Cascaded high-gain amp, 1984 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 Mesa/Boogie Mark IIC+.

A first stage drives a treble-mid-bass stack straight from its plate, then a second stage. In lead mode three more triodes run in cascade before the shared stages and the master. Optical switches change channels. On the bench the Lead button does it.

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 tone stack comes right after the first stage, ahead of most of the gain, so the EQ shapes the signal before it distorts, and the bass and treble act differently from an amp with the stack at the end. In lead mode the signal goes through a 680 kΩ/1 MΩ drive network into V3B, a 270 kΩ/68 kΩ divider into V4A, then 220 kΩ with 250 pF into 100 kΩ with 500 pF. Each step throws away some level and some treble, so the next stage clips cleanly instead of fizzing. Probe the V3B and V4A plates with the lead drive up to see where the clipping piles up. Switch Lead off to hear the clean channel, which reaches V2B only through 3.3 MΩ.

Download.

Parts and what they do

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

R85, 5.6 kΩ
Dropping resistor to the D node.
C84, 30 µF electrolytic
D node filter cap.
R86, 1 kΩ
Dropping resistor from D to D+.
C87, 30 µF electrolytic
D+ node filter cap.
R1, 1 MΩ
Input grid leak. There is no grid stopper: the jack goes straight to the grid.
V1A, 12AX7 / ECC83 / 7025
First stage, with a 150 kΩ plate load. It drives the tone stack directly, with no cathode follower.
R4, 150 kΩ
V1A plate load.
R2, 1.5 kΩ
V1A cathode resistor.
C1, 470 nF
Small bypass cap. The stage has full gain only above about 230 Hz.
C2, 22 µF electrolytic
Big bypass cap, in series with R3 unless the bass shift is pulled.
R3, 15 kΩ
In series with C2, so it barely bypasses anything until the bass shift shorts it out.
C5, 250 pF
Treble cap.
R5, 100 kΩ
Slope resistor. Because the stack is driven from a 150 kΩ plate, the plate’s own impedance is part of the stack.
C4, 100 nF
Bass cap.
C3, 47 nF
Mid cap.
Treble, 250 kΩ pot, audio taper
Treble pot. Its wiper is the stack’s output.
Bass, 250 kΩ pot, audio taper
Bass pot, a variable resistor.
Middle, 10 kΩ pot, linear taper
Mid pot, a variable resistor to ground.
Volume 1, 1 MΩ pot, audio taper
Volume 1. In lead mode it sets how hard V1B and the whole lead cascade are driven.
V1B, 12AX7 / ECC83 / 7025
Second stage, fully bypassed. Shared by both channels.
R8, 100 kΩ
V1B plate load.
R7, 1.5 kΩ
V1B cathode resistor.
C13, 22 µF electrolytic
V1B bypass cap.
C7, 100 nF
V1B coupling cap, feeding both the clean path and the lead input.
R9, 91 kΩ
Load on V1B’s output. 91 kΩ on RP10A boards, 100 kΩ on RP11A.
C21, 20 nF
Coupling cap into the lead circuit.
LDR2A, 1 kΩ
Lead input switch: an LED-lit photoresistor, modeled as 1 kΩ when lit.
R21, 680 kΩ
Top of the lead drive divider (1 MΩ on the ++ mod).
Lead drive, 1 MΩ pot, audio taper
Lead drive. With R21 it sets how hard the lead cascade is hit.
R22, 475 kΩ
From V3B’s grid to ground, loading the drive wiper.
C22, 120 pF
Grid to cathode: a small high-frequency bypass of the stage’s input.
V3B, 12AX7 / ECC83 / 7025
First lead stage, 82 kΩ plate load.
R26, 82 kΩ
V3B plate load.
R23, 1.5 kΩ
V3B cathode resistor.
C23, 2.2 µF electrolytic
V3B bypass. The stage has full gain above about 50 Hz.
C25, 22 nF
Coupling cap from V3B.
R25, 270 kΩ
Top of the 270 kΩ/68 kΩ divider into V4A: about a fifth of the signal gets through.
R24, 68 kΩ
Bottom of the divider.
C24, 1 nF
With the divider, rolls off treble above about 3 kHz going into V4A.
V4A, 12AX7 / ECC83 / 7025
Second lead stage: 274 kΩ plate load, 3.3 kΩ cathode, only partly bypassed.
