Tweed tremolo amp with a paraphase inverter, 1957 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 5E9-A Tremolux.

The 5E3 Deluxe’s two-channel 12AY7 front end, a 12AX7 paraphase inverter and two cathode-biased 6V6GTs with no feedback. The tremolo’s oscillator drives a cathode follower that pushes a swinging current into the inverter’s cathode, so the inverter’s gain rises and falls.

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 front end is the 5E3’s: two 12AY7 channels on one cathode resistor, four jacks, and one tone control where the channels meet. The power amp has a paraphase inverter, the 5C3’s form: the 12AX7’s first half drives tube A, and its second half is fed from 6800 Ω under tube A’s 220 kΩ grid resistor. No feedback. The tremolo works on that inverter. A cathode follower, about 220 V up, copies the oscillator’s swing and pushes a current through the Depth pot into the inverter’s shared cathode resistor. More current raises the cathode and cuts the inverter’s gain, so as the current swings, the volume rises and falls, about 4 to 9 times a second. Depth sets the current: the cathode sits at about 2.3 V with Depth at 0 and 4.5 V at full, near the drawing’s 2.1 and 4.5 V. On a clean A2 the level swings about 0.8 dB at 0, 2 dB at 5 and close to 30 dB at full, where the inverter nearly cuts off at the bottom of each swing. The tremolo works only with the Tremolux power amp on. The transformers are typical figures, not measured.

Download.

