EF86 tremolo channel and triode normal 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 AC10.

Two channels. An EF86 pentode whose plate supply is swung by a pentode phase shift oscillator, so its gain rises and falls, and a triode normal channel. Each channel feeds one grid of the long-tail pair inverter. Two EL84s in cathode bias with no feedback, on an EZ81.

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 tremolo works on the EF86 itself. The oscillator’s plate reaches the EF86’s plate through 330 kΩ, so the plate’s voltage rises and falls a few times a second and the stage’s gain follows. The Amp. pot sets the screen’s voltage: low, the gain depends more on the plate, and the swing is deeper. Speed runs the oscillator from about 4.7 to 7.8 times a second. Three high-pass steps, .005 µF and 1 MΩ each, strip the slow swing off the signal before the volume. The normal channel is one triode, the ECF82’s other half, into its own volume. Each channel goes to one grid of the inverter. The Normal vol. at 0 holds the second grid at ground for signal. The EL84s run in cathode bias with no feedback, and the Tone sets how much treble cancels between their grids.

Download.

Parts and what they do

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

220k, 220 kΩ
From tremolo jack 1 to the EF86’s grid.
220k, 220 kΩ
From tremolo jack 2 to the EF86’s grid. With nothing plugged in, the jack’s switch grounds its tip, so this goes to ground and makes a divider with the other 220 kΩ. The guitar loses about 7 dB here.
1M, 1 MΩ
Grid leak, 1 MΩ to ground.
V1, EF86 / 6267
EF86 pentode, the tremolo channel’s only gain stage. Its plate supply swings with the oscillator, so its gain rises and falls.
470k, 470 kΩ
Plate resistor from the 255 V node. With the 330 kΩ from the oscillator it sets the plate’s DC, which the drawing shows swinging from 60 to 160 V.
750k, 750 kΩ
Screen resistor from the 255 V node.
150k, 150 kΩ
From the screen down to the Amp. pot. With it the screen is a divider, so the pot sets the screen’s voltage.
Amp., 500 kΩ pot, linear taper
Tremolo Amplitude, 500 kΩ linear, a variable resistor to ground. Turned up, its resistance falls and the screen drops toward 40 V, where the plate’s swing moves the gain more. Turned down, the screen sits near 80 V. The drawing shows those two figures.
0.1µ, 100 nF
Across the Amp. pot. It bypasses the screen only through the 150 kΩ, so the screen isn’t held still at audio and the stage’s gain stays lower.
1.5k, 1.5 kΩ
Cathode resistor shared by the EF86 and the oscillator. The drawing shows 2 V here.
25µ, 25 µF electrolytic
Bypass on the shared cathode, 25 µF.
.005µ, 5 nF
First of three high-pass steps, .005 µF with 1 MΩ, each about 32 Hz. Together they strip the plate’s slow swing off the signal.
1M, 1 MΩ
First high-pass step’s resistor to ground.
.005µ, 5 nF
Second high-pass step.
1M, 1 MΩ
Second high-pass step’s resistor to ground.
.005µ, 5 nF
Third high-pass step.
1M, 1 MΩ
Third high-pass step’s resistor to ground.
.005µ, 5 nF
Into the Tremolo Volume.
Trem. vol., 500 kΩ pot, audio taper
Tremolo Volume, 500 kΩ log.
.01µ, 10 nF
Coupling cap into the inverter’s first grid.
V3, 6AU6 / EF94
The pentode half of the ECF82, the tremolo’s 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.
220k, 220 kΩ
Oscillator plate resistor from the 255 V node. The drawing shows 150 V on the plate.
330k, 330 kΩ
Oscillator screen, from the 255 V node.
100k, 100 kΩ
Oscillator screen, to ground. The divider puts the screen near 45 V, as the drawing shows.
.01µ, 10 nF
First cap of the ladder.
470k, 470 kΩ
The ladder’s first leg, 470 kΩ above the Speed pot.
Speed, 2 MΩ pot, linear taper
Speed, 2 MΩ linear, a variable resistor to ground. Turned up, its resistance falls and the oscillator runs faster.
.01µ, 10 nF
Second cap of the ladder.
1M, 1 MΩ
The ladder’s middle leg, 1 MΩ to ground through the footswitch.
.01µ, 10 nF
Last cap of the ladder, into the oscillator’s grid.
1M, 1 MΩ
The oscillator’s grid leak.
330k, 330 kΩ
From the oscillator’s plate toward the EF86’s plate.
.05µ, 50 nF
From the middle of the two 330 kΩ to ground. It passes the slow swing and shorts the audio, so neither tube’s signal gets to the other.
330k, 330 kΩ