R27, 274 kΩ
V4A plate load.
C27, 1 nF
Across the plate load: rolls off V4A’s treble. Deleted on later Simul-class amps.
R30, 3.3 kΩ
V4A cathode resistor.
C29, 220 nF
Partial bypass through R29. The lead bright pull shorts R29.
R29, 6.8 kΩ
In series with C29 (15 kΩ also seen).
C30, 47 nF
Coupling cap from V4A.
R31, 220 kΩ
Top of the lead output divider.
C31, 250 pF
Treble bypass across R31.
R32, 100 kΩ
Bottom of the lead output divider.
C32, 500 pF
Treble cut at the divider output.
LDR3A, 1 kΩ
Lead output switch into V2B’s grid.
R10, 3.3 MΩ
The clean path into V2B: 3.3 MΩ against R11 lets through about a sixth of the signal.
C10, 10 pF
Treble bypass across R10 (20 pF on later amps).
R11, 680 kΩ
V2B grid resistor.
C11, 47 pF
Small treble cut at V2B’s grid.
V2B, 12AX7 / ECC83 / 7025
Shared stage after the channels meet. Its cathode isn’t bypassed.
R13, 100 kΩ
V2B plate load.
R16, 1.5 kΩ
V2B cathode resistor (bypassed with 15 µF on the ++ mod).
R105, 47 kΩ
Series resistor out of V2B.
C9, 47 nF
Coupling cap out of V2B.
Lead master, 250 kΩ pot, audio taper
Lead master: a variable resistor to ground, switched in by LDR4A in lead mode only.
LDR4A, 1 kΩ
Puts the lead master in circuit in lead mode.
R102, 150 kΩ
Top of the send divider. With R101 it drops the level about 33 times before V2A.
R101, 4.7 kΩ
Bottom of the send divider. The reverb recovery also mixes in here.
R12, 2.2 kΩ
Send resistor, a later factory addition. The return jack is normalled to the send, so it feeds V2A.
R103, 47 kΩ
V2A grid resistor.
V2A, 12AX7 / ECC83 / 7025
Recovery stage after the send divider: 120 kΩ plate load, 1 kΩ cathode.
R19, 120 kΩ
V2A plate load.
R104, 1 kΩ
V2A cathode resistor (680 Ω also seen).
C16, 470 nF
Small bypass, for treble gain in V2A.
C15, 22 µF electrolytic
Big bypass in series with R20 unless the pull deep switch is pulled.
R20, 68 kΩ
In series with C15 (15 kΩ also seen).
C12, 47 nF
Coupling cap out of V2A.
R106, 15 kΩ
Series resistor into the master, added to smooth the taper of the master pot.
Master, 1 MΩ pot, audio taper
Master volume. Its wiper feeds the graphic EQ, or the phase inverter with the EQ out.
.1µ, 100 nF
Coupling cap into the phase inverter. With the grid resistor it sets how low the inverter goes, around 1 Hz.
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.
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.
100k, 100 kΩ
Grid resistor for the driven half, returned to the bias point above the tail, not to ground.
150k, 150 kΩ
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.
22k, 22 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 bottom of the leg for signal, .1µF. The feedback arrives above it, at the top of the leg, so it moves the tail more than the second grid and acts across the pair.
82.5k, 82.5 kΩ
Plate resistor for the driven half. It’s smaller than the other so the two outputs come out about even.
90.9k, 90.9 kΩ
Plate resistor for the other half.
56k, 56 kΩ
The feedback resistor from the speaker. With the 1.5k Ω below it, it sends about 3 % of the speaker voltage back to the inverter, which lowers the gain, flattens the response and tightens the bass.
Presence, 250 kΩ pot, linear taper
Presence, a 250 kΩ pot wired as a variable resistor in series with the feedback resistor, its resistance rising as it’s turned up. Turned up, less is fed back overall, and the treble most of all, since the cap across the feedback resistor then counts for less of the path. The gain rises and the top end comes up most: from 0 to full, about 3 dB at 110 Hz, 4.5 dB at 1 kHz and 6 dB at 5 kHz. Taper assumed linear.
1.5k, 1.5 kΩ
