Top-boost tube amp, 1963 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 Vox AC30/6 Top Boost.

Two channels mixed at the phase inverter. The brilliant channel adds a second gain stage and a cathode follower driving treble and bass controls. The normal channel is one triode straight to its volume. No master volume and no negative feedback, so the chime and the breakup come from the stages themselves.

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 guitar plugs into the brilliant channel. A 500 pF coupling cap after the first triode throws away most of the bass, and that’s where the chime starts. The top-boost stage and cathode follower make up the level. The two-knob stack is a Bassman stack with a fixed 10 kΩ for the middle pot, so the mids are always scooped slightly. The bass pot’s wiper sits on that 10 kΩ, its far end on ground. All the way up, it shorts the 10 kΩ, and the mids near 1 kHz come out about 10 dB lower than at 9. Both volumes meet the phase inverter through 220 kΩ, so the normal channel’s volume loads the brilliant channel even with nothing plugged in. Pick a Jumpered choice in the Input menu to hear both together.

Download.

Parts and what they do

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

R2, 68 kΩ
The brilliant channel’s 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.
R2B, 68 kΩ
The brilliant channel’s 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.
R1, 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.
R3, 68 kΩ
The normal channel’s 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.
R3B, 68 kΩ
The normal channel’s 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.
RN, 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.
RDN, 22 kΩ
Dropping resistor from the screen node to V1, which runs both input stages. Each branch of the AC30’s supply fans out from the screen node on its own resistor.
CDN, 8 µF electrolytic
Filter cap on V1’s supply.
RDT, 10 kΩ
Dropping resistor from the screen node to the top-boost tube. It’s smaller than V1’s, since the cathode follower alone pulls about 3 mA.
CDT, 32 µF electrolytic
Filter cap on the top-boost tube’s supply.
V1b, 12AX7 / ECC83 / 7025
Brilliant channel input stage, a common-cathode triode with a 220 kΩ plate load. Gain about 75 before the volume control.
RP2, 220 kΩ
Brilliant stage plate load.
V1a, 12AX7 / ECC83 / 7025
Normal channel input stage, the other half of the same tube.
RP1, 220 kΩ
Normal stage plate load.
RK1, 1.5 kΩ
Cathode resistor shared by both V1 triodes. It carries two triodes’ current, so it biases each one as 3 kΩ would, slightly cool.
CK1, 25 µF electrolytic
Shared cathode bypass cap, for full gain down into the bass. The cathode is shared, so a big signal in one channel moves the other channel’s bias slightly.
C3, 500 pF
Brilliant channel coupling cap. 500 pF into the 500 kΩ volume is a high-pass near 600 Hz, about 17 dB down at the low E. Most of the top-boost brightness comes from this cap, not the tone controls.
Brilliant vol., 500 kΩ pot, audio taper
Brilliant channel volume, ahead of the top-boost stage, so it sets how hard that stage and the follower are driven.
C4, 100 pF
Bright cap across the top of the volume, for more treble with the volume below full.
V2a, 12AX7 / ECC83 / 7025
Top-boost gain stage, a 100 kΩ plate load and an unbypassed cathode. Gain about 30, and the cathode feedback keeps it cleaner than a bypassed stage.
RP3, 100 kΩ
Top-boost plate load. It also sets the follower’s grid voltage, since the two are wired directly.
RK3, 1.5 kΩ
Top-boost cathode resistor.
V2b, 12AX7 / ECC83 / 7025
Cathode follower, DC-coupled from the top-boost plate. With only 56 kΩ below it (Marshall used 100 kΩ), it runs about 3 mA with its grid right at its cathode voltage, so it starts drawing grid current with almost no signal. That makes a soft, lopsided squash that’s part of the top-boost sound. In the model the grid idles about 0.6 V positive and the grid current cuts the top-boost stage’s gain from about 31 to 23. A measured AC30 reissue shows the grid a few tenths below the cathode, so the effect comes out too strong in the model.
RK4, 56 kΩ
Cathode follower load.
CT, 50 pF
Treble cap. 50 pF in early amps, 47 pF later.
R15, 100 kΩ
Slope resistor.
C7, 22 nF
Bass cap.
C8, 22 nF
Mid cap, to the bass pot’s wiper and the fixed 10 kΩ.
Treble, 1 MΩ pot, audio taper
Treble pot. Its wiper is the channel’s output.
Bass, 1 MΩ pot, audio taper
Bass pot. 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. The rest of the track sits across the 10 kΩ. That barely matters up to about 9, but at full it shorts the 10 kΩ and the mids near 1 kHz drop about 10 dB.
R16, 10 kΩ
Fixed resistor where a Bassman has its middle pot, from the bass pot’s wiper to ground. 10 kΩ is about a Fender middle control at 4. The bass pot’s lower track sits across it, which barely matters until the bass is near full.
R20, 220 kΩ
Mixing resistor from the brilliant channel.
C2, 47 nF
Normal channel coupling cap. It passes the full bass.
Normal vol., 500 kΩ pot, audio taper
Normal channel volume. Even unused, it and R19 load the mixing node: turning it down pulls some of the brilliant channel to ground.
R19, 220 kΩ
Mixing resistor from the normal channel.
C9, 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.
.15µ, 150 nF
Coupling cap to tube A’s grid. The drawings show .15 µF. AC30s also turn up with .047 and .1 µF, modern Vox amps use .1 µF, and some treble versions are said to use .01 µF. Smaller caps thin the low end into the output tubes.
.15µ, 150 nF
Coupling cap to tube B’s grid. The drawings show .15 µF. AC30s also turn up with .047 and .1 µF, modern Vox amps use .1 µF, and some treble versions are said to use .01 µF. Smaller caps thin the low end into the output tubes.
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.
250µ, 250 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.
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.
150, 150 Ω
The choke’s winding resistance, typical. It’s all the DC drop the screens see from B+.
16µ, 16 µF electrolytic
Filter cap on the screen supply.
22k, 22 kΩ
Dropping resistor from the screen supply node to the node for the inverter, as on the Vox AC30/6 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.
8µ, 8 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.
15k, 15 kΩ
Dropping resistor from the screen supply node to the node for the vibrato channel, as on the Vox AC30/6 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.
32µ, 32 µF electrolytic
Filter cap at the node for the vibrato channel. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
Vibrato channel, 47 kΩ
Stands for the vibrato channel and its oscillator on the node for the vibrato channel. It pulls about the current worked out from the voltages on the drawing. That part of the amp isn’t modeled here. A typical value.
Modulator, 68 kΩ
Stands for the vibrato’s modulator on the screen supply node. It pulls about the current worked out from the voltages on the drawing. That part of the amp isn’t modeled here. A typical value.
Jumper, 50 mΩ
Joins the preamp’s supply to the screen supply node. This preamp carries its own dropping resistors and filter caps for the normal and brilliant channels and the top boost, as on the Vox AC30/6, so they hang from here and those branches are left out of the chain. It’s drawn as plain wire, and the preamp’s rail starts at the flag.
OT, 4 kΩ to 8 Ω, 20 H primary, 5 mH leakage
The output transformer, 4 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, 295 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 294.5 V ac from the grounded center tap, as a sine source at the mains frequency. The value is typical of the part.
PT, 295 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, 60 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, 60 Ω
Resistance of the other half. Typical of the part.
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.
16µ, 16 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

ECC83 throughout
Two 12AX7s. V1 runs the two input stages, and the top-boost tube the gain stage and cathode follower. 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. Caps are ideal, and the battery or HT is a voltage behind a resistance.

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