, Q8, three 4069 gates, C47) and the 4013 dividers. The clock runs at Q6’s current over C47’s 20 pF times 6.4 V, the drop from the gate’s input clamp to its switch point, and stops at 1.55 MHz, where the listed shortest delay, 0.33 ms, puts it. The SAD1024 gets half that on each phase, the R5101 a quarter.
R93, 13 kΩ
Top of the LFO’s 6.6 V divider.
R94, 15 kΩ
Bottom of the LFO’s divider. 15 kΩ in the model, not the drawing’s 16 kΩ: with 16 kΩ the triangle’s upper turn sits near 11.2 V, more than U11 reaches on 12 V, so the LFO stops.
U11a, MC1458
Comparator with hysteresis. Its output flips each time the triangle reaches a turn. A 1458 on 0 and +12 V.
R96, 100 kΩ
Positive feedback that holds the comparator where it last flipped.
R97, 82 kΩ
From the triangle to the comparator. With R96 it sets the triangle’s turns, about 3.4 and 10.8 V by hand.
R95, 100 kΩ
Holds the comparator’s − input at the LFO’s 6.6 V.
C49, 1 nF
From the comparator’s output to its − input.
R98, 680 Ω
Sets the fastest sweep, with Speed at full.
Speed, 500 kΩ pot, reverse audio taper
Speed, 500 kΩ, used as a variable resistor. Its resistance falls along an audio curve as it turns up, so the triangle ramps faster, about 0.02 to 16 sweeps a second. The taper is assumed.
C43, C44, 23.5 µF
Two 47 µF electrolytics back to back, drawn as one 23.5 µF. The integrator’s cap: a larger one slows every Speed setting.
U11b, MC1458
Integrator. The square wave in makes the triangle out. A 1458 on 0 and +12 V.

About this version

As drawn
Mains powered: the battery part stands in for the +12 V regulator. The op-amps run on +12 and −12 V. The transformer, rectifier and regulators are not drawn.

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.