Two-mode amp with LED clipping, 1987 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 Silver Jubilee 2550 (25/50 W).

One channel with two modes switched by a relay. In lead, V1b drives a clipper of red LEDs and diodes, then the lead master and a feedback stage. In rhythm, the relay grounds the clipper node and the signal goes around it, part of it fed back to V1b’s grid. A send divider, one more triode and the treble-middle-bass stack follow.

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. The clipper after V1b clips at about 3 V positive and 2.4 V negative: a red LED and two diodes one way, a red LED and one diode the other. The uneven clipping adds even harmonics as well as odd. A 1 MΩ from V2a’s output back to its grid keeps its gain low, so it stays clean while the LEDs clip. In rhythm the relay grounds the clipper node, shorting the LEDs and the lead master. The signal goes from V1b through R7 to the switching node, then through R8 and R10 to V2a, and R6 brings part of it back to V1b’s grid, for less gain. The Rhythm clip switch puts two plain diodes on the switching node, clipping at about 0.35 V. In lead that node is on ground, so the switch grounds the diodes and leaves R9 across V2a’s grid, about 3 dB quieter, as wired on the drawing. Probe the clipper node to see the LEDs clip, or the switching node in rhythm.

Download.

Parts and what they do

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

C34a, 50 µF electrolytic
Filter cap on node A, which feeds V2.
R49, 10 kΩ
Dropping resistor, 10 kΩ 1 W, to V1’s supply node.
C34b, 50 µF electrolytic
Filter cap on V1’s supply node.
C2, 47 nF
Input coupling cap, .047 µF.
R2, 1 MΩ
Grid return, ahead of the grid stopper.
R25, 68 kΩ
Grid stopper, 68 kΩ. With the tube’s input capacitance it trims the top end.
V1a, 12AX7 / ECC83 / 7025
Input stage.
R26, 100 kΩ
Plate load.
R1, 2.7 kΩ
Cathode resistor.
C1, 680 nF
Cathode bypass, 0.68 µF. The stage has full gain above about 90 Hz.
C33, 47 pF
From plate to cathode. With the cathode bypassed, it works as a small cap to ground.
C3, 22 nF
Coupling cap to the gain pot.
Input gain, 1 MΩ pot, audio taper
Input gain, 1 MΩ log, with the Rhythm clip pull switch on its shaft.
C5, 1 nF
Bright cap, 1 nF across the top of the gain pot. It lifts the treble most with the gain low.
R27, 100 kΩ
Series resistor into V1b. With R6 it sets how much feedback reaches the grid in rhythm mode.
R6, 220 kΩ
V1b’s grid return to the switching node. In lead the node is on ground and this is a plain grid resistor. In rhythm it feeds part of V1b’s output back to the grid.
V1b, 12AX7 / ECC83 / 7025
Second gain stage. It drives the clipper.
R28, 100 kΩ
Plate load.
C19, 470 pF
From the plate to the supply, which is ground for the signal. It cuts the treble above about 5 kHz.
R3, 1.5 kΩ
Cathode resistor.
C4, 10 µF electrolytic
Cathode bypass, full gain down through the bass.
C20, 22 nF
Coupling cap.
R7, 220 kΩ
From after C20 to the switching node. In lead it’s a load to ground. In rhythm it carries the signal to the switching node.
R29, 47 kΩ
Series resistor into the clipper, limiting the LEDs’ and diodes’ current. In rhythm, with the clipper node on ground, it’s a 47 kΩ load on V1b.
LED2, LED3, D1–D3, Red LED + 2 × 1N4001 one way, Red LED + 1N4001 the other way
The clipper: LED3, D1 and D2 in series on the positive half, about 3 V, and LED2 and D3 on the negative, about 2.4 V. The diodes are 1N4007s.
C21, 1 nF
Across the clipper. With R29 it cuts above about 3.4 kHz.
Lead master, 220 kΩ pot, linear taper
Lead master, 220 kΩ linear, across the clipper. In rhythm both ends are on ground.
R11, 47 kΩ
From the lead master to V2a’s grid.
R8, 10 kΩ
From the switching node toward V2a’s grid.
R10, 100 kΩ
In lead, with R8, a load from V2a’s grid to ground. In rhythm, the path from the switching node to V2a.
R9, 47 kΩ
From V2a’s grid to D4 and D5. With C6, a treble cut. With the Rhythm clip pulled in lead it goes to ground.
C6, 2.2 nF
With R9, a treble cut at V2a’s grid.
D4, D5, 1N4001, one each way
Two 1N4007s back to back to ground.
R12, 1 MΩ
Feedback from V2a’s output, after C24, to its grid. It holds the stage’s gain down to a few times.
V2a, 12AX7 / ECC83 / 7025
Third stage, with plate-to-grid feedback and an unbypassed cathode.
R35, 100 kΩ
Plate load.
R32, 1.5 kΩ
Cathode resistor, unbypassed.
C24, 100 nF
Coupling cap.
R34, 100 kΩ
Top of the send divider.
R33, 10 kΩ
Bottom of the send divider. The send sits about 21 dB below V2a’s plate.
C25, 100 nF
Coupling cap from the effects return (normalled to the send) to V2b.
R36, 1 MΩ
Grid leak.
V2b, 12AX7 / ECC83 / 7025
Return stage. Its plate drives the tone stack.
R38, 220 kΩ
Plate load, 220 kΩ.
R37, 470 Ω
Cathode resistor, 470 Ω unbypassed, for a warm bias.
C27, 100 nF
Feeds the bass pot’s back gang.
Bass, 1 MΩ pot, audio taper
Bass, back gang, 1 MΩ log. Turned up, it lets more of the plate’s signal reach the stack through the 0.1 µF cap, and that cap passes the bass.
R16, 100 kΩ
Below the bass pot’s back gang.
C26, 4.7 nF
Feeds the stack input directly. At 4.7 nF it passes the mids and treble at any Bass setting.
C8, 220 pF
Treble cap.
R13, 100 kΩ
Slope resistor to the mid node.
