Pentode channel and vibrato channel, 1960 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 AC15.

Two channels. One EF86 pentode straight to its volume and the phase inverter, with no tone controls. Beside it a vibrato channel: two phase-shift networks, a modulator tube that slides between them, and an oscillator that runs it, for vibrato or tremolo.

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 EF86’s screen grid, fed through 1 MΩ, makes the plate current almost independent of the plate voltage, so the stage acts like a current source into its 220 kΩ load, with huge gain, about 45 dB. The plate idles low, around 90 V, and the screen sags when the stage is driven, so it compresses and then breaks up softly. Probe the screen and play hard to see it move. With one stage, the pickup, the guitar’s volume and your picking all come straight through. The vibrato channel splits its signal into two networks that shift its phase by different amounts. Two halves of a 12AU7, swung opposite ways by the oscillator, slide the output from one network to the other, so the note’s pitch wavers. On Tremolo one network is grounded and the other’s level swings instead. Each channel goes to one grid of the inverter.

Download.

Parts and what they do

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

R6, 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.
R5, 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.
R7, 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.
R27, 68 kΩ
The vib-trem 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.
R26, 68 kΩ
The vib-trem 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.
R28, 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.
R14, 22 kΩ
Dropping resistor from the 315 V node to the EF86’s supply.
C3, 8 µF electrolytic
Filter cap on the EF86 supply node.
V1, EF86 / 6267
EF86 pentode. The suppressor grid is tied to the cathode inside the model.
R15, 220 kΩ
Plate load. With a pentode, gain rises almost in step with this resistor.
R20, 1 MΩ
Screen resistor, dropping the screen to about 105 V. It’s so large that the screen voltage falls when the stage pulls more current, and that’s where much of the compression comes from.
C8, 100 nF
Screen bypass cap. It holds the screen steady at audio frequencies, above about 2 Hz, so the sag only moves with the playing dynamics.
R13, 2.2 kΩ
Cathode resistor.
C5, 25 µF electrolytic
Cathode bypass cap. With it the stage has full gain down into the bass.
C12, 10 nF
Coupling cap to the volume control. With the 500 kΩ it makes a high-pass near 30 Hz.
Volume, 500 kΩ pot, audio taper
Normal channel volume.
C27, 10 nF
Coupling cap into the phase inverter grid.
R33, 22 kΩ
Dropping resistor from B+ to the vib-trem channel’s supply node, 210 V on the drawing. Every tube in the channel hangs on that node.
C20, 32 µF electrolytic
Filter cap on the vib-trem channel’s supply node, 32 µF.
V5a, 12AX7 / ECC83 / 7025
First half of V5, an ECC83: the vib-trem channel’s gain stage.
R31, 150 kΩ
V5a’s plate resistor, 150 kΩ.
R29, 2.2 kΩ
V5a’s cathode resistor, 2.2 kΩ.
C10, 25 µF electrolytic
Bypass on V5a’s cathode, 25 µF.
C19, 10 nF
Coupling cap into V5b.
V5b, 12AX7 / ECC83 / 7025
Second half of V5, a split-load phase splitter: 22 kΩ on the plate and about 22 kΩ under the cathode, so the plate and the cathode swing equal and opposite. The two feed the phase-shift networks.
R36, 1 MΩ
V5b’s grid resistor, 1 MΩ to the junction under its bias resistor, so the grid sits about 1.5 V under the cathode.
R38, 1.5 kΩ
V5b’s bias resistor, 1.5 kΩ.
R37, 22 kΩ
V5b’s cathode load, 22 kΩ to ground. The drawing shows 22 V at its top.
R39, 22 kΩ
V5b’s plate load, 22 kΩ. The drawing shows 190 V on the plate.
C27, 500 pF
From V5b’s plate into network A, 500 pF in series with 68 kΩ. It passes the treble of the plate’s signal.
R49, 68 kΩ
In series with the 500 pF into network A.
R45, 100 kΩ
Network A to ground, 100 kΩ, with 200 pF across it.
C23, 200 pF
Across the 100 kΩ of network A.
R46, 330 kΩ
From V5b’s cathode side into network A, 330 kΩ, with 100 pF across it.
C24, 100 pF
Across the 330 kΩ into network A.
C28, 2 nF
From V5b’s plate into network B, .002 µF in series with 68 kΩ. It passes more of the lows than network A’s 500 pF.
