Two-channel 6L6GC amp with a 6FQ7 cathodyne, 1964

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 Silvertone 1484 Twin Twelve.

Two 12AX7 channels, each with Bass, Treble and Volume and a second stage of its own, mixed at their plates into a 6FQ7 driver and cathodyne and two fixed-bias 6L6GCs off a silicon supply.

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 is one 12AX7 stage, a Bass and Treble network, a Volume and a second stage. The two first stages share a cathode resistor. Channel 1 has 1 MΩ from the far side of its coupling cap back to its grid, which lowers the gain somewhat. Channel 2 has 330 kΩ in series instead. The two second stages have 4.7 kΩ under their cathodes and meet at their plates through 68 kΩ each, under one shared 220 kΩ. The reverb isn’t modeled. The tremolo works on channel 2: a 12AX7 phase-shift oscillator, about 5 to 12 times a second, swings a second half that lights a neon lamp, and the lamp’s photocell pulls channel 2 down from after its 330 kΩ to ground. The tube models are typical of the types, not measured.

Download.

Parts and what they do

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

1.5k, 1.5 kΩ
Cathode resistor shared by both halves of V1 (+1.1 V on the drawing).
25µ, 25 µF electrolytic
Bypass for V1’s shared cathode, full gain down to a few hertz.
68k, 68 kΩ
Input resistor for channel 1’s first jack.
68k, 68 kΩ
Input resistor for channel 1’s second jack, the same.
1M, 1 MΩ
Grid resistor for V1b.
V1b, 12AX7 / ECC83 / 7025
Half of a 12AX7, channel 1’s first stage. The two halves share one cathode resistor.
220k, 220 kΩ
Plate resistor for V1b (+100 V on the drawing).
.02µ, 20 nF
Coupling cap into channel 1’s tone network.
1M, 1 MΩ
From channel 1’s grid to the far side of the coupling cap, 1 MΩ. It feeds some of the plate’s signal back to the grid, turned the other way, so it lowers the stage’s gain, more as the source gets weaker.
100k, 100 kΩ
The tone network’s slope resistor, 100 kΩ down to the Bass pot.
680p, 680 pF
Treble cap, 680 pF to the top of the Treble pot.
Treble 1 1M, 1 MΩ pot, audio taper
Treble, 1 MΩ. Its wiper is the Volume’s top. Turned up, the wiper moves toward the 680 pF and more treble gets through. Taper and direction aren’t marked, so audio and brighter clockwise are assumed.
.01µ, 10 nF
From the Treble pot’s bottom to ground.
Bass 1 1M, 1 MΩ pot, audio taper
Bass, 1 MΩ, between the 100 kΩ and the 68 kΩ to ground. Its wiper feeds the Volume’s top through 100 kΩ. Turned up, the wiper moves toward the 100 kΩ end and the lows come up. Audio taper and more bass clockwise are assumed.
.0015µ, 1.5 nF
From the Bass pot’s top to its wiper.
.02µ, 20 nF
From the Bass pot’s wiper to its bottom.
68k, 68 kΩ
The bottom of the tone network, 68 kΩ to ground.
100k, 100 kΩ
From the Bass wiper to the Treble wiper, the Volume’s top. The two controls mix here.
Vol 1 1M, 1 MΩ pot, audio taper
Channel 1’s Volume, 1 MΩ. Its wiper is the second stage’s grid. Audio taper assumed.
V2a, 12AX7 / ECC83 / 7025
Half of the second 12AX7, channel 1’s second stage, its cathode not bypassed.
4.7k, 4.7 kΩ
Cathode resistor for V2a, 4.7 kΩ with no bypass, so the stage runs at low gain (+1.1 V).
68k, 68 kΩ
From V2a’s plate to the mixing node. Both second stages share the 220 kΩ above it, so each is part of the other’s plate load.
68k, 68 kΩ
Input resistor for channel 2’s first jack.
68k, 68 kΩ
Input resistor for channel 2’s second jack, the same.
1M, 1 MΩ
Grid resistor for V1a.
V1a, 12AX7 / ECC83 / 7025
Half of a 12AX7, channel 2’s first stage. The two halves share one cathode resistor.
220k, 220 kΩ
Plate resistor for V1a (+100 V on the drawing).
.02µ, 20 nF
Coupling cap out of channel 2’s stage.
330k, 330 kΩ
In series ahead of channel 2’s tone network, 330 kΩ. Channel 1 has none here, so channel 2 comes out quieter.
100k, 100 kΩ
The tone network’s slope resistor, 100 kΩ down to the Bass pot.
680p, 680 pF
Treble cap, 680 pF to the top of the Treble pot.
Treble 2 1M, 1 MΩ pot, audio taper
Treble, 1 MΩ. Its wiper is the Volume’s top. Turned up, the wiper moves toward the 680 pF and more treble gets through. Taper and direction aren’t marked, so audio and brighter clockwise are assumed.
.01µ, 10 nF
From the Treble pot’s bottom to ground.
Bass 2 1M, 1 MΩ pot, audio taper