Parts and what they do

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

R1A, 68 kΩ
The instrument channel’s input 1 resistor into the grid. With nothing in input 2 it sits in parallel with input 2’s resistor, so a guitar in input 1 sees 34 kΩ to the grid: enough to stop radio pickup, not enough to cut treble.
R1B, 68 kΩ
The instrument channel’s input 2 resistor into the grid. With nothing in input 1, input 1’s tip is grounded, so the two 68 kΩ make a divider: input 2 is about 6 dB quieter and loads the guitar with 136 kΩ.
RG1, 1 MΩ
Input 1’s 1 MΩ to ground. It sets the load a guitar in input 1 sees, and with a patch lead in input 1 it is the end of the lead’s path.
R1C, 68 kΩ
The mic channel’s input 1 resistor into the grid. With nothing in input 2 it sits in parallel with input 2’s resistor, so a guitar in input 1 sees 34 kΩ to the grid: enough to stop radio pickup, not enough to cut treble.
R1D, 68 kΩ
The mic channel’s input 2 resistor into the grid. With nothing in input 1, input 1’s tip is grounded, so the two 68 kΩ make a divider: input 2 is about 6 dB quieter and loads the guitar with 136 kΩ.
RG2, 1 MΩ
Input 1’s 1 MΩ to ground. It sets the load a guitar in input 1 sees, and with a patch lead in input 1 it is the end of the lead’s path.
V1a, 12AY7 / 6072
First half of the 12AY7, the instrument channel’s input stage. With a mu of about 45 it gains about 25. The drawing reads +150 V on the plate.
100k, 100 kΩ
Plate resistor for the instrument channel.
V1b, 12AY7 / 6072
Second half of the 12AY7, the mic channel. With nothing plugged in it still pulls current through the shared cathode resistor, and its plate still hangs on the tone network through the mic volume.
100k, 100 kΩ
Plate resistor for the mic channel.
820, 820 Ω
Cathode resistor shared by both 12AY7 halves, +1.4 V on the drawing. It carries both currents, so each half sees about twice its value.
25µ, 25 µF electrolytic
Cathode bypass for the 12AY7, full gain down to about 15 Hz.
.1µ, 100 nF
Coupling cap from the instrument channel’s plate to its volume pot’s wiper, .1 µF 400 V.
Vol 1M, 1 MΩ pot, audio taper
Instrument channel volume, 1 MΩ. The signal comes in on the wiper. The track above it is in series with the next grid, and the track below shunts signal to ground. There’s no master, so it sets how hard the inverter and output tubes are pushed.
500p, 500 pF
Bright cap, 500 pF mica, from the instrument channel ahead of its volume to the tone pot. It carries treble around the volume, so the channel gets brighter turned down.
Tone 1M, 1 MΩ pot, audio taper
Tone, 1 MΩ, wiper on the second stage’s grid. Up puts the grid near the 500 pF. Down puts the .005 µF to ground on it, which cuts treble. Clockwise is brighter, as on every amp on the bench.
.005µ, 5 nF
Treble cut cap, .005 µF 600 V to ground from the other end of the tone pot.
Vol 1M, 1 MΩ pot, audio taper
Mic channel volume. It matters even with nothing plugged in: turned up, it hangs the mic channel’s plate on the second grid through less track, which loads the instrument channel.
.1µ, 100 nF
The mic channel’s coupling cap, .1 µF 400 V.
220k, 220 kΩ
Plate resistor for the oscillator, from the screen node.
V3b, 12AX7 / ECC83 / 7025
Half of a 12AX7, the tremolo’s oscillator. The three caps after its plate turn the signal around at one frequency, and it runs at that frequency, a few times a second.
3300, 3.3 kΩ
Cathode resistor for the oscillator.
25µ, 25 µF electrolytic
Bypass for the oscillator’s cathode. At a few hertz it isn’t a full short, and it adds some of the phase shift that keeps the loop oscillating.
.02µ, 20 nF
First ladder cap, .02 µF 400 V.
Speed 2M, 2 MΩ pot, linear taper
Speed, 2 MΩ, the ladder’s first leg with the 220 kΩ on its wiper. Turned up, the wiper moves toward the ladder, so less track sits in series and the oscillator runs faster. The drawing doesn’t mark the taper or the direction, so linear and up faster are assumed.
220k, 220 kΩ
From the Speed pot’s wiper to ground. With Speed on 10 it’s across the whole pot, and it sets the top speed.
.01µ, 10 nF
Second ladder cap.
1M, 1 MΩ
Middle leg of the ladder, to ground. The footswitch grounds this point.
10M, 10 MΩ
From the 290 V node to the ladder’s middle. With the 1 MΩ below it, it holds that point at about +25 V, as the layout reads with the footswitch open, so the caps there are already charged when the switch opens.
.01µ, 10 nF
Third ladder cap, into the grid.
1M, 1 MΩ
Grid leak for the oscillator, and the ladder’s last leg.
V3a, 12AX7 / ECC83 / 7025
The other half of V3, a cathode follower straight off the oscillator’s plate. Its cathode sits a volt or so above the oscillator’s plate, about 220 V, and copies the swing, so the Depth network barely loads the oscillator.
100k, 100 kΩ
From the follower’s cathode to the Depth pot’s wiper, across the track above the wiper. At full Depth it’s all that’s left in the path, with the whole pot beside it, about 71 kΩ, so it sets the most current.
Depth 250k, 250 kΩ pot, linear taper
Depth, 250 kΩ, between the follower’s cathode and the inverter’s cathode. The follower sits about 220 V up, so a current of about 0.9 to 3 mA flows into the inverter’s 1500 Ω and raises its cathode, which cuts its gain, and the oscillator swings that current. Turned up, the resistance drops from 250 kΩ to 71 kΩ, so more current flows and the swing is deeper. The layout reads +2.1 V on the inverter’s cathode at minimum and +4.5 V at maximum.
V2a, 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.
220k, 220 kΩ
Plate resistor for the first half, the output to tube A.
1.5k, 1.5 kΩ
Cathode resistor for both halves. The tube has one cathode for its two triodes, so this carries both currents and sets both biases.
25µ, 25 µF electrolytic
Bypass cap across the shared cathode resistor, for full gain above a few hertz.
220k, 220 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.
6.8k, 6.8 kΩ
The tap, 6.8k 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.
V2b, 12AX7 / ECC83 / 7025
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.
100p, 100 pF
A small cap from plate to plate. It rolls off the far top end the inverter passes on.
.02µ, 20 nF
Coupling cap to tube A’s grid.
.02µ, 20 nF
Coupling cap to tube B’s grid.
V4, 6V6GT
6V6GT, tube A of the push-pull pair, its cathode on the shared cathode resistor, so it biases itself.
V5, 6V6GT
6V6GT, tube B of the pair.
220k, 220 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.
2.5k, 2.5 kΩ
Dropping resistor from B+ to the screen supply. The screens sit slightly below the plates.
16µ, 16 µF electrolytic
Filter cap on the screen supply.
22k, 22 kΩ
Dropping resistor from the screen supply node to the node for the inverter and preamp, as on the Fender 5E9-A Tremolux 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, 8 kΩ to 8 Ω, 25 H primary, 220 mH leakage
The output transformer, 8 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, 344 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 344 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, 344 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, not measured.
PT R, 60 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 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, 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
The tube models are fitted to their data sheets: RCA 12AY7, Brimar 12AX7, Tung-Sol 6V6GT. The 5U4GA runs on the 5U4G model. The transformers are typical figures, 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.

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.

Product and company names are trademarks of their owners. They are used here only to identify the circuits these models are based on; this site is not affiliated with or endorsed by any of them.

From Swell Amplification, swellamps.com.