Five-stage cascaded overdrive amp, 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 Soldano SLO-100.

Two channels. The overdrive channel runs four gain stages and a cathode follower in cascade, then a 2 % send divider, a recovery stage and a second follower into the tone stack. The normal channel is one stage that joins it at the fourth stage’s grid.

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 overdrive path has four gain stages. V1b and V3b have 220 kΩ plate loads and 1 µF cathode caps, so their gain falls below about 84 Hz, and V2a’s below about 140 Hz. This is how the amp trims bass. The 470 kΩ resistors ahead of V2a and V2b divide the level and act as large grid stoppers. V2b is a cold clipper with 39 kΩ unbypassed and gains only a few dB, with 1 nF across its plate load trimming the treble. Most of the overdrive happens in V3b and its cathode follower, V3a. Probe V3b’s plate and turn up the OD volume. The send divider passes only about 2 %, so the recovery stage, tone stack and phase inverter stay clean. The normal channel is V1a alone, fed from V1b through S1 and joined to the mix node through 2.2 MΩ. Its signal is there in Overdrive too, far under the overdrive’s.

Download.

Parts and what they do

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

R61, 15 kΩ
Dropping resistor from B2 to B4, which feeds V1b and V1a. The drawing marks B4 at 359 V.
C37, 10 µF electrolytic
B4 filter cap, 10 µF.
R62, 15 kΩ
Dropping resistor from B2 to B3, which feeds V2a and V2b. The drawing marks B3 at 350 V.
C38, 10 µF electrolytic
B3 filter cap, 10 µF.
R1, 1 MΩ
Input grid leak.
R2, 68 kΩ
Input grid stopper.
V1b, 12AX7 / ECC83 / 7025
Input stage, shared by both channels. It has a 220 kΩ plate load.
R4, 220 kΩ
V1b plate load.
R3, 1.8 kΩ
V1b cathode resistor.
C1, 1 µF
Only 1 µF. The stage’s gain is 3 dB under full at about 84 Hz and falls further below. This is one place the amp trims bass.
C2, 20 nF
Coupling cap. Both channels take their signal from its far side.
R5, 470 kΩ
Top of the first divider, with the OD volume as the bottom.
C3, 2 nF
Treble peaker, .002 µF. It passes the highs around R5 above about 170 Hz.
OD volume, 500 kΩ pot, audio taper
The overdrive channel’s gain, 500 kΩ (taper assumed audio).
C100, 1 nF
Bright cap, .001 µF, across the top of the OD volume. It lifts the treble most with the volume low. Taking it out is the mod the drawing’s note names after Warren Haynes.
R8, 470 kΩ
A 470 kΩ grid stopper. With V2a’s input capacitance it cuts treble, and it keeps the grid from drawing much current.
V2a, 12AX7 / ECC83 / 7025
Second gain stage, with a 100 kΩ plate load and a partly bypassed cathode.
R11, 100 kΩ
V2a plate load.
R9, 1.8 kΩ
V2a cathode resistor.
C4, 1 µF
V2a bypass, 1 µF. With the 100 kΩ plate load the stage is 3 dB under full gain at about 140 Hz.
C6, 20 nF
Coupling cap.
R12, 470 kΩ
Top of the second divider and V2b’s grid stopper.
R13, 1 MΩ
Bottom of the second divider. About two thirds of the signal gets through.
V2b, 12AX7 / ECC83 / 7025
The cold clipper. Its 39 kΩ cathode resistor is unbypassed, and it idles near 0.1 mA. It gains only a few dB but rounds one side of the wave. In Clean, VR1 grounds its grid.
R19, 100 kΩ
V2b plate load.
C9, 1 nF
Across the plate load. It rolls this stage’s output off above about 1.6 kHz.
R14, 39 kΩ
The 39 kΩ cold-clipper cathode resistor.
C10, 20 nF
Coupling cap out of V2b.
R20, 2.2 MΩ
Holds C10’s far side at ground for DC while VR2 is dark.
R6, 470 kΩ
Normal channel series resistor, in circuit with S1 on Clean and shorted on Crunch.
R10, 470 kΩ
Normal channel series resistor into the normal volume.
C5, 470 pF
Bright cap, 470 pF. S2 puts it across R10.
R7, 39 kΩ
With S1 on Clean it hangs from the normal volume’s top to ground and cuts the level about 28 dB.
Normal volume, 500 kΩ pot, audio taper
The normal channel’s volume, 500 kΩ (taper assumed audio). Its wiper is V1a’s grid.
V1a, 12AX7 / ECC83 / 7025
The normal channel’s one gain stage, 100 kΩ plate load and 2.2 kΩ unbypassed cathode.
R15, 100 kΩ
V1a plate load.
R16, 2.2 kΩ
V1a cathode resistor, unbypassed.
C7, 20 nF
Coupling cap out of V1a.
R17, 2.2 MΩ
Normal channel’s mixing resistor, 2.2 MΩ. With R18 it passes about an eighth of V1a’s output at low frequencies.
C8, 120 pF
Across R17. It passes more of the highs around it.
R18, 330 kΩ
Mix node to ground. It loads V2b’s output in Overdrive.
R21, 220 kΩ
Grid stopper into V3b.
V3b, 12AX7 / ECC83 / 7025
Fourth gain stage, shared by both channels. It has a 220 kΩ plate load and 1.8 kΩ with 1 µF on the cathode, and it’s DC-coupled into V3a.
R22, 220 kΩ
V3b plate load, from B2.
R23, 1.8 kΩ
V3b cathode resistor.
C11, 1 µF
V3b bypass, 1 µF. The stage is 3 dB under full gain at about 84 Hz.
V3a, 12AX7 / ECC83 / 7025
Send follower, DC-coupled to V3b’s plate. When pushed it draws grid current and loads V3b, which squares off both sides of the wave.
R25, 100 kΩ
Top of the send divider, from V3a’s cathode.
R24, 2.2 kΩ
RL on the drawing, the bottom of the send divider. It lets about 2 % through. The drawing’s note puts 1 kΩ across it for a lower send level.
C12, 1 µF
Send coupling cap, 1 µF. It keeps the tap’s 4 V off the send jack and the recovery grid.
R26, 1 MΩ