On to the EF86’s plate. The oscillator’s swing moves the plate’s voltage, and the EF86’s gain with it. At audio it’s 330 kΩ to ground beside the 470 kΩ, so the plate’s load is about 190 kΩ.
220k, 220 kΩ
From normal jack 1 to the triode’s grid.
220k, 220 kΩ
From normal jack 2 to the triode’s grid. With nothing plugged in, the jack’s switch grounds its tip, so this makes a divider with the other 220 kΩ.
1M, 1 MΩ
The normal channel’s grid leak, 1 MΩ to ground.
V3t, 6DJ8 / ECC88
The triode half of the ECF82, the normal channel’s only gain stage. Its signal goes through the Normal volume to the inverter’s second grid.
220k, 220 kΩ
Plate resistor from the 255 V node. The drawing shows 40 V on the plate.
820, 820 Ω
Cathode resistor. The drawing shows 1 V here.
25µ, 25 µF electrolytic
Bypass on the triode’s cathode, 25 µF, so the stage has its full gain down into the bass.
.022µ, 22 nF
Coupling cap from the triode’s plate to the Normal volume.
Normal vol., 500 kΩ pot, audio taper
Normal 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.
V4a, 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.
V4b, 12AX7 / ECC83 / 7025
The other half of the inverter. Its grid takes the normal 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 normal 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.
.05µ, 50 nF
Coupling cap to tube A’s grid.
.05µ, 50 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.
V5, EL84 / 6BQ5
EL84, tube A of the push-pull pair, its cathode on the shared cathode resistor, so it biases itself.
V6, EL84 / 6BQ5
EL84, tube B of the pair.
100, 100 Ω
Plate resistor for tube A, 100 Ω between the plate and the output transformer. It damps oscillation at radio frequencies. The drop across it is the plate current times 100 Ω, a few volts.
100, 100 Ω
Plate resistor for tube B, the same.
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.
Tone, 250 kΩ pot, audio taper
Tone, a 250k pot in series with the .005µF cap, the pair between the two output grids. The cap lets only the treble through, so the treble on one grid cancels the treble on the other. Its resistance rises as it’s turned up, so less treble cancels and the amp gets brighter. At 0 the cap goes straight across the grids.
.005µ, 5 nF
The Cut cap. The smaller it is, the higher the treble it cancels.
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.
8 H, 8 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.
210, 210 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 AC10 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.
22k, 22 kΩ
Dropping resistor from the screen supply node to the node for the preamp, as on the Vox AC10 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.
8µ, 8 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.
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, 260 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, 260 Ω
Resistance of the other half. Typical of the part.
V7, 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.
V7, 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

As drawn
The EF86 preset is fitted to the data sheet. The ECF82’s pentode is stood in for by a 6AU6, a small pentode of about the same transconductance, and its triode by a 6DJ8 half, a triode with a mu in the 30s. A typical value.

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.

Spot an error, or want a circuit added? Email info@swellamps.com. A schematic or a clear photo of one helps. Anything you send may be used to improve these models and our desktop app.

© 2026 Swell Amplification LLC. All rights reserved. The simulator, its circuit models, drawings, presets and text are the property of Swell Amplification LLC and may not be copied, reproduced, redistributed or used to build another product without written permission. The original circuits are historical designs documented in public sources, and no claim is made to them. Everything here is provided as is, without warranty of any kind, for study and personal use. Results are simulations of typical parts, not measurements of any real unit. Don’t rely on them for the safety of any build or repair. Tube amplifiers carry lethal voltages.

Product and company names are trademarks of their owners. They are used here only to identify the circuits these models are based on; this site is not affiliated with or endorsed by any of them.

From Swell Amplification, swellamps.com.