Op-amp gain, diode clipping and one feedback triode stage, 1990 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 Marshall JCM900 4100 and 4500 (dual reverb), channel B.

The lead channel of a two-channel amp. Two op-amp gain stages drive a diode clipper made from a bridge rectifier and one more diode. Then a passive pre-EQ and op-amp buffer, and one triode stage and a cathode follower in a feedback loop. The follower drives the treble, bass and middle stack.

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. Most of the gain and all of the deliberate clipping happen in the op-amps. The first stage gains about 22 in the mids and less in the bass. The second gains up to 100 with the Gain pot, and its 220 pF rolls the treble off lower as the gain goes up. The clipper holds its node near ±2 V. The pre-EQ after it cuts the bass and shapes the mids. A 220 kΩ from the cathode follower’s output back to the first grid holds the triode pair to a gain of about 3. No more than about 1.7 V peak reaches the pair, so V1a’s grid moves only about 0.15 V and the tube stays linear at every Gain setting. The triodes set the level and impedance, and the op-amps and diodes make the distortion. Probe the clipper node, then V1a’s plate, to compare.

Download.

Parts and what they do

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

R5, 10 kΩ
Dropping resistor from the HT to node A, which feeds V1. It also feeds the reverb and return stages, which aren’t modeled here.
C3, 33 µF electrolytic
Filter cap at node A.
R35, 10 kΩ
Input series resistor.
C23, 100 pF
With R35 it makes a low-pass at about 160 kHz, far above the audio band, to filter out RF.
C5, 100 nF
Input coupling cap.
R6, 1 MΩ
Input bias resistor. It sets the input impedance near 1 MΩ.
IC1b, M5201 (one half)
First gain stage, the channel B half of a switching op-amp. Both halves get the same input, and the channel switch sets which one drives the shared output.
R10, 100 kΩ
Feedback resistor.
C8, 470 pF
Across R10, so the gain falls above about 3.4 kHz.
R11, 4.7 kΩ
To ground through C26. It sets the gain at 1 + 100 k / 4.7 k, about 22.
C26, 47 nF
In series with R11. Below about 720 Hz the gain falls toward 1, so the bass gets less gain.
C9, 220 nF
Coupling cap into the gain stage.
R42, 10 kΩ
Input resistor of the inverting stage.
Gain, 1 MΩ pot, audio taper
Gain, 1 MΩ audio, wired as a variable feedback resistor. The stage’s gain is the pot’s resistance over 10 k, up to 100.
C35, 220 pF
Across the Gain pot. At full gain it rolls the stage off above about 720 Hz. At lower settings the corner moves up.
IC6b, M5201 (one half)
Inverting gain stage, the channel B half of a second switching op-amp. The other half has its input grounded, so in channel A this stage passes nothing.
C36, 220 nF
Coupling cap. The sheet prints the labels C36 and R58 the other way round.
R58, 4.7 kΩ
Series resistor into the clipper. The op-amp swings about ±14 V and the clipper holds the node near ±2 V, so most of the swing drops across this resistor.
BR2, D10, 3 × 1N4001, one each way
A W005F bridge rectifier with a 1N4007 across its DC terminals, from the clipper node to ground. Either way the current passes two bridge diodes and D10, so it clips evenly at about three diode drops.
C27, 4.7 nF
Part of the pre-EQ, the treble path to R41.
R43, 4.7 kΩ
Main path through the pre-EQ, to C10 and C1.
R41, 10 kΩ
Part of the pre-EQ, from C27 to the op-amp input.
C10, 220 nF
In the pre-EQ, from R43 to the R40/R12 junction.
C1, 220 nF
In the pre-EQ, from R43 to the R12/R7 junction.
R40, 10 kΩ
In the pre-EQ, from the R40/R12 junction to the op-amp input.
R12, 100 kΩ
Part of the pre-EQ, between the two cap junctions.
R7, 4.7 kΩ
Part of the pre-EQ, to ground. With R43 it forms a divider that cuts most of the signal below the midrange.
R55, 100 kΩ
Bias resistor at the op-amp input.
IC4b, M5201 (one half)
Pre-EQ buffer, the channel B half of the third switching op-amp. Its other half is channel A’s LED clipper.
R38, 10 kΩ
Feedback resistor. It sets the gain at 1 + 10 k / 22 k, about 1.45.
C24, 100 pF
Across R38. Its effect is far above the audio band.
R37, 22 kΩ
Feedback resistor to ground.
R36, 100 kΩ
To ground at the shared output. Channel A’s volume pot, LEDs and caps also hang here, but the op-amp drives the node directly, so they’re left out.
R39, 2.2 kΩ
Series resistor toward V1a. With nothing plugged in, the input jack’s shorting contact (F on the sheet) grounds its far end.
R1, 68 kΩ
Input resistor of the feedback pair around V1. With R13 it sets the gain of V1a and V1b together near 220 k / 68 k, about 3.
C2, 22 pF
From V1a’s grid to its cathode.
V1a, 12AX7 / ECC83 / 7025
The only triode gain stage in the channel, inside the R13 feedback loop.
R2, 1.5 kΩ
Unbypassed cathode resistor.
150K, 150 kΩ
Plate load, on the tube base.
V1b, 12AX7 / ECC83 / 7025
