Stack-first tube amp with a small coupling cap, 1989 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 Liverpool.

Three 12AX7 stages with the tone stack straight off the first plate and a 1 M volume after it. A .0022 µF cap into 68 kΩ cuts the bass ahead of the third stage, which drives the phase inverter. No master volume.

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 first triode drives the tone stack straight from its plate, with no coupling cap. The stack’s own caps keep the plate’s DC off the pots. The volume sits after the stack, ahead of V1b, so it sets how hard the last two stages and the power amp get pushed. The bright switch puts 100 pF or 500 pF across the volume, which adds treble with the volume below full. Between V1b and V2, .0022 µF into 68 kΩ cuts the bass: from V1b’s grid to V2’s, the response is 3 dB down near 700 Hz and about 18 dB down at the low E. Mark Abbott calls these two parts where the amp is tuned. V2 has 10 kΩ on its cathode with no bypass, so it adds a gain of only about 7.5. The tube models are typical of the type, not measured.

Download.

Parts and what they do

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

1M, 1 MΩ
Input resistor, 1 MΩ from the jack’s tip to ground. It’s V1a’s grid leak. There’s no grid stopper on the drawing.
9.1k, 9.1 kΩ
Dropping resistor from B+4 to B+5, the last step of the supply chain. Mark Abbott measured 256 V and 244 V idle on either side.
20µ, 20 µF electrolytic
Filter cap on B+5, which feeds both halves of V1.
V1a, 12AX7 / ECC83 / 7025
Input stage. Its plate drives the tone stack directly, with no coupling cap.
100k, 100 kΩ
V1a plate resistor. With the tube’s plate resistance it’s also what the tone stack is driven from.
1.5k, 1.5 kΩ
V1a cathode resistor. Its drop is the stage’s bias.
22µ, 22 µF electrolytic
V1a cathode bypass, for full gain down through the bass.
50p, 50 pF
Treble cap, 50 pF, from V1a’s plate to the top of the treble pot. It carries V1a’s plate voltage on one side.
100k, 100 kΩ
Slope resistor, 100 kΩ, from V1a’s plate to the bass and mid caps. With the treble cap it sets how deep the mid scoop goes.
.022µ, 22 nF
Bass cap, .022 µF, to the bottom of the treble pot and the top of the bass pot.
.022µ, 22 nF
Mid cap, .022 µF, to the bottom of the bass pot and the top of the middle pot.
Treble, 1 MΩ pot, audio taper
Treble, 1 MΩ. Its wiper is the stack’s output and feeds the volume pot. The drawing marks no taper, so audio is assumed.
Bass, 250 kΩ pot, linear taper
Bass, 250 kΩ linear, a variable resistor between the treble pot’s bottom and the mid cap. Turned up, it adds resistance there and lets more bass through.
Middle, 25 kΩ pot, linear taper
Middle, 25 kΩ, a variable resistor from the mid cap to ground. Turned down, it shorts the bottom of the stack and the mids drop. The drawing marks no taper, so linear is assumed.
Volume, 1 MΩ pot, audio taper
Volume, 1 MΩ, after the tone stack and ahead of V1b. It sets how hard V1b, V2 and the power amp get driven. The drawing marks no taper, so audio is assumed.
100p, 100 pF
Bright cap, 100 pF, from the volume’s top to the bright switch. With the switch on 100 pF it bypasses the volume for the top end.
500p, 500 pF
Bright cap, 500 pF, the switch’s other position. It lifts from lower in the mids than the 100 pF.
2.2k, 2.2 kΩ
Grid stopper on V1b, mounted at the tube socket. Mark Abbott saw 820 Ω to 2.2 kΩ and notes it can be left out. 2.2 kΩ is the one he saw.
V1b, 12AX7 / ECC83 / 7025
Second gain stage. Its grid returns to ground through the volume pot.
100k, 100 kΩ
V1b plate resistor.
2.7k, 2.7 kΩ
V1b cathode resistor, 2.7 kΩ. It runs V1b slightly cooler than V1a, so it clips sooner on the bottom of the wave.
22µ, 22 µF electrolytic
V1b cathode bypass.
C8, 2.2 nF
Coupling cap into V2, .0022 µF. With R7 and V1b’s own output resistance it’s a high-pass near 700 Hz. Mark Abbott says the amps run .001 to .01 µF here and recommends .002 to .0047 µF. Both amps he saw had .001 µF ceramic plus .001 µF film in parallel.
R7, 68 kΩ
V2’s grid resistor, 68 kΩ. With C8 it sets the high-pass. Mark Abbott says values from 33 kΩ up are found, and recommends 56 kΩ or 68 kΩ, preferring 68 kΩ.
V2, 12AX7 / ECC83 / 7025
Third gain stage, with no cathode bypass. Its plate feeds the phase inverter.
100k, 100 kΩ
V2 plate resistor, from B+4.
10k, 10 kΩ
V2 cathode resistor, 10 kΩ, not bypassed. It biases V2 cold, about 2.5 V, and its feedback holds the stage’s gain to about 7.5.
.1µ, 100 nF
The phase inverter’s input cap, .1 µF, from V2’s plate. It’s on the output stage page of the drawing.
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.
1k, 1 kΩ
Bias resistor for both halves. The cathodes sit this far above the grids’ return, a volt or so.
39k, 39 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.
100k, 100 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.
100p, 100 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, 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.
.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, EL84 / 6BQ5
EL84, tube A of the push-pull pair, its cathode on the shared cathode resistor, so it biases itself. Two tubes in parallel, drawn as one.
V5, V7, EL84 / 6BQ5
EL84, tube B of the pair. Two tubes in parallel, drawn as one.
220k, 220 kΩ
Grid leak for tube A, to ground. The bias comes from the cathode resistor. 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.
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.
65, 65 Ω
The shared cathode resistor. Every output tube’s current runs through it and lifts the cathodes above the grids, and that’s the bias. Driven hard, the pair draws more current on average, so the cathodes climb and the tubes run colder, which softens the clipping.
440µ, 440 µF electrolytic
Bypass cap across the cathode resistor, so the cathodes hold still at the signal’s frequencies. It charges and discharges slowly as the average current changes on a hard note.
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 Liverpool 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 Liverpool 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.
20µ, 20 µF electrolytic
Filter cap at the node for the preamp. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
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 1.6 mA through the 1 kΩ and moves that node under 2 V.
OT, 4.3 kΩ to 8 Ω, 20 H primary, 5 mH leakage
The output transformer, 4.3 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, not measured. Liverpools used about 4.3 kΩ plate to plate, some the Express’s 6.6 kΩ. The core figures are typical, not measured.
PT, 264 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 263.5 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, 264 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, 40 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, 40 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.
80µ, 80 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.
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

As drawn
Three 12AX7 sections, the values from Mark Abbott’s drawing with his picks for Rgs, R7 and C8. The tube models are typical of the type, not measured.

Sources

  • Mark Abbott, hand-drawn Liverpool preamp, output stage and power supply

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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