Recovery grid leak at the return jack. With nothing plugged in, the send’s switch contact feeds the return.
V4b, 12AX7 / ECC83 / 7025
Effects recovery stage. It brings back much of the level the send divider threw away, and it stays clean.
R28, 220 kΩ
Recovery plate load, from B2.
R27, 2.2 kΩ
Recovery cathode resistor.
C13, 25 µF electrolytic
Recovery bypass. The drawing marks it with a star and no value, and its note says to add 25 µF, the value used here. The stage has full gain down to the lowest notes.
V4a, 12AX7 / ECC83 / 7025
Tone-stack follower, DC-coupled to the recovery plate.
R29, 100 kΩ
Tone-stack follower cathode resistor.
C14, 470 pF
Treble cap, 470 pF.
R30, 47 kΩ
Slope resistor, 47 kΩ.
C15, 20 nF
Bass cap, .02 µF.
C16, 20 nF
Mid cap, .02 µF, on the mid pot’s wiper.
Treble, 250 kΩ pot, audio taper
Treble pot, 250 kΩ. Its wiper feeds both masters.
Bass, 1 MΩ pot, audio taper
Bass pot, 1 MΩ, a variable resistor.
Middle, 25 kΩ pot, linear taper
Mid pot, 25 kΩ, with the mid cap on the wiper and the bass pot returning to ground through the whole track. Turned down, it leaves slightly more bass than a pot wired as a variable resistor.
OD master, 1 MΩ pot, audio taper
Overdrive master, 1 MΩ, reached through VR3 in Overdrive.
Normal master, 1 MΩ pot, audio taper
Normal master, 1 MΩ, reached through VR4 in Clean. It loads the treble wiper in both modes.
C17, 20 nF
Coupling cap into the phase inverter grid.
V5a, 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.
V5b, 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.
39k, 39 kΩ
The feedback resistor from the speaker. With the 4.7k Ω below it, it sends about 11 % 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, 25 kΩ pot, linear taper
Presence, a 25k 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.
.047µ, 47 nF
Coupling cap to tube A’s grid.
.047µ, 47 nF
Coupling cap to tube B’s grid.
2.2k, 2.2 kΩ
Grid stopper for tube A. It keeps the tube from oscillating and softens grid current. One 2.2k Ω serves both tubes on this side, as on the drawing.
2.2k, 2.2 kΩ
Grid stopper for tube B. One 2.2k Ω serves both tubes on this side, as on the drawing.
V6, V8, 6L6GC
6L6GC, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply. Two tubes in parallel, drawn as one.
V7, V9, 6L6GC
6L6GC, tube B of the pair. Two tubes in parallel, drawn as one.
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.
475, 475 Ω
Screen resistor for tube A. It limits the screen current when the tube is driven hard. One 475 Ω serves both tubes on this side, as on the drawing.
475, 475 Ω
Screen resistor for tube B. One 475 Ω serves both tubes on this side, as on the drawing.
10 H, 10 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.
100, 100 Ω
The choke’s winding resistance, typical. It’s all the DC drop the screens see from B+.
100µ, 100 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 Soldano SLO-100 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.
40µ, 40 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, 2 kΩ to 8 Ω, 20 H primary, 5 mH leakage
The output transformer, 2 kΩ plate to plate into 8 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, 378 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 378 V ac from the grounded center tap, as a sine source at the mains frequency. The value is typical of the part.
PT, 378 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, 30 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.
PT R, 30 Ω
Resistance of the other half. 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.
200µ, 200 µF electrolytic
The reservoir cap. 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.
Bias tap, 49 V ac, 60 Hz
The power transformer’s bias tap, 49 V ac, as a sine source at the mains frequency. Typical of the part.
tap R, 50 Ω
The bias tap’s own resistance, 50 Ω, typical. The drawing has no resistor between the tap and the diode.
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 25k, 25 kΩ pot, linear taper
Bias adjust, a 25k 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, 8.69 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 8 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, 256 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, 464 µH
With Loss R across it, the treble rises and then levels off, like the losses in a real coil.
Loss R, 14.3 Ω
Limits what Loss L adds to the treble rise.
Loss L2, 2.13 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, 2.19 Ω
Limits what Loss L2 adds.
Tank R, 83.7 Ω
Height of the bass resonance, about 92 Ω in all at its peak. The feedback holds the output steadier against that than it would be without feedback.
Tank L, 21.1 mH
With Tank C, sets the resonance: 74 Hz.
Tank C, 220 µF
With Tank L, sets the resonance. With Tank R, it sets how narrow the peak is.
4 Ω tap, 1 V
The 4 Ω tap the feedback comes from, assumed to be an ideal tap on the same winding: 0.707 times the speaker voltage. The load the feedback network puts on the winding is left out. It’s tens of kilohms on a winding of a few ohms.

About this version

12AX7 throughout
Eight 12AX7 triodes. 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. Caps are ideal, and the battery or HT is a voltage behind a resistance.

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