Cathode follower, direct-coupled from V1a’s plate, driving the tone stack.
R14, 100 kΩ
Cathode load.
C11, 100 nF
Coupling cap from the cathode follower to the tone stack.
R13, 220 kΩ
Negative feedback from the tone-stack input back to V1a’s grid. V1a inverts and V1b follows, so this is shunt feedback: it lowers the gain from about 40 to about 3 and holds the tone stack’s input at 0 V DC.
C37, 470 pF
Treble cap.
Treble, 220 kΩ pot, linear taper
Treble, 220 kΩ linear. Its wiper is the stack’s output.
R61, 33 kΩ
Slope resistor.
C39, 22 nF
Bass cap.
Bass, 1 MΩ pot, audio taper
Bass, 1 MΩ audio, as a variable resistor.
C38, 22 nF
Mid cap, to the Mid pot’s wiper.
Middle, 22 kΩ pot, linear taper
Middle, 22 kΩ linear.
R59, 470 kΩ
With R60 it halves the signal into the buffer.
R60, 470 kΩ
Bottom of the divider.
IC5b, MC1458
Half an MC1458 as a follower, driving the effects send.
C13, 10 µF electrolytic
Output coupling cap to the send (B1). On the bench it drives a standard phase-inverter load. In the amp the loop, reverb mix and master volumes follow.
V2a, 12AX7 / ECC83 / 7025
12AX7, the driven half of the long-tail pair phase inverter. Its plate swings opposite the input and drives tube A. Its grid is fed through the preamp’s coupling cap, so there’s no second one here.
V2b, 12AX7 / ECC83 / 7025
The other half of the inverter. Its grid sits at the feedback point for signal, so it’s driven through the shared cathodes, and its plate swings with the input. It drives tube B.
1M, 1 MΩ
Grid resistor for the driven half, returned to the bias point above the tail, not to ground.
1M, 1 MΩ
Grid resistor for the other half.
1k, 1 kΩ
Bias resistor for both halves. The cathodes sit this far above the grids’ return, a volt or so.
15k, 15 kΩ
The tail resistor. It’s large, so the pair shares a nearly fixed current. When one half conducts more, the other conducts less, which splits the phase.
.1µ, 100 nF
Ties the second grid to the feedback point for signal, so the feedback acts across the pair.
82k, 82 kΩ
Plate resistor for the driven half. It’s smaller than the other so the two outputs come out about even.
100k, 100 kΩ
Plate resistor for the other half.
100p, 100 pF
A small cap from plate to plate. It rolls off the far top end the inverter passes on, which keeps the feedback loop steady.
47k, 47 kΩ
The feedback resistor from the speaker. With the 4.7k Ω below it, it sends about 9 % of the speaker voltage back to the inverter, which lowers the gain, flattens the response and tightens the bass.
100p, 100 pF
A small cap across the feedback resistor. It feeds back more of the top end, which keeps the loop steady out where the output transformer turns the phase round.
4.7k, 4.7 kΩ
The bottom of the feedback loop, 4.7k Ω to ground. The fed-back speaker voltage arrives here and moves the tail.
.022µ, 22 nF
In series with the resistor below it, across the leg. Above a few kilohertz it lowers the leg, so less treble is fed back and the top end comes up slightly. There’s no presence pot in the power amp.
4.7k, 4.7 kΩ
With the cap above it, across the leg. It sets how far the top end comes up, a fixed presence lift.
.047µ, 47 nF
Coupling cap to tube A’s grid.
.047µ, 47 nF
Coupling cap to tube B’s grid.
750, 750 Ω
Grid stopper for tube A. It keeps the tube from oscillating and softens grid current. Each of the two tubes has its own 1.5k. The resistors are drawn as one part, the two in parallel.
750, 750 Ω
Grid stopper for tube B. Each of the two tubes has its own 1.5k. The resistors are drawn as one part, the two in parallel.
V3, V5, 5881
n5881, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply. Two tubes in parallel, drawn as one.
V4, V6, 5881
n5881, tube B of the pair. Two tubes in parallel, drawn as one.
150k, 150 kΩ
Grid leak for tube A, returned to the bias supply. Driven into grid current, the coupling cap charges through it and the bias goes colder for a moment. That’s blocking distortion.
150k, 150 kΩ
Grid leak for tube B.
235, 235 Ω
Screen resistor for tube A. It limits the screen current when the tube is driven hard. Each of the two tubes has its own 470. The resistors are drawn as one part, the two in parallel.
235, 235 Ω
Screen resistor for tube B. Each of the two tubes has its own 470. The resistors are drawn as one part, the two in parallel.
High, 50 mΩ
The Hi/Low switch’s High contact for tube A’s screen resistor, to the screen supply.
High, 50 mΩ
The Hi/Low switch’s High contact for tube B’s screen resistor, to the screen supply.
100, 100 Ω
Dropping resistor from B+ to the screen supply. The screens sit slightly below the plates.
50µ, 50 µF electrolytic
Filter cap on the screen supply.
10k, 10 kΩ
