Two-channel 6L6 amp with cathode tremolo and a paraphase, late 1950s

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 Supro 1690T.

Channel 1 is one grid-leak biased 12AX7 stage, channel 2 two stages with tremolo on its first cathode. The two Volumes mix through 220 kΩ into a Tone, then a 12AX7 paraphase drives two cathode-biased 6L6s.

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. Each channel has a Treble jack through .005 µF and a Norm jack through 100 kΩ. Channel 1 is one 12AX7 half, grid-leak biased through 6.8 MΩ. Channel 2 is two stages, with .001 µF and a 470 kΩ over 100 kΩ divider between them. Its second stage runs off a 270 kΩ and 180 kΩ divider, at about 30 V on the plate, so it clips early. Each Volume has a .001 µF bright cap and feeds the mix through 220 kΩ, where the Tone hangs a 500 kΩ variable resistor and .01 µF to ground. A 12AX7 paraphase drives two cathode-biased 6L6s, no feedback. The tremolo moves channel 2’s first cathode. Speed and Intensity are wired faster and deeper clockwise. The redraw notes that real ones run the other way.

Download.

Parts and what they do

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

.005µ, 5 nF
Channel 1’s Treble jack cap. It cuts the bass on the way in.
100k, 100 kΩ
Channel 1’s Norm jack resistor. With nothing in that jack its switch grounds this end, so it’s 100 kΩ to ground behind the Treble jack.
.05µ, 50 nF
Coupling cap to V1A’s grid (.02 µF on some). It also holds the grid-leak bias.
6.8M, 6.8 MΩ
V1A’s grid leak. The cathode is grounded, so grid current through it sets the bias, about a volt negative.
500p, 500 pF
From V1A’s grid to ground. Behind 100 kΩ it trims the top end.
V1A, 12AX7 / ECC83 / 7025
Half of a 12AX7, channel 1’s only stage. Its cathode is grounded.
100k, 100 kΩ
Dropping resistor from C to V1A’s own node.
.05µ, 50 nF
Filter cap on V1A’s node.
270k, 270 kΩ
Plate resistor for V1A.
.005µ, 5 nF
Coupling cap to Volume 1 (.01 µF on some). Into the Volume it thins the lows a little.
Vol 1 500k, 500 kΩ pot, linear taper
Volume 1, 500 kΩ linear (audio on amps after about 1960).
.001µ, 1 nF
Bright cap across the top of Volume 1. It lets treble around the pot, more as the Volume comes down.
220k, 220 kΩ
Channel 1’s mixing resistor (470 kΩ on some).
.005µ, 5 nF
Channel 2’s Treble jack cap.
100k, 100 kΩ
Channel 2’s Norm jack resistor, with the same grounding switch.
470k, 470 kΩ
Grid resistor for V1B. There’s no coupling cap: the jacks go straight to the grid.
500p, 500 pF
From V1B’s grid to ground, against the 100 kΩ.
V1B, 12AX7 / ECC83 / 7025
The other half of V1, channel 2’s first stage. The tremolo moves its cathode.
3.9k, 3.9 kΩ
Cathode resistor for V1B. The oscillator’s cathode current comes in through 15 kΩ and runs through it too, so the LFO moves V1B’s bias.
35µ, 35 µF electrolytic
Bypass on V1B’s cathode. At a few hertz it only partly holds the cathode still.
100k, 100 kΩ
Dropping resistor from C to V1B’s own node.
.05µ, 50 nF
Filter cap on V1B’s node.
270k, 270 kΩ
Plate resistor for V1B.
.01µ, 10 nF
Coupling cap after V1B.
470k, 470 kΩ
To ground after the .01 µF.
.001µ, 1 nF
Small coupling cap (.005 µF on some). Into the 570 kΩ behind it, it cuts the bass below about 280 Hz.
470k, 470 kΩ
Series resistor into V2B’s grid. With the 100 kΩ below it, it drops the level to about a sixth.
100k, 100 kΩ
V2B’s grid resistor, the lower leg of the divider.
V2B, 12AX7 / ECC83 / 7025
Half of the second 12AX7, channel 2’s second stage.
2.2k, 2.2 kΩ
Cathode resistor for V2B, with no bypass.
270k, 270 kΩ
Upper leg of V2B’s supply divider, from C.
180k, 180 kΩ
Lower leg of V2B’s supply divider. With the 270 kΩ it feeds V2B from a lower, stiffer node.
220k, 220 kΩ
Plate resistor for V2B.
.01µ, 10 nF
Coupling cap to Volume 2.
Vol 2 500k, 500 kΩ pot, linear taper
Volume 2, 500 kΩ linear (audio on amps after about 1960).
.001µ, 1 nF
Bright cap across the top of Volume 2.
220k, 220 kΩ
Channel 2’s mixing resistor (470 kΩ on some).
Tone 500k, 500 kΩ pot, audio taper
Tone, 500 kΩ audio, a variable resistor from the mixing node to the .01 µF. Turned down, its resistance drops and the cap pulls more treble to ground. Brighter clockwise is assumed.
.01µ, 10 nF
The Tone’s cut cap (.005 µF on some).
V2A, 12AX7 / ECC83 / 7025
The other half of V2, the tremolo’s phase-shift oscillator. Its cathode current returns through 15 kΩ and V1B’s cathode resistor.
150k, 150 kΩ
Feeds the oscillator from C, with Intensity across it.
Int 500k, 500 kΩ pot, linear taper
Intensity, 500 kΩ linear, as a variable resistor across the 150 kΩ, wired so it’s deeper clockwise. Turned up, its resistance drops, the oscillator runs on more voltage and swings harder. The redraw notes that real ones run the other way, deeper turned down.
270k, 270 kΩ
