Bass amp with sweep and an active tone stage, 1981 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 JCM800 1992 Super Bass (100 W).

Two triode stages with a mid sweep between them, a cathode follower, an active bass and treble stage, and a three-section high-pass Slope control into the phase inverter.

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. A bass amp with more tone shaping than Marshall’s guitar amps. After the first triode the signal splits: treble goes to the volume through 100 pF, the rest through 220 kΩ, the Sweep pot and 470 kΩ. The Mid pot and a small cap to ground hang off the middle of that path, so Mid cuts a band whose center the Sweep pot moves. The second triode has 10 kΩ on its cathode and a gain near 8, then a cathode follower drives the tone stage. Bass and Treble are a Baxandall built round V2b, with its plate feeding back into the network, so each knob boosts above its middle and cuts below. In the model, with both at the middle the stage loses about 2 dB at 1 kHz and 9 dB at 100 Hz, since the two bass caps read as different values. Last, three .022 µF caps in a row with 100 kΩ shunts to the Slope pot set how steeply the low end rolls off. The power amp is the 2203’s with 27 kΩ of feedback from the 16 Ω tap and no presence control.

Download.

Parts and what they do

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

R2, 68 kΩ
The input 1 resistor into the grid. With nothing in input 2 it sits in parallel with input 2’s resistor, so a guitar in input 1 sees 34 kΩ to the grid: enough to stop radio pickup, not enough to cut treble.
R1, 68 kΩ
The input 2 resistor into the grid. With nothing in input 1, input 1’s tip is grounded, so the two 68 kΩ make a divider: input 2 is about 6 dB quieter and loads the guitar with 136 kΩ.
RIN, 1 MΩ
Input 1’s 1 MΩ to ground. It sets the load a guitar in input 1 sees, and with a patch lead in input 1 it is the end of the lead’s path.
V1a, 12AX7 / ECC83 / 7025
First gain stage, half an ECC83. Its cathode isn’t bypassed, so the 820 Ω lowers the gain and keeps it clean.
R5, 100 kΩ
V1a’s plate resistor.
R3, 820 Ω
V1a’s cathode resistor, 820 Ω, not bypassed.
C1, 100 nF
Coupling cap off V1a’s plate into the sweep network, .1 µF.
C2, 100 pF
The treble path to the volume, 100 pF straight across. Everything below the top end goes through the resistor path through the sweep network.
R6, 220 kΩ
From C1 into the Sweep pot, 220 kΩ.
Sweep, 1 MΩ pot, linear taper
Sweep, 1 MΩ linear, a variable resistor (one end free on the drawing). It sets how much resistance sits ahead of the mid cap, so it moves the frequency the mid control works at. Which way it turns isn’t marked, so up is assumed to be less resistance.
R7, 470 kΩ
From the volume’s top to the sweep node, 470 kΩ.
Mid, 1 MΩ pot, linear taper
Mid, 1 MΩ linear, a variable resistor from the sweep node to the mid cap. Turned down it lets the cap shunt the sweep node to ground, which cuts the mids. Up puts more resistance in the way, for more mids.
CMID, 2 nF
The mid cap to ground under the Mid pot. Its value is smudged on the drawing and read as .002 µF.
Volume, 1 MΩ pot, audio taper
Volume, 1 MΩ log, ahead of V1b. Treble goes through the 100 pF and the rest through the sweep network.
V1b, 12AX7 / ECC83 / 7025
Second gain stage. 10 kΩ on its cathode with no bypass holds its gain to about 8, so it stays clean on a bass.
R8, 100 kΩ
V1b’s plate resistor.
R4, 10 kΩ
V1b’s cathode resistor, 10 kΩ, not bypassed.
C3, 22 nF
Coupling cap into the cathode follower, .022 µF.
V2a, 12AX7 / ECC83 / 7025
Cathode follower. It drives the tone stage from a low impedance.
R9, 1 MΩ
The follower’s grid resistor, 1 MΩ to the point between its two cathode resistors. That point follows the signal, so the 1 MΩ looks far bigger to C3.
R11, 1 kΩ
The follower’s bias resistor, 1 kΩ, between its cathode and the grid resistor’s return.
R10, 100 kΩ
The follower’s load, 100 kΩ to ground.
C4, 100 nF
Coupling cap from the follower into the tone network, .1 µF.
R13, 33 kΩ
Input resistor of the bass section, 33 kΩ.
Bass, 1 MΩ pot, linear taper
Bass, 1 MΩ linear, between the two 33 kΩ. Up moves the wiper to the input side, so more bass reaches V2b’s grid and less comes back from its plate: boost. Down does the opposite: cut.
R17, 33 kΩ
Feedback resistor of the bass section, 33 kΩ, from V2b’s plate side.
C9, 4.7 nF
Across the input half of the bass pot. Marked .004 µF on the drawing, assumed to be .0047 µF. Above the bass it shorts that half out, so the pot only acts on low notes.
