Single-channel top-boost amp, 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 Rocket.

An AC30-style brilliant channel on its own: one triode, a small coupling cap into the volume, a second triode and a cathode follower driving treble and bass controls, then the AC30’s Cut on the power amp. Four EL84s, no master and no negative feedback.

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. One triode, then 500 pF into the 500 kΩ volume, which cuts most of the bass before the second stage. That stage and the cathode follower make up the level and do most of the breakup. The follower drives the AC30’s two-knob stack: 50 pF to the treble pot, 100 kΩ, and .022 µF with 10 kΩ on the bass side. A 220 kΩ in series with the treble pot’s wiper and the inverter’s 1 MΩ make a divider, so the inverter gets about four fifths of the stack’s output. With no master, the volume and the stack set how hard the inverter and the four EL84s get pushed, and Cut rolls off highs between the inverter and the output tubes.

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 V1a and the follower run from.
9.1k, 9.1 kΩ
Dropping resistor from B+4 to B+5, 9.1 kΩ 2 W.
20µ, 20 µF electrolytic
Filter cap on B+5, V1b’s supply.
1M, 1 MΩ
Input resistor to ground at the jack.
33k, 33 kΩ
Grid stopper into the first stage, 33 kΩ. With the tube’s input capacitance it keeps radio and fizz out.
V1b, 12AX7 / ECC83 / 7025
First stage, a common-cathode triode with a 220 kΩ plate load and a bypassed cathode.
220k, 220 kΩ
First stage plate resistor, from B+5.
1.5k, 1.5 kΩ
First stage cathode resistor. The drawing’s note: 1.5 kΩ to 4.7 kΩ.
22µ, 22 µF electrolytic
First stage bypass cap, for full gain down to about 30 Hz.
500p, 500 pF
Coupling cap into the volume, 500 pF. Into 500 kΩ it’s a high-pass near 640 Hz, so most of the bass is gone before the second stage. The drawing’s note: 500 pF to 4.7 nF.
Volume, 500 kΩ pot, audio taper
Volume, 500 kΩ audio. It sits ahead of the second stage, so it sets how hard that stage and the follower get driven.
100p, 100 pF
Bright cap across the top of the volume, for more treble with the volume below full.
V1a, 12AX7 / ECC83 / 7025
Second stage, a 100 kΩ plate load and an unbypassed cathode. It and the follower clip first.
100k, 100 kΩ
Second stage plate resistor, from B+4. It also sets the follower’s grid voltage, since the two are wired directly.
1.5k, 1.5 kΩ
Second stage cathode resistor, no bypass cap. The drawing shows a 22 µF across it dashed, as an option for more gain.
V2a, 12AX7 / ECC83 / 7025
Cathode follower, wired straight to the second stage’s plate, its plate on B+4. It drives the tone stack from a low impedance.
56k, 56 kΩ
Cathode follower load, 56 kΩ as on the AC30. The drawing’s note: up to 68 kΩ if 56 kΩ is too much for the preamp tubes.
50p, 50 pF
Treble cap, 50 pF, as on the AC30.
100k, 100 kΩ
Slope resistor.
.022µ, 22 nF
Bass cap, to the treble pot’s bottom and the bass pot’s top.
.022µ, 22 nF
Mid cap, .022 µF, to the bass pot’s wiper and the 10 kΩ.
Treble, 1 MΩ pot, audio taper
Treble pot, 1 MΩ audio. Its wiper is the stack’s output.
Bass, 1 MΩ pot, audio taper
Bass pot, 1 MΩ audio. Top on the treble pot’s bottom, wiper on the mid cap and the 10 kΩ, far end on ground. Turning it up puts more track between the treble pot and the wiper, for more bass.
10k, 10 kΩ
From the bass pot’s wiper to ground, 10 kΩ, as on the AC30.
220k, 220 kΩ
In series from the treble pot’s wiper to the inverter. With the inverter’s 1 MΩ grid resistor it passes about four fifths of the signal, and it keeps the stack from being loaded by the inverter.
.047µ, 47 nF
Coupling cap into the phase inverter.
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.
1.2k, 1.2 kΩ
Bias resistor for both halves. The cathodes sit this far above the grids’ return, a volt or so.
47k, 47 kΩ
The tail resistor, to ground. It’s large, so the pair shares a nearly fixed current. When one half conducts more, the other conducts less, which splits the phase.
.047µ, 47 nF
Ties the second grid to ground for signal, so that half is driven only through the shared cathodes. There’s no feedback loop in this style.
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.
.047µ, 47 nF
Coupling cap to tube A’s grid.
.047µ, 47 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.
Cut, 250 kΩ pot, audio taper
Cut, a 250k pot in series with the .0047µF cap, the pair between the two output grids. It works like the master across the grids, but the cap lets only the treble through, so turning it up cancels treble. The resistance falls as it’s turned up. Turned down, the pot’s resistance leaves a small treble shelf.
.0047µ, 4.7 nF
The Cut cap. The smaller it is, the higher the treble it cancels.
50, 50 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 100. The resistors are drawn as one part, the two in parallel.
50, 50 Ω
Screen resistor for tube B. Each of the two tubes has its own 100. The resistors are drawn as one part, the two in parallel.
50, 50 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. 50 Ω 25 W under all four cathodes, as on the AC30.
220µ, 220 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.
15 H, 15 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.
411, 411 Ω
The choke’s winding resistance, typical, not measured. It’s all the DC drop the screens see from B+.
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 Rocket 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 Rocket 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.
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. 4.3 kΩ plate to plate. Some of these amps used 6.6 kΩ.
PT, 250 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 250 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, 250 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, 18.5 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, 18.5 Ω
Resistance of the other half. Typical of the part, not measured.
V8, GZ34
One plate of the GZ34 rectifier. It conducts near the peak of its half of the winding, and its own drop rises with the current, which adds to the sag.
V8, GZ34
The rectifier’s other plate, 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. With no feedback, the voltage at the speaker rises and falls with its impedance.
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. With no feedback, the voltage at the speaker rises and falls with its impedance.
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 (2008)
The values on the 2008 drawing. Three 12AX7 triodes, typical of the type, not measured. Real tubes spread 20 % or more.

Sources

  • Mark Abbott, original drawing of the Rocket circuit, from notes on cloned amps
  • Matt Taylor, redrawn Rocket schematic, Version A0 (2004, updated 2008)

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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