R50, 68 kΩ
In series with the .002 µF into network B.
R47, 180 kΩ
Network B to ground, 180 kΩ, with 750 pF across it.
C25, 750 pF
Across the 180 kΩ of network B.
R48, 330 kΩ
From V5b’s cathode side into network B, 330 kΩ, with 500 pF across it.
C26, 500 pF
Across the 330 kΩ into network B.
C21, 100 nF
Network A into V6a’s grid.
C22, 100 nF
Network B into V6b’s grid.
C30, 100 nF
From network B to the Vib-trem switch. On Tremolo the switch grounds it, so network B is shorted for signal and only network A plays, its level swung by V6a: tremolo.
V6a, 12AU7 / ECC82
First half of V6, an ECC82, the modulator. Its gain follows the oscillator on its grid.
V6b, 12AU7 / ECC82
Second half of V6. Its grid gets the oscillator turned the other way, so as one half’s gain rises the other’s falls. Their plate currents add in the 12 kΩ, so the output slides between network A and network B, and the note’s phase moves: vibrato.
R40, 470 kΩ
From the oscillator’s feed to V6a’s grid, 470 kΩ.
R41, 1 MΩ
V6a’s grid return, 1 MΩ to ground.
R43, 470 kΩ
From the oscillator’s other feed to V6b’s grid, 470 kΩ.
R44, 1 MΩ
V6b’s grid return, 1 MΩ to ground.
R42, 1.5 kΩ
Cathode resistor for both halves of V6, not bypassed. The drawing shows 3.9 V here.
R34, 10 kΩ
V6a’s plate resistor, 10 kΩ. The drawing shows 80 V on each plate.
R35, 10 kΩ
V6b’s plate resistor, 10 kΩ.
R32, 47 kΩ
From the output node to both plate resistors, 47 kΩ.
C42, 33 nF
Across the 47 kΩ, .033 µF, added on the drawing’s 1961 change note.
R30, 12 kΩ
The modulator’s load, 12 kΩ from the supply node. Both halves’ plate currents run through it, so the signal here is their sum. The oscillator’s swing on the two grids is turned opposite ways, so most of it cancels here.
C13, 5 nF
First of four high-pass steps, .005 µF with 1 MΩ, each about 32 Hz. They strip what’s left of the oscillator’s slow swing off the signal.
R12, 1 MΩ
First high-pass step’s resistor to ground.
C14, 5 nF
Second high-pass step.
R13, 1 MΩ
Second high-pass step’s resistor to ground.
C15, 5 nF
Third high-pass step.
R14, 1 MΩ
Third high-pass step’s resistor to ground.
C16, 5 nF
Into the Vib-trem volume. The drawing’s fifth step, C17 and R15, came out on its 1961 change note.
Vib-trem vol., 500 kΩ pot, audio taper
Vib-trem volume, 500 kΩ log. Its wiper goes through .01 µF to the inverter’s second grid, so at 0 it holds that grid at ground for signal.
V7a, 12AX7 / ECC83 / 7025
First half of V7, an ECC83: the oscillator. Its plate feeds a ladder of three caps back to its grid, which turns the phase round at a few hertz, so it runs on its own.
R56, 270 kΩ
Oscillator plate resistor, 270 kΩ. The drawing shows 90 V (130 V) on the plate.
R60, 3.9 kΩ
Oscillator cathode resistor, 3.9 kΩ. The drawing shows 2.2 V here.
C37, 25 µF electrolytic
Bypass on the oscillator’s cathode.
C34, 10 nF
First cap of the ladder.
R54, 1 MΩ
The ladder’s first leg, 1 MΩ to ground through the footswitch.
C35, 10 nF
Second cap of the ladder.
R52, 560 kΩ
The ladder’s middle leg on Medium, 560 kΩ.
C36, 10 nF
Last cap of the ladder, into the oscillator’s grid.
R55, 1 MΩ
The oscillator’s grid leak, 1 MΩ to ground through the footswitch.
R58, 220 kΩ
From the oscillator’s plate, 220 kΩ into 0.1 µF: a low-pass that smooths the oscillator’s wave.
C33, 100 nF
The low-pass’s cap to ground.
C32, 100 nF
Coupling cap toward the depth divider.
R59, 1 MΩ
Top of the depth divider, 1 MΩ.
C39, 20 nF
From the depth divider to ground, .02 µF. It smooths the oscillator’s wave further.
Depth, 500 kΩ pot, audio taper
Depth preset, VR4, 500 kΩ log as a variable resistor, the bottom of the depth divider with 150 kΩ. It sits inside the chassis, not on the panel. Turned up, its resistance rises, more of the oscillator reaches V7b and the effect gets deeper.
R64, 150 kΩ
Under the depth preset, 150 kΩ, to B+ as drawn. B+ is ground for the slow swing, so it’s the bottom of the divider.
C38, 100 nF
From the depth divider into V7b’s grid.
V7b, 12AX7 / ECC83 / 7025
Second half of V7, a split-load phase splitter for the oscillator: its plate and cathode swing opposite ways, one to each half of the modulator.
R57, 22 kΩ
V7b’s plate load, 22 kΩ. The drawing shows 200 V on the plate.