C10, 22 nF
Across the plate half of the bass pot, read as .022 µF.
R16, 47 kΩ
From the bass wiper to the middle of the network, 47 kΩ. It keeps the bass section from loading the treble section.
Treble, 250 kΩ pot, linear taper
Treble, 250 kΩ linear, straight across from the input to V2b’s plate side. Up moves its wiper to the input side, for treble boost.
C6, 1 nF
Treble cap from the treble wiper to the middle of the network, read as .001 µF. Only treble gets through it, so the treble pot only acts on high notes.
C7, 22 nF
Coupling cap from the network’s middle into V2b’s grid, read as .022 µF.
R12, 1 MΩ
V2b’s grid resistor, 1 MΩ.
V2b, 12AX7 / ECC83 / 7025
The tone stage. Its plate feeds back into the network through C8, so with both knobs at the middle the network’s two sides balance and the response is close to flat. Turning a knob unbalances them for boost or cut.
R14, 100 kΩ
V2b’s plate resistor, 100 kΩ.
R15, 1.5 kΩ
V2b’s cathode resistor, read as 1.5 kΩ (the last figure is unclear).
C5, 25 µF electrolytic
V2b’s cathode bypass, 25 µF.
C8, 100 nF
Feedback cap from V2b’s plate to the network’s plate side, .1 µF.
C11, 22 nF
First cap of the Slope high-pass, read as .022 µF.
R18, 100 kΩ
First shunt of the Slope high-pass, 100 kΩ to the Slope pot.
C12, 22 nF
Second cap of the high-pass.
R19, 100 kΩ
Second shunt, 100 kΩ.
C13, 22 nF
Third cap of the high-pass.
R20, 100 kΩ
Third shunt, 100 kΩ.
C14, 22 nF
Last cap of the high-pass, into the inverter’s grid.
Slope, 1 MΩ pot, linear taper
Slope, 1 MΩ linear, a variable resistor from the three shunts’ common end to ground. At zero the shunts go straight to ground and the three sections cut the low end steeply. Turned up, the pot sits in series with all three, so less is cut.
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.
220p, 220 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.
27k, 27 kΩ
The feedback resistor from the speaker. With the 4.7k Ω below it, it sends about 15 % 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.
.022µ, 22 nF
Coupling cap to tube A’s grid.
.022µ, 22 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.
V4, V6, EL34 / 6CA7
EL34, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply. Two tubes in parallel, drawn as one.
V5, V7, EL34 / 6CA7
EL34, 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, returned to its own bias pot.
500, 500 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 1k. The resistors are drawn as one part, the two in parallel.
500, 500 Ω
Screen resistor for tube B. Each of the two tubes has its own 1k. The resistors are drawn as one part, the two in parallel.
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 and the preamp, as on the Marshall JCM800 1992 Super Bass 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.
100µ, 100 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, not measured.
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, not measured.
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, not measured.
PT R, 40 Ω
The same winding’s resistance, on the other half cycle. 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.
60µ, 60 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, 96 V ac, 60 Hz
The power transformer’s bias tap, 96 V ac, as a sine source at the mains frequency. Typical of the part, not measured.
22k, 22 kΩ
Series resistor into the bias rectifier, 22k Ω. It limits the charging current.
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. With the cap after it, it filters the ripple a second time.
10µ, 10 µF electrolytic
Second filter cap, its plus side to ground. The bias pot hangs from here.
Bias A, 22 kΩ pot, linear taper
Bias adjust for tube A, 22k Ω linear, from the second cap down to the resistor below it. Its wiper feeds tube A’s grid through its grid leak. Toward the cap end the grid goes more negative and the tube idles cooler.
Bias B, 22 kΩ pot, linear taper
Bias adjust for tube B, the same, beside the first. With one for each tube, the two can be set to idle alike.
47k, 47 kΩ
The bottom of the bias divider, 47k Ω to ground, under both pots. It sets the least bias the pots can set.
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.
16 Ω tap, 2 V
The 16 Ω tap the feedback comes from, assumed to be an ideal tap on the same winding: 2 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

As drawn, 1981
ECC83 (12AX7) tubes, as drawn. The tube models are typical of the type, 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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