Dropping resistor from the screen supply node to the node for the inverter and the preamp, as on the Marshall JCM900 4100 drawing. The current drawn past it drops the voltage, so each stage down the chain runs lower than the one before, and a hard note that pulls B+ down pulls these down too. The preamp runs off this node instead of its own HT supply.
50µ, 50 µF electrolytic
Filter cap at the node for the inverter and the preamp. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
OT, 1.7 kΩ to 4 Ω, 20 H primary, 5 mH leakage
The output transformer, 1.7 kΩ plate to plate into 4 Ω, center tap on B+. Tap any piece of it to change its figures. The core saturates as the flux nears its knee. Every figure is typical of the part.
PT, 360 V ac, 60 Hz
The power transformer’s high-voltage winding, 360 V ac end to end, into a bridge of four silicon diodes with the center tap unused. It’s drawn as two sources of 360 V ac a half cycle apart into two diodes, which charges the reservoir the same way at one diode drop (under a volt) higher. The value is typical of the part.
PT, 360 V ac, 60 Hz
The same winding on the other half cycle. It runs opposite the first source, so the bridge charges the reservoir on every half cycle.
PT R, 40 Ω
Resistance of the whole winding, plus the primary’s share. In a bridge every charging pulse runs through the whole winding. With the diodes’ drop it sets how far B+ sags on a hard note. Typical of the part.
PT R, 40 Ω
The same winding’s resistance, on the other half cycle. Typical of the part.
D1, 1N4001
One of the silicon rectifier diodes. It drops under a volt, so B+ holds up better under load than with a tube rectifier.
D2, 1N4001
The other rectifier diode, on the other half cycle.
50µ, 50 µF electrolytic
The reservoir caps’ first. It feeds the output transformer’s center tap. It charges in short pulses at each peak of the mains and drains between them, so B+ carries ripple, and on a hard note it sags.
50µ, 50 µF electrolytic
Another reservoir cap, in parallel with the first.
Pump, 202 V ac, 60 Hz
Stands in for the bias pump with the part below it. On the drawing, a .047µF cap from one leg of the high-voltage winding, 56k Ω to ground and the diode make the bias. The operating point can’t be solved through a cap in series, so this 201.72 V ac source and its resistor replace it. They were fitted to a simulation of the real pump on this winding: the same bias, and the same shift when the grids pull current on a hard note. It moves with the wall voltage, as the pump does.
pump R, 84.6 kΩ
Stands in for the pump with the source beside it, 84.6k Ω, fitted. It sets how far the bias moves under grid current. The amp has a .047µF cap here and 56k Ω from the diode end to ground.
Bias diode, 1N4001
The bias rectifier, a silicon diode turned so the supply comes out negative.
10µ, 10 µF electrolytic
Filter cap on the bias supply, its plus side to ground.
8.2k, 8.2 kΩ
Second resistor of the bias supply, 8.2k Ω to the bias line. With the cap after it, it filters the ripple a second time.
10µ, 10 µF electrolytic
Second filter cap, on the bias line, its plus side to ground.
47k, 47 kΩ
From the bias line toward ground, 47k Ω, in series with the bias pot. These two and the 8.2k Ω above them divide the supply down to the bias.
Bias 47k, 47 kΩ pot, linear taper
Bias adjust, a 47k Ω trimmer used as a variable resistor, the bottom of the divider. More resistance puts more of the supply on the grids, so turned up the tubes idle cooler. Which way it turns isn’t marked on the drawing.
Coil R, 4.34 Ω
Voice coil resistance, the floor of the speaker’s impedance. The shape of a 1x12 open-back cab with an alnico 12 in, measured (a 93 Ω peak near 74 Hz on 8 Ω), scaled to the 4 Ω tap. One real cab, not every speaker. The Speaker menu by the play buttons is separate. It plays the note through a speaker and cab impulse response, with either load.
Coil L, 128 µH
Voice coil inductance. It makes the impedance rise through the treble. The feedback holds the output steadier against that than it would be without feedback.
Loss L, 232 µH
With Loss R across it, the treble rises and then levels off, like the losses in a real coil.
Loss R, 7.15 Ω
Limits what Loss L adds to the treble rise.
Loss L2, 1.06 mH
A second step in the treble rise, with Loss R2 across it: a real voice coil’s losses keep climbing from a few hundred hertz.
Loss R2, 1.09 Ω
Limits what Loss L2 adds.
Tank R, 41.9 Ω
Height of the bass resonance, about 46 Ω in all at its peak. The feedback holds the output steadier against that than it would be without feedback.
Tank L, 10.6 mH
With Tank C, sets the resonance: 74 Hz.
Tank C, 440 µF
With Tank L, sets the resonance. With Tank R, it sets how narrow the peak is.

About this version

ECC83, M5201
One ECC83 and three M5201 halves. Real tubes spread 20 % or more. The HT voltage is assumed, since the drawing shows none.

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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