Tube amp with the tone stack after the first 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 Trainwreck Express.

One gain stage drives the treble, middle and bass stack straight off its plate. The volume after the stack sets how hard the next two stages clip. A .002 µF cap between them cuts the bass ahead of the last stage.

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 sits right after the first triode, ahead of the volume and the two stages that clip. So Treble, Middle and Bass set what drives the clipping, not only what comes out of it: turn up the bass and the low strings break up sooner. V1b drives the stack straight off its plate with no coupling cap. After the volume, V1a has a gain of about 47 and is the first stage to clip. The .002 µF cap into V2a cuts below about 530 Hz, so the last stage gets less low end and its clipping stays tight. V2a runs cold on an unbypassed 10 kΩ, a gain of about 7.5. There’s no master, so the volume and the power amp do the rest.

Download.

Parts and what they do

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

20µ, 20 µF electrolytic
Filter cap on B+4, the node V2a runs from.
9100, 9.1 kΩ
Dropping resistor from B+4 to B+5, 9.1 kΩ 2 W. V1’s two halves pull about 1.6 mA through it, so B+5 sits about 15 V lower.
20µ, 20 µF electrolytic
Filter cap on B+5, V1’s node. With the dropping resistor it keeps the first two stages from coupling to the later ones through the supply.
1M, 1 MΩ
The input’s 1 MΩ to ground. It’s V1b’s grid leak and the load the guitar sees. There’s no grid stopper on the drawing.
V1b, 12AX7 / ECC83 / 7025
First gain stage, 100 kΩ plate and a bypassed 1.5 kΩ cathode. Its plate drives the tone stack directly, and the stack’s load lowers its gain from about 63 to about 46.
100k, 100 kΩ
V1b plate load.
1.5k, 1.5 kΩ
V1b cathode resistor. 1.5 kΩ runs the stage warm, about 1.4 V of bias and 0.94 mA.
22µ, 22 µF electrolytic
V1b cathode bypass. With 22 µF against 1.5 kΩ the stage has full gain down to about 5 Hz.
500p, 500 pF
Treble cap, 500 pF, from V1b’s plate to the top of the treble pot. With the 1 MΩ pot it lets more of the upper mids through than a Marshall’s 470 pF into 220 kΩ.
100k, 100 kΩ
Slope resistor, 100 kΩ. Bigger than a Marshall’s 33 kΩ, so the stack drops more of the bass and mids and leaves more scoop.
.022µ, 22 nF
Bass cap, .022 µF.
.022µ, 22 nF
Mid cap, .022 µF, to the bass pot’s bottom and the mid pot.
Treble, 1 MΩ pot, audio taper
Treble pot, 1 MΩ audio. Its wiper is the tone stack’s output.
Bass, 1 MΩ pot, audio taper
Bass pot, 1 MΩ audio, as a variable resistor from the treble pot’s bottom to the mid pot. Its top lug isn’t connected.
Middle, 25 kΩ pot, linear taper
Mid pot, 25 kΩ linear, as a variable resistor from the bass pot’s bottom to ground, its top lug not connected. Turned down, it shorts the bottom of the stack to ground and the mids drop out.
Volume, 1 MΩ pot, audio taper
Volume, 1 MΩ audio, after the tone stack. There’s no master, so it sets how hard V1a, V2a and the power amp are pushed.
V1a, 12AX7 / ECC83 / 7025
Second gain stage, 100 kΩ plate and a bypassed 2.7 kΩ cathode, a gain of about 47 at 1 kHz. With the volume up, it’s the first stage to clip.
100k, 100 kΩ
V1a plate load.
2.7k, 2.7 kΩ
V1a cathode resistor. 2.7 kΩ biases it slightly colder than V1b, about 1.8 V and 0.68 mA.
22µ, 22 µF electrolytic
V1a cathode bypass, full gain down to a few hertz.
.002µ, 2 nF
Coupling cap, .002 µF, into V2a. With the 150 kΩ below it, it rolls off below about 530 Hz, so the low end reaches the last stage about 11 dB down at 160 Hz and its clipping stays tight.
150k, 150 kΩ
V2a grid resistor, 150 kΩ to ground. Some builders added an 820 Ω grid stopper at V2a. It isn’t on the drawing, so it isn’t modeled.
V2a, 12AX7 / ECC83 / 7025
Third stage, with 10 kΩ under its cathode and no bypass cap. It idles cold, about 0.28 mA, and the unbypassed resistor holds its gain near 7.5. Driven hard, it cuts off on the negative peaks.
100k, 100 kΩ
V2a plate load, from B+4.
10k, 10 kΩ
V2a cathode resistor, 10 kΩ, not bypassed. It sets the cold bias and the low gain.