Bass, 1 MΩ, between the 100 kΩ and the 68 kΩ to ground. Its wiper feeds the Volume’s top through 100 kΩ. Turned up, the wiper moves toward the 100 kΩ end and the lows come up. Audio taper and more bass clockwise are assumed.
.0015µ, 1.5 nF
From the Bass pot’s top to its wiper.
.02µ, 20 nF
From the Bass pot’s wiper to its bottom.
68k, 68 kΩ
The bottom of the tone network, 68 kΩ to ground.
100k, 100 kΩ
From the Bass wiper to the Treble wiper, the Volume’s top. The two controls mix here.
Vol 2 1M, 1 MΩ pot, audio taper
Channel 2’s Volume, 1 MΩ. Its wiper is the second stage’s grid. Audio taper assumed.
V2b, 12AX7 / ECC83 / 7025
Half of the second 12AX7, channel 2’s second stage, its cathode not bypassed.
4.7k, 4.7 kΩ
Cathode resistor for V2b, 4.7 kΩ with no bypass, so the stage runs at low gain (+1.1 V).
68k, 68 kΩ
From V2b’s plate to the mixing node. Both second stages share the 220 kΩ above it, so each is part of the other’s plate load.
220k, 220 kΩ
The two second stages’ shared plate resistor, from node D, one 68 kΩ step past the screens. Both plates sit at 93 V on the drawing.
560k, 560 kΩ
From the mixing node to the reverb’s return. The reverb isn’t modeled, so it goes to ground here.
.01µ, 10 nF
Coupling cap from the mixing node to the driver’s grid.
V6a, 12AX7 / ECC83 / 7025
Half of V6, a 12AX7, the tremolo’s oscillator. Its cathode is grounded and its grid sits on −1.5 V through 2.2 MΩ. A ladder of three caps from its plate back to its grid turns the phase round at a few hertz.
330k, 330 kΩ
The oscillator’s plate resistor, from the screen supply (+340 V). The drawing shows 205 V on the plate.
.02µ, 20 nF
First cap of the ladder, from the oscillator’s plate.
Speed 1M, 1 MΩ pot, linear taper
Trem Speed, 1 MΩ, a variable resistor from the ladder’s first node toward ground through 68 kΩ. Turned up, there’s less track in the circuit and the oscillator runs faster. Taper and direction aren’t marked, so linear and faster clockwise are assumed.
68k, 68 kΩ
The fixed 68 kΩ under the Speed pot. It sets the top speed.
.01µ, 10 nF
Second cap of the ladder.
560k, 560 kΩ
The ladder’s second leg, to ground.
.01µ, 10 nF
Last cap of the ladder, into the oscillator’s grid.
2.2M, 2.2 MΩ
The oscillator’s grid leak, 2.2 MΩ to the −1.5 V tap.
100k, 100 kΩ
From the bias supply (−36 V) to the tap for the oscillator’s grid.
2.2k, 2.2 kΩ
The tap’s bottom, 2.2 kΩ to ground. With 100 kΩ from −36 V it comes to about −0.8 V, where the drawing marks −1.5 V.
.05µ, 50 nF
Couples the oscillator’s swing from its plate to the Strength pot.
560k, 560 kΩ
With the Strength pot below it, a divider. It cuts the oscillator’s 100-odd volts of swing to a few volts at the lamp driver.
.5µ, 500 nF
Across the Strength pot, 0.5 µF. With the 560 kΩ and 100 kΩ it rounds the swing off, a filter at about 3.7 Hz, so the drive falls as the speed goes up. Builders put a cap like this on the LFO’s output to damp the tick.
Strength 100k, 100 kΩ pot, linear taper
Tremolo Strength, 100 kΩ, to ground. Its wiper drives the lamp driver’s grid: turned up, more of the swing reaches it and the lamp flashes harder. Taper and direction aren’t marked, so linear and more clockwise are assumed.
V6b, 12AX7 / ECC83 / 7025
The other half of V6, the lamp driver. Its plate current runs through the neon from the preamp supply, so the lamp lights on the upswing.
68k, 68 kΩ
The lamp driver’s cathode resistor, 68 kΩ with no bypass. It holds the idle current to a few tens of microamps (the drawing shows 1.5 V here).
Neon, NE-2 neon
The neon lamp in the tremolo module (306-2H), from the preamp supply to the lamp driver’s plate. It strikes at about 76 V and once lit holds about 58 V, and its light follows its current. NE-2 class figures, not measured on the module.
LDR, Ampeg TM-1 cell
The photocell in the tremolo module, from channel 2’s tone network input (after the 330 kΩ) to ground. Lit, it pulls channel 2 down. The tremolo works on channel 2 only. The module’s cell isn’t documented. A Fender roach cell would let the lamp’s dim idle glow cut channel 2 about 39 dB, so a high-resistance cell (the Ampeg TM-1 class: 35 MΩ dark, 20 kΩ at the neon’s 0.3 mA) stands in, typical, not measured.
V3b, 6CG7
6CG7, the driver. A cathodyne has no gain of its own, so this stage raises the preamp’s signal enough to drive the output tubes. Its grid is fed through the preamp’s coupling cap, so there’s no second one here.
1M, 1 MΩ
Grid resistor for the driver.
100k, 100 kΩ
Plate resistor for the driver.
1.5k, 1.5 kΩ
