Tweed four-input 6L6 amp with a self-balancing paraphase, 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 Fender 5D5 Pro.

The tweed Pro between the 6SC7 era and the 5E5, with the 5D4 Super as a build. A 12AY7 for each channel, its halves paralleled from jack 1, straight into the volumes and one tone control, then a 12AX7 paraphase that evens itself out from the two output grids, into two cathode-biased 6L6s with a little 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 channel’s 12AY7 has both halves on one 100 kΩ plate resistor, and the signal comes off the volumes’ wipers with no mixing resistors, so the two channels and the tone pot all meet on the inverter’s grid. The inverter’s second half has its grid at the point where tube A’s 220 kΩ and tube B’s 270 kΩ grid resistors meet, 50 kΩ above ground, so it drives tube B with whatever keeps the two grids even. A 1 MΩ from the speaker line to tube A’s grid feeds a little of the output back. The transformers are typical figures, not measured.

Download.

Parts and what they do

Stock values for the 5D5 Pro version. Tap a part on the schematic to change it.

1M, 1 MΩ
Grid resistor for jack 2’s grid, 1 MΩ to ground. With the guitar in jack 1 the two jacks are tied, so the guitar sees this beside the other 1 MΩ, 500 kΩ in all.
1M, 1 MΩ
Grid resistor for jack 1’s grid, 1 MΩ to ground.
V1a, 12AY7 / 6072
Channel 1’s jack 2 half of the 12AY7. With the guitar in jack 1 it runs beside the other half. With the guitar in jack 2 it runs alone.
V1b, 12AY7 / 6072
Channel 1’s jack 1 half of the 12AY7, its plate tied to the other half’s. With the guitar in jack 2 its grid is grounded, and its plate current still flows through the shared plate resistor.
100k, 100 kΩ
Plate resistor shared by both halves of channel 1’s 12AY7, 100 kΩ. Both plate currents run through it, so the plates sit low, near +100 V.
.05µ, 50 nF
Coupling cap from channel 1’s plates into its volume’s wiper, .05 µF 600 V.
Vol 1M, 1 MΩ pot, audio taper
Channel 1 volume, 1 MΩ. The signal comes in on the wiper. One end goes to ground and the other to V3a’s grid, so up moves the wiper toward the grid. Down, the wiper grounds the channel. The drawing doesn’t mark the taper, so audio is assumed.
1M, 1 MΩ
Grid resistor for jack 2’s grid, 1 MΩ to ground. With the guitar in jack 1 the two jacks are tied, so the guitar sees this beside the other 1 MΩ, 500 kΩ in all.
1M, 1 MΩ
Grid resistor for jack 1’s grid, 1 MΩ to ground.
V2a, 12AY7 / 6072
Channel 2’s jack 2 half of the 12AY7. With the guitar in jack 1 it runs beside the other half. With the guitar in jack 2 it runs alone.
V2b, 12AY7 / 6072
Channel 2’s jack 1 half of the 12AY7, its plate tied to the other half’s. With the guitar in jack 2 its grid is grounded, and its plate current still flows through the shared plate resistor.
100k, 100 kΩ
Plate resistor shared by both halves of channel 2’s 12AY7, 100 kΩ. Both plate currents run through it, so the plates sit low, near +100 V.
.05µ, 50 nF
Coupling cap from channel 2’s plates into its volume’s wiper, .05 µF 600 V.
Vol 1M, 1 MΩ pot, audio taper
Channel 2 volume, 1 MΩ. The signal comes in on the wiper. One end goes to ground and the other to V3a’s grid, so up moves the wiper toward the grid. Down, the wiper grounds the channel. The drawing doesn’t mark the taper, so audio is assumed.
470, 470 Ω
Cathode resistor shared by all four input triodes, 470 Ω. It carries all four currents, so each half sees about four times its value.
25µ, 25 µF electrolytic
Bypass for the shared cathode, 25 µF 25 V.
500p, 500 pF
From channel 1’s volume wiper to the bright end of the tone pot, 500 pF (.0005 µF on the drawing).
Tone 1M, 1 MΩ pot, audio taper
Tone, 1 MΩ, its wiper on the inverter’s grid with both volumes. Clockwise moves the wiper toward the 500 pF from channel 1, which carries treble around volume 1. Counterclockwise moves it toward the .005 µF to ground, which cuts treble from both channels. The drawing marks no taper or direction. Audio taper, clockwise brighter.
.005µ, 5 nF
From the tone pot’s far end to ground, .005 µF.
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.