C10, 10 nF
From the mid node to the mid pot’s wiper.
Middle, 100 kΩ pot, audio taper
Middle, 100 kΩ log. Its top goes through C11 to the top of the treble pot.
C11, 470 pF
From the top of the mid pot to the top of the treble pot.
C9, 100 nF
Bass cap from the mid node toward the treble pot’s bottom.
R14, 10 kΩ
In series with C9.
C7, 2.2 nF
From the treble pot’s bottom to ground.
Treble, 220 kΩ pot, linear taper
Treble, 220 kΩ linear. Its wiper is the stack’s output.
R15, 100 kΩ
Above the bass pot’s front gang.
Bass, 1 MΩ pot, audio taper
Bass, front gang, 1 MΩ log, a variable resistor. Turned up, it puts the bottom of the treble pot further from ground.
Output master, 1 MΩ pot, audio taper
Output master, 1 MΩ log. In the amp a pull switch on it also works the relay.
C28, 22 nF
Coupling cap into the phase inverter.
V3a, 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.
V3b, 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.
470, 470 Ω
Bias resistor for both halves. The cathodes sit this far above the grids’ return, a volt or so.
10k, 10 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.
47p, 47 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.
100k, 100 kΩ
The feedback resistor from the speaker. With the 4.7k Ω below it, it sends about 4 % of the speaker voltage back to the inverter, which lowers the gain, flattens the response and tightens the bass.
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.
.1µ, 100 nF
The presence cap, in series with the pot across the leg. With the pot it sets where the lift starts.
Presence, 22 kΩ pot, linear taper
Presence, a 22k linear pot as a variable resistor, its resistance falling as it’s turned up, in series with the cap across the leg. Turned up, the cap shunts more of the leg’s treble, so less treble is fed back and the top end rises.
.022µ, 22 nF
Coupling cap to tube A’s grid.
.022µ, 22 nF
Coupling cap to tube B’s grid.
5.6k, 5.6 kΩ
Grid stopper for tube A. It keeps the tube from oscillating and softens grid current.
5.6k, 5.6 kΩ
Grid stopper for tube B.
V4, EL34 / 6CA7
EL34, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply.
V5, EL34 / 6CA7
EL34, tube B of the pair.
220k, 220 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.
220k, 220 kΩ
Grid leak for tube B.
1.5k, 1.5 kΩ
Screen resistor for tube A. It limits the screen current when the tube is driven hard.
1.5k, 1.5 kΩ
Screen resistor for tube B.
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.
5 H, 5 H
A choke from B+ to the screen supply, as in the amp this style is based on. It passes DC with little drop, so the screens sit close to B+. With the cap after it, it would filter ripple, but ripple isn’t modeled here. Typical of the part, not measured.
100, 100 Ω
The choke’s winding resistance, typical, not measured. It’s all the DC drop the screens see from B+.
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, as on the Marshall Silver Jubilee 2550 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.
50µ, 50 µF electrolytic
Filter cap at the node for the inverter. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
10k, 10 kΩ
Dropping resistor from the node for the inverter to the node for the preamp, as on the Marshall Silver Jubilee 2550 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.
OT, 3.4 kΩ to 4 Ω, 40 H primary, 10 mH leakage
The output transformer, 3.4 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, not measured.
PT, 350 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 350 V ac from the grounded center tap, as a sine source at the mains frequency. The value is typical of the part, not measured.
PT, 350 V ac, 60 Hz
The other half of the winding, from the grounded center tap. It runs opposite the first half, so the rectifier charges the reservoir on every half cycle.
PT R, 50 Ω
Resistance of one half of the winding, plus the primary’s share. With the rectifier’s own drop it sets how far B+ sags on a hard note. Typical of the part, not measured.
PT R, 50 Ω
Resistance of the other half. Typical of the part, not measured.
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.
PT, 350 V ac, 60 Hz
The end of the high-voltage winding the bias comes off, 350 V ac, the same end as the rectifier’s first plate. It’s drawn as its own source so the bias supply’s average can be found. Keep it equal to the winding beside the rectifier.
220k, 220 kΩ
Series resistor into the bias rectifier, 220k Ω. There’s no bias tap, so the bias comes straight off the high-voltage winding, and this resistor drops most of it.
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.
15k, 15 kΩ
Second resistor of the bias supply, 15k 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.
56k, 56 kΩ
From the bias line toward ground, 56k Ω, in series with the bias pot. These two and the 15k Ω above them divide the supply down to the bias.
Bias 22k, 22 kΩ pot, linear taper
Bias adjust, a 22k 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 throughout
Two 12AX7s, the factory tubes, and red LEDs, typical of their types, not measured. Real tubes spread 20 % or more.

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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