R61, 1.5 kΩ
V7b’s bias resistor, 1.5 kΩ. The drawing shows 18.5 V on the cathode.
R63, 22 kΩ
V7b’s cathode load, 22 kΩ.
R62, 560 kΩ
V7b’s grid resistor, 560 kΩ to the junction under its bias resistor.
C31, 100 nF
The oscillator’s swing from V7b’s plate to V6a’s grid feed.
C29, 100 nF
The oscillator’s swing from V7b’s cathode side to V6b’s grid feed, turned the other way from the plate’s.
V8a, 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.
V8b, 12AX7 / ECC83 / 7025
The other half of the inverter. Its grid takes the vib-trem channel through its own coupling cap. It’s also driven through the shared cathodes, so each channel comes out of both plates, turned opposite ways. 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.
.01µ, 10 nF
Coupling cap from the vib-trem channel’s volume wiper into the second half’s grid, .01µF. With that volume at 0 the wiper is at ground, so the grid is held at ground for signal and that half is driven only through the shared cathodes.
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.
.01µ, 10 nF
Coupling cap to tube A’s grid.
.01µ, 10 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.
V9, EL84 / 6BQ5
EL84, tube A of the push-pull pair, its cathode on the shared cathode resistor, so it biases itself.
V10, EL84 / 6BQ5
EL84, tube B of the pair.
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 .005µF 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.
.005µ, 5 nF
The Cut cap. The smaller it is, the higher the treble it cancels.
100, 100 Ω
Screen resistor for tube A. It limits the screen current when the tube is driven hard.
100, 100 Ω
Screen resistor for tube B.
130, 130 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.
50µ, 50 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
The choke from the reservoir cap to B+, as on the amp’s drawing. The output transformer’s center tap, the screens and everything after them hang past it, so all of their current runs through it. It passes DC with little drop. 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. With the output tubes’ whole current through it, it’s most of the DC drop between the reservoir and B+.
16µ, 16 µF electrolytic
Filter cap on B+ after the choke, where the output transformer’s center tap and the screens hang.
22k, 22 kΩ
Dropping resistor from the screen supply node to the node for the inverter, as on the Vox AC15 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.
OT, 8 kΩ to 15 Ω, 40 H primary, 10 mH leakage
The output transformer, 8 kΩ plate to plate into 15 Ω, center tap on B+ after the choke. 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, 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.
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, 225 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, 225 Ω
Resistance of the other half. Typical of the part.
V11, EZ81
One plate of the EZ81 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.
V11, EZ81
The rectifier’s other plate, on the other half cycle.
16µ, 16 µF electrolytic
The reservoir cap. It feeds the choke, and past it the output transformer’s center tap and everything else. 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, 16.3 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 15 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, 480 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, 870 µH
With Loss R across it, the treble rises and then levels off, like the losses in a real coil.
Loss R, 26.8 Ω
Limits what Loss L adds to the treble rise.
Loss L2, 3.99 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, 4.11 Ω
Limits what Loss L2 adds.
Tank R, 157 Ω
Height of the bass resonance, about 173 Ω in all at its peak. With no feedback, the voltage at the speaker rises and falls with its impedance.
Tank L, 39.6 mH
With Tank C, sets the resonance: 74 Hz.
Tank C, 117 µF
With Tank L, sets the resonance. With Tank R, it sets how narrow the peak is.

About this version

EF86
The EF86 preset is fitted to the datasheet. Real EF86s vary, and they’re known for being microphonic, which isn’t in the model.

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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