Single-ended EL84 amp with tremolo, late 1960s 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 Kalamazoo Model Two.

Half a 12AX7 with an unbypassed cathode, Volume and a treble-cut Tone, then the other half driving one 6BQ5 (EL84), single-ended with no feedback, on a 6X4 rectifier. The tremolo oscillator shares the driver’s cathode resistor and swings its bias.

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 Model One’s circuit with a tremolo added. The first stage has 4.7 kΩ on its cathode, unbypassed, so it has about half a 12AX7’s full gain. The oscillator is half a second 12AX7, a phase-shift oscillator, and its cathode sits on the same 4.7 kΩ and 10 µF as the driver’s. As its current swings, the driver’s bias swings with it, so the driver’s gain rises and falls a few times a second. There’s no depth control, only Speed and the on switch. Miles O’Neal redrew the amp with voltages.

Download.

Parts and what they do

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

47k, 47 kΩ
Input 1’s resistor into the grid (R1). With a guitar in input 1 it’s a grid stopper against radio pickup.
47k, 47 kΩ
Input 2’s resistor into the grid (R2). With nothing in input 1 that tip is grounded, so the two 47 kΩ make a divider and input 2 is about 6 dB quieter.
V1b, 12AX7 / ECC83 / 7025
Half a 12AX7A, the only gain stage ahead of the volume. There’s no grid resistor, so the grid’s path to ground is the guitar, or input 1’s contact.
4.7k, 4.7 kΩ
Cathode resistor for V1b (R3), 4.7 kΩ and not bypassed, so the stage has about half its full gain. The Model One has 2.2 kΩ.
100k, 100 kΩ
Plate resistor for V1b (R4), from the +230 V node. The redraw reads +183 V on the plate.
15p, 15 pF
A small cap from V1b’s plate to ground (C10). It trims the highest treble, above about 100 kHz, against radio pickup and oscillation.
.01µ, 10 nF
Coupling cap to the volume pot (C1). Into 1 MΩ it passes everything above about 16 Hz.
Vol 1M, 1 MΩ pot, audio taper
Volume, 1 MΩ audio (R6). Its wiper is the driver’s grid, and the pot is that grid’s only path to ground.
.0047µ, 4.7 nF
The tone control’s cap (C2), from the plate to the tone pot.
Tone 1M, 1 MΩ pot, audio taper
Tone, 1 MΩ audio (R5), wired as a variable resistor from the cap to ground. Up puts more resistance in series with the cap, so less treble is bled away. Up is assumed to be brighter.
150k, 150 kΩ
Plate resistor for the oscillator (R11), from the screen node. The redraw reads +230 V on the plate.
V4a, 12AX7 / ECC83 / 7025
Half a 12AX7A, the tremolo oscillator. Its cathode is V1a’s, so its current swings the driver’s bias.
.033µ, 33 nF
First cap of the oscillator’s phase-shift ladder (C6).
68k, 68 kΩ
In series with the speed pot, the ladder’s first leg (R14). It sets the top speed.
Speed, 500 kΩ pot, reverse audio taper
Speed, 500 kΩ reverse audio (R17), as a variable resistor in the ladder’s first leg. Turned up, its resistance drops and the oscillator runs faster. Its far end goes through S2 to ground.
.02µ, 20 nF
Second ladder cap (C7).
510k, 510 kΩ
Second ladder leg to ground (R15).
.02µ, 20 nF
Third ladder cap, into the grid (C8). The three caps and legs turn the plate’s swing round by half a cycle at one frequency, so it oscillates there.
1.5M, 1.5 MΩ
Grid leak for the oscillator, and the ladder’s last leg (R16).
V1a, 12AX7 / ECC83 / 7025
12AX7, the driver. With one output tube there’s no phase inverter, only a gain stage to raise the preamp’s signal enough to drive the output grid. Its grid connects straight to the preamp’s output with no coupling cap, so its path to ground is through the preamp’s volume and tone network.
R7, 100 kΩ
Plate resistor for the driver. Its plate drives the output tube’s grid through the coupling cap.
R13, 4.7 kΩ
Cathode resistor for the driver. It sets the driver’s bias, a volt or two.
C5, 10 µF electrolytic
Bypass cap across the driver’s cathode resistor, for full gain above a few hertz.
C3, 10 nF
Coupling cap to tube A’s grid.
V2, EL84 / 6BQ5 (Philips curves)
EL84, the output tube, single-ended: it carries the whole signal, and its idle current runs through the transformer all the time. Its cathode resistor biases it.
R8, 330 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.
R9, 150 Ω
The cathode resistor. The tube’s current runs through it and lifts the cathode above the grid, and that’s the bias. Driven hard, the tube draws more current on average, so the cathode climbs and the bias goes colder.
R12, 2.2 kΩ
Dropping resistor from B+ to the screen supply. The screens sit slightly below the plates.
C4b, 10 µF electrolytic
Filter cap on the screen supply.
R10, 100 kΩ
Dropping resistor from the screen supply node to the node for the driver and preamp, as on the Kalamazoo Model Two 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.
C4c, 10 µF electrolytic
Filter cap at the node for the driver and preamp. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
OT, 5 kΩ to 8 Ω, 14.3 H primary, 54.3 mH leakage
The output transformer, 5 kΩ from the plate to B+ 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.
PT, 252 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 252 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, 252 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, 100 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, 100 Ω
Resistance of the other half. Typical of the part, not measured.
V3, 6X4
One plate of the 6X4 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.
V3, 6X4
The rectifier’s other plate, on the other half cycle.
C4a, 20 µF electrolytic
The reservoir cap. It feeds the output transformer’s B+ end. 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
Miles O’Neal’s redraw. The 12AX7A uses the 12AX7 model. The output transformer and the 6X4 are typical, not on the drawing.

Sources

  • Miles O’Neal, redrawn Model Two schematic with voltages, 2004

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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