Plate resistor for the oscillator (220 kΩ on some).
.02µ, 20 nF
First cap of the oscillator’s ladder.
100k, 100 kΩ
In series with Speed, the ladder’s first leg to ground. It sets the top speed.
Speed 500k, 500 kΩ pot, linear taper
Speed, 500 kΩ linear, as a variable resistor in the first leg, wired so it’s faster clockwise. Turned up, its resistance drops and the oscillator runs faster. The redraw notes that real ones run the other way, faster turned down.
.05µ, 50 nF
Second ladder cap.
270k, 270 kΩ
Second ladder leg to ground.
.02µ, 20 nF
Third ladder cap, into the oscillator’s grid.
470k, 470 kΩ
Third ladder leg, the oscillator’s grid resistor.
.001µ, 1 nF
From the oscillator’s grid to ground. It keeps the oscillator off audio frequencies.
.001µ, 1 nF
From the oscillator’s cathode to ground, a small bypass for clicks.
15k, 15 kΩ
From the oscillator’s cathode to V1B’s (6.8 kΩ on some). With the tremolo off it keeps about the same current flowing, so V1B’s bias doesn’t jump, and its feedback holds the oscillator still. The switch shorts it.
V3A, 12AX7 / ECC83 / 7025
12AX7, the first half of the paraphase inverter. It’s a plain gain stage: its plate swings opposite the input and drives tube A. 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.
270k, 270 kΩ
Plate resistor for the first half, the output to tube A.
2.2k, 2.2 kΩ
Cathode resistor for both halves. The tube has one cathode for its two triodes, so this carries both currents and sets both biases.
270k, 270 kΩ
Tube A’s grid resistor, down to the tap. With the resistor under the tap it feeds a small share of tube A’s grid signal to the second half’s grid. The share is about one over the second half’s gain, so the two output grids swing about the same.
4.7k, 4.7 kΩ
The tap, 4.7k Ω to ground under tube A’s grid resistor, and the second half’s grid resistor. The two sides only match while the share times the second half’s gain is 1, so they drift apart as the tube ages.
V3B, 12AX7 / ECC83 / 7025
The second half. It turns the divided signal over again, so its plate swings with the input. It drives tube B.
270k, 270 kΩ
Plate resistor for the second half, the output to tube B.
.02µ, 20 nF
Coupling cap to tube A’s grid.
.02µ, 20 nF
Coupling cap to tube B’s grid.
V4, 6L6GC
6L6GC, tube A of the push-pull pair, its cathode on the shared cathode resistor, so it biases itself.
V5, 6L6GC
6L6GC, tube B of the pair.
270k, 270 kΩ
Grid leak for tube B.
250, 250 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.
25µ, 25 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.
10k, 10 kΩ
Dropping resistor from B+ to the screens. It’s large, so the screens sit about 30 V below the plates and drop further when they draw more current on a hard note.
10µ, 10 µF electrolytic
Filter cap on the screen supply.
10k, 10 kΩ
Dropping resistor from the screen supply node to the node for the inverter and the preamp, as on the Supro 1690T 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.
10µ, 10 µF electrolytic
Filter cap at the node for the inverter and 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, 9.5 kΩ to 1.6 Ω, 12 H primary, 15 mH leakage
The output transformer, 9.5 kΩ plate to plate into 1.6 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, 310 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 310 V ac from the grounded center tap, as a sine source at the mains frequency. The value is typical of the part.
PT, 310 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.
PT R, 100 Ω
Resistance of the other half. Typical of the part.
V6, 5V4G
One plate of the 5V4G 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.
V6, 5V4G
The rectifier’s other plate, on the other half cycle.
20µ, 20 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, 1.74 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 1.6 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, 51.2 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, 92.8 µH
With Loss R across it, the treble rises and then levels off, like the losses in a real coil.
Loss R, 2.86 Ω
Limits what Loss L adds to the treble rise.
Loss L2, 426 µH
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, 438 mΩ
Limits what Loss L2 adds.
Tank R, 16.7 Ω
Height of the bass resonance, about 18 Ω in all at its peak. With no feedback, the voltage at the speaker rises and falls with its impedance.
Tank L, 4.22 mH
With Tank C, sets the resonance: 74 Hz.
Tank C, 1.1 mF
With Tank L, sets the resonance. With Tank R, it sets how narrow the peak is.

About this version

As drawn
The plain values on Eric Kettenburg’s redraw. The 12AX7 model is fitted to its data sheet (Brimar).

Sources

  • Eric Kettenburg, redrawn schematic with variants, 2013

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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