In series with the Presence pot and the feedback resistor, 1.5k Ω. With the pot at 0 and the cap across the feedback resistor shorting it in the treble, this is nearly all that’s left, so it sets how much treble is fed back at most.
.0047µ, 4.7 nF
A .0047µF cap across the feedback resistor. Above about 605 Hz it bypasses the resistor, so more treble is fed back and the top end is held down, more so with the Presence low.
1.5k, 1.5 kΩ
The top of the feedback loop’s leg, 1.5k Ω from the feedback point down to the 3.3k Ω and the .047µF cap below it. The fed-back speaker voltage arrives at its top and moves the tail. The second grid is tied to its bottom, so it sees only part of the feedback.
3.3k, 3.3 kΩ
The bottom of the feedback loop’s leg, 3.3k Ω to ground. The inverter’s current runs through it and the two resistors above it.
.047µ, 47 nF
Across the bottom of the leg, .047µF. Above about 1026 Hz it shorts that resistor, so the second grid, tied here, is held at ground for the treble. Lower down the second grid rises with the feedback, so the pair rejects more of it and less bass is fed back.
.1µ, 100 nF
Coupling cap to tube A’s grid.
.1µ, 100 nF
Coupling cap to tube B’s grid.
1.1k, 1.1 kΩ
Grid stopper for tube A. It keeps the tube from oscillating and softens grid current. Each of the two tubes has its own 2.2k. The resistors are drawn as one part, the two in parallel.
1.1k, 1.1 kΩ
Grid stopper for tube B. Each of the two tubes has its own 2.2k. The resistors are drawn as one part, the two in parallel.
V9, 6L6GC
6L6GC, tube A of the pair, cathode grounded, biased from the fixed bias supply. V7 runs beside it on the same grid, plate and screen.
V8, 6L6GC
6L6GC, tube B of the pair. V9 runs beside it.
V6, 6L6GC
6L6GC, the second tube on side A, in parallel with V6 on 100 W. On 60 W the switch lifts its cathode off ground, so it stops conducting.
V7, 6L6GC
6L6GC, the second tube on side B, in parallel with V8 on 100 W. On 60 W the switch lifts its cathode off ground, so it stops conducting.
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.
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.
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+.
30µ, 30 µF electrolytic
Filter cap on the screen supply.
5.6k, 5.6 kΩ
Dropping resistor from the screen supply node to the node for the inverter and the preamp, as on the Mesa/Boogie Mark IIC+ 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.
30µ, 30 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, 348 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 348 V ac from the grounded center tap, as a sine source at the mains frequency. The value is typical of the part.
PT, 348 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.
110µ, 110 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, 46 V ac, 60 Hz
The power transformer’s bias tap, 46 V ac, as a sine source at the mains frequency. Typical of the part.
470, 470 Ω
Series resistor into the bias rectifier, 470 Ω. It limits the charging current.
Bias diode, 1N4001
The bias rectifier, a silicon diode turned so the supply comes out negative.
47µ, 47 µF electrolytic
Filter cap on the bias supply, its plus side to ground.
15.2k, 15.2 kΩ
The bias supply’s load, 15.2k Ω to ground. With the 470 Ω above it, it sets the bias. There’s no bias pot, so change this resistor to change the bias. Bigger runs the tubes colder.
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.

About this version

12AX7 throughout
Six 12AX7 triodes. Real tubes spread 20 % or more, and the first two tube positions change this amp more than most.

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.

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