Two-channel 6L6 amp with bass and treble, 1954 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 Gibson GA-55.

Four jacks on four 12AY7 triodes, two to a channel, each channel’s pair on one plate resistor. Two volumes mixed into a bass and treble network, then a 6SC7 paraphase into two 6L6GAs with no feedback.

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 jack has its own 12AY7 triode with a 10 MΩ grid leak and its cathode on ground, and each channel’s two triodes share one 220 kΩ plate resistor. The two volumes meet, then 470 kΩ runs to the 6SC7’s grid with the tone network around it: Treble a pot between a .001 µF from the mixing node and ground, its wiper on the grid through another .001 µF, and Bass a 1 MΩ variable resistor with .001 µF across it, under 100 kΩ from the grid. The 6SC7 is a paraphase inverter into two 6L6GAs in cathode bias with no feedback. At the default settings a steady A2 puts about 7.5 V on the 4 Ω load, about 14 W, at under 2 % harmonics. The 6L6GA runs on the 6L6GC’s model.

Download.

Parts and what they do

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

.05µ, 50 nF
Input cap from jack 1. With nothing plugged in, the jack’s switch grounds the tip.
10M, 10 MΩ
Grid leak, 10 MΩ. The cathodes are grounded, so the grid biases itself a little negative from the grid current through it.
V1a, 12AY7 / 6072
One half of the first 12AY7, for jack 1. Its plate is tied to the jack 2 triode’s.
.05µ, 50 nF
Input cap from jack 2. Jack 2 is empty on the bench, so its switch grounds this end.
10M, 10 MΩ
Grid leak for jack 2’s triode.
V1b, 12AY7 / 6072
The first 12AY7’s other half, for jack 2. Empty, it still pulls current and loads the shared plate.
220k, 220 kΩ
Plate resistor for the instrument channel’s two triodes. The table reads +40 V on the plates.
.05µ, 50 nF
Input cap from jack 3. With nothing plugged in, the jack’s switch grounds the tip.
10M, 10 MΩ
Grid leak, 10 MΩ.
V2a, 12AY7 / 6072
One half of the second 12AY7, for jack 3. Its plate is tied to the jack 4 triode’s.
.05µ, 50 nF
Input cap from jack 4. Jack 4 is empty on the bench, so its switch grounds this end.
10M, 10 MΩ
Grid leak for jack 4’s triode.
V2b, 12AY7 / 6072
The second 12AY7’s other half, for jack 4.
220k, 220 kΩ
Plate resistor for the mic channel’s two triodes. The table reads +40 V on the plates.
.05µ, 50 nF
Coupling cap to the instrument volume’s wiper.
Vol 1M, 1 MΩ pot, audio taper
Instrument volume, 1 MΩ. The signal comes in on the wiper and leaves from the top end, the mixing node. The taper isn’t marked, so audio is assumed.
.01µ, 10 nF
Coupling cap to the mic volume’s wiper, smaller than the instrument channel’s, so less bass gets through.
250p, 250 pF
Bright cap, .00025 µF from the mic volume’s wiper to its top. Below full volume it lets treble past the pot’s upper part.
Vol 1M, 1 MΩ pot, audio taper
Mic volume, 1 MΩ, wired like the instrument volume. The taper isn’t marked, so audio is assumed.
470k, 470 kΩ
From the mixing node to the inverter’s grid. With the bass leg below the grid it’s a divider for the lows, and the treble caps go around it.
.001µ, 1 nF
Treble cap from the mixing node to the treble pot’s top end.
Treble, 1 MΩ pot, audio taper
Treble, 1 MΩ. Its wiper goes through .001 µF to the inverter’s grid. Turned up, the wiper moves toward the cap from the mixing node, so treble goes around the 470 kΩ. Turned down, it moves toward ground, and the .001 µF bleeds treble off the grid. The taper isn’t marked, so audio is assumed.
.001µ, 1 nF
From the treble wiper to the inverter’s grid.
100k, 100 kΩ
The bass leg’s top, from the inverter’s grid. Below it the bass pot and a .001 µF go to ground.
.001µ, 1 nF
Across the bass pot. It shorts the treble at the bottom of the leg, so the 100 kΩ always cuts the treble some.
Bass, 1 MΩ pot, audio taper
Bass, 1 MΩ as a variable resistor under the 100 kΩ. Turned down, its resistance drops and the leg pulls the grid down at every pitch. Turned up, the lows see over 1 MΩ and come through. The taper isn’t marked, so audio is assumed.
V3a, 6SC7
6SC7, 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.
220k, 220 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.
330k, 330 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.
8.2k, 8.2 kΩ
The tap, 8.2k 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, 6SC7
The second half. It turns the divided signal over again, so its plate swings with the input. It drives tube B.
220k, 220 kΩ
Plate resistor for the second half, the output to tube B.
.05µ, 50 nF
Coupling cap to tube A’s grid.
.05µ, 50 nF
Coupling cap to tube B’s grid.
V4, 6L6GA / 6L6G
6L6GA, tube A of the push-pull pair, its cathode on the shared cathode resistor, so it biases itself.
V5, 6L6GA / 6L6G
6L6GA, tube B of the pair.
330k, 330 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.
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.
7.5k, 7.5 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.
20µ, 20 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 preamp, as on the Gibson GA-55 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 inverter 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 4 Ω, 10 H primary, 10 mH leakage
The output transformer, 5 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.
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, 60 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, 60 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, 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 12AY7 model is fitted to the RCA, GE and JJ plate charts.

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.

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