Cathode resistor for the driver. It sets the driver’s bias, a volt or so.
.01µ, 10 nF
Coupling cap from the driver to the cathodyne.
V3a, 6CG7
The cathodyne, or split-load inverter. The same current runs through its plate resistor and its cathode resistors, which are about equal, so the plate swings one way and the cathode the other by about the same amount. The cathode drives tube A, the plate tube B. Its gain is slightly under 1 on each side.
68k, 68 kΩ
The cathodyne’s plate resistor, the output to tube B.
2.2k, 2.2 kΩ
The cathodyne’s bias resistor. The grid returns below it, so the cathode sits this far above the grid.
68k, 68 kΩ
The cathodyne’s cathode load, the same value as its plate resistor. The cathode above the bias resistor is the output to tube A.
1M, 1 MΩ
Grid resistor for the cathodyne, returned to the junction of its cathode resistors so the grid sits near the cathode’s high DC voltage.
.02µ, 20 nF
Coupling cap to tube A’s grid.
.02µ, 20 nF
Coupling cap to tube B’s grid.
V7, 6L6GC
6L6GC, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply.
V8, 6L6GC
6L6GC, tube B of the pair.
330k, 330 kΩ
Grid leak for tube A, returned to the bias supply. Driven into grid current, the coupling cap charges through it and the bias goes colder for a moment. That’s blocking distortion.
330k, 330 kΩ
Grid leak for tube B.
Screens, 340 V
The screens’ supply, 340 V on the drawing. In the amp it comes off the middle of the doubler stack through a choke to 5 µF. Here it’s a steady source behind 300 Ω, so it doesn’t sag the way the real one would.
68k, 68 kΩ
Dropping resistor from the screen supply node to the node for the mixer, as on the Silvertone 1484 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.
10µ, 10 µF electrolytic
Filter cap at the node for the mixer. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
68k, 68 kΩ
Dropping resistor from the node for the mixer to the node for the preamp, as on the Silvertone 1484 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.
20µ, 20 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, 4 kΩ to 4 Ω, 30 H primary, 10 mH leakage
The output transformer, 4 kΩ plate to plate into 4 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, 346 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 346 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, 346 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, 12 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, 12 Ω
Resistance of the other half. Typical of the part, not measured.
D1, 1N4001
One of the silicon rectifier diodes. It drops under a volt, so B+ holds up better under load than with a tube rectifier.
D2, 1N4001
The other rectifier diode, on the other half cycle.
50µ, 50 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.
Bias tap, 26 V ac, 60 Hz
The power transformer’s bias tap, 26 V ac, as a sine source at the mains frequency. Typical of the part, not measured.
tap R, 4 Ω
The bias tap’s own resistance, 4 Ω, typical, not measured. The drawing has no resistor between the tap and the diode.
Bias diode, 1N4001
The bias rectifier, a silicon diode turned so the supply comes out negative.
100µ, 100 µF electrolytic
Filter cap on the bias supply, its plus side to ground.
102k, 102 kΩ
The bias supply’s load, 102k Ω to ground. With the 4 Ω above it, it sets the bias. There’s no bias pot, so change this resistor to change the bias. Bigger runs the tubes colder.
Coil R, 4.34 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 4 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, 128 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, 232 µH
With Loss R across it, the treble rises and then levels off, like the losses in a real coil.
Loss R, 7.15 Ω
Limits what Loss L adds to the treble rise.
Loss L2, 1.06 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, 1.09 Ω
Limits what Loss L2 adds.
Tank R, 41.9 Ω
Height of the bass resonance, about 46 Ω in all at its peak. With no feedback, the voltage at the speaker rises and falls with its impedance.
Tank L, 10.6 mH
With Tank C, sets the resonance: 74 Hz.
Tank C, 440 µF
With Tank L, sets the resonance. With Tank R, it sets how narrow the peak is.

About this version

As drawn
The 12AX7 model is fitted to its data sheet (Brimar). Values from the Sears drawing.

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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