100k, 100 kΩ
Plate resistor for the first half, the output to tube A.
1.5k, 1.5 kΩ
Cathode resistor for the first half. It sets its bias, a volt or so.
25µ, 25 µF electrolytic
Bypass cap across the first half’s cathode resistor, for full gain above a few hertz.
220k, 220 kΩ
Tube A’s grid resistor, 220k Ω down to the tap. Tube B’s grid resistor comes up to the same tap, and the tap is the second half’s grid. The two output grids swing opposite ways, so the tap gets only what’s left over when they don’t match. The second half turns that over and drives tube B with it, so the inverter keeps the two sides close to even as the tubes age.
50k, 50 kΩ
The tap, 50k Ω to ground, and the second half’s grid resistor. Both output tubes’ grid resistors return through it, so it’s part of each one’s grid leak.
V3b, 12AX7 / ECC83 / 7025
The second half. Its grid is the tap between the two output grids. It turns what’s there over and drives tube B, so tube B’s grid swings opposite tube A’s.
100k, 100 kΩ
Plate resistor for the second half, the output to tube B.
1.5k, 1.5 kΩ
Cathode resistor for the second half, straight to ground.
100p, 100 pF
A small cap from plate to plate. It rolls off the far top end the inverter passes on.
1M, 1 MΩ
Feedback, 1M Ω from the speaker to tube A’s grid. The first half’s plate drives that grid through its own plate resistance, a few tens of kΩ, so only a little feedback gets in. Through the tap it reaches the second half too.
.05µ, 50 nF
Coupling cap to tube A’s grid.
.05µ, 50 nF
Coupling cap to tube B’s grid.
V4, 5881
n5881, tube A of the push-pull pair, its cathode on the shared cathode resistor, so it biases itself.
V5, 5881
n5881, tube B of the pair.
270k, 270 kΩ
Tube B’s grid resistor, 270k Ω up to the tap, the inverter’s second grid, which sits near ground. With tube A’s resistor and the one under the tap, it sets how much of tube B’s swing reaches the second half’s grid.
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.
16µ, 16 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 Fender 5D5 Pro 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.
16µ, 16 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, 4 kΩ to 8 Ω, 40 H primary, 10 mH leakage
The output transformer, 4 kΩ plate to plate 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, 350 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 350 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, 350 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, 50 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, 50 Ω
Resistance of the other half. Typical of the part, not measured.
V6, 5U4G
One plate of the 5U4G 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, 5U4G
The rectifier’s other plate, on the other half cycle.
16µ, 16 µF electrolytic
The reservoir caps’ first. 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.
16µ, 16 µF electrolytic
Another reservoir cap, in parallel with the first.
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. The feedback holds the output steadier against that than it would be without feedback.
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. The feedback holds the output steadier against that than it would be without feedback.
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.

Versions

5D5 Pro
The 5D5 drawing: a 12AY7 for each channel, both halves on the same plate resistor, the volumes and tone pot straight on the inverter’s grid. The 6L6Gs run on the 5881 model, the closest model on the bench. The tube models are fitted to their data sheets, not measured. The transformers are typical figures.
5D4 Super, modeled after the Fender 5D4 Super
The 5D4 Super’s drawing is the 5D5’s part for part, with two speakers in parallel on the secondary, a 2x10. On the bench it stays on the 5D5’s matched 8 Ω tap, so the plates see the same load. The output transformer is typical, not measured.

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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© 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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