.1µ, 100 nF
Coupling cap from V2a’s plate to the inverter’s grid.
V3b, 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.
V3a, 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.
50p, 50 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, 100 kΩ from the 8 Ω tap. With the presence pot it sends about 5 % of the speaker voltage back to the inverter, which lowers the gain, flattens the response and tightens the bass.
Presence, 5 kΩ pot, linear taper
Presence, 5 kΩ linear, the bottom of the feedback loop. Its wiper goes to ground through the cap. Turned up, the cap shunts more of the fed-back treble to ground, so less treble is fed back and the top end rises, a few dB around 5 kHz.
.1µ, 100 nF
The presence cap. With the pot it sets where the lift starts.
.022µ, 22 nF
Coupling cap to tube A’s grid.
.022µ, 22 nF
Coupling cap to tube B’s grid.
1.5k, 1.5 kΩ
Grid stopper for tube A. It keeps the tube from oscillating and softens grid current.
1.5k, 1.5 kΩ
Grid stopper for tube B.
V6, EL34 / 6CA7
EL34, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply.
V7, 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.
1k, 1 kΩ
Screen resistor for tube A. It limits the screen current when the tube is driven hard.
1k, 1 kΩ
Screen resistor for tube B.
1k, 1 kΩ
Dropping resistor from B+ to the screen supply. The screens sit slightly below the plates.
40µ, 40 µF electrolytic
Filter cap on the screen supply.
18.2k, 18.2 kΩ
Dropping resistor from the screen supply node to the node for the inverter, as on the Trainwreck Express 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.
20µ, 20 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.
9.1k, 9.1 kΩ
Dropping resistor from the node for the inverter to the node for the preamp, as on the Trainwreck Express 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.
200k, 200 kΩ
The bleeder, two 100 kΩ 3 W in series, which drains the filter caps after the amp is switched off. On the drawing it sits across the reservoir caps ahead of the standby switch. Here it hangs on the screen node, which puts its 2 mA through the 1 kΩ and moves that node about 2 V.
OT, 6.6 kΩ to 8 Ω, 22.5 H primary, 8.5 mH leakage
The output transformer, 6.6 kΩ plate to plate into 8 Ω, as the amp had, center tap on B+. Tap any piece of it to change its figures. Measured on a real one: 79 Ω across the primary, 0.29 Ω on the secondary, 22.5 H, 8.5 mH of leakage and 1.17 nF across the winding. A real core’s inductance falls as the frequency rises, from about 25 H at 100 Hz to about 19 H at 1 kHz. Here it’s one figure, fitted across the band. The core saturates as the flux nears its knee. That part is typical, not measured.
PT, 300 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 300 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, 300 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, 28 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, 28 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.
40µ, 40 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.
40µ, 40 µF electrolytic
Another reservoir cap, in parallel with the first.
PT, 300 V ac, 60 Hz
The end of the high-voltage winding the bias comes off, 300 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.
47k, 47 kΩ
From the bias line toward ground, 47k 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. On the drawing the 47 kΩ goes into this pot’s wiper and the Bias line comes off the pot’s far end. Wired that way, the supply can’t reach the printed −33 V, so here it’s wired the way Marshall’s 1987 wires the same parts, with the Bias line at the second cap. It starts near the middle, at −30 V, where real ones idle at about 49 mA a tube. Near the top of its travel the line reads the drawing’s −33 V.
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
Two 12AX7s, as on the drawing. The tube models are typical of the type, not measured. Real tubes spread 20 % or more in current at the same bias.

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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