Push-pull 6V6 amp with a paraphase inverter, 1952 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 5B3 Deluxe.

The Deluxe before the 5C3, almost the same amp. Two 6SC7s do all the small-signal work: the first is the two input channels, with grid-leak bias, and the second is a gain stage plus a paraphase inverter, fed from a tap on the output tube’s grid resistor. No feedback. Two cathode-biased 6V6GTs and a 5Y3GT rectifier.

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 guitar goes in through a 50 kΩ divider, so the first stage gets half the signal and the pickup sees 100 kΩ, which rolls off some top end right away. The mic input, on the Input menu, skips the divider and goes straight to the other half of the first tube. The first tube has no cathode resistor: its grids bias themselves through 5 MΩ. The inverter’s second half is fed from a tap on tube A’s grid resistor, 7000 Ω under 250 kΩ, so the output tubes only match while that share and the second half’s gain line up. There’s no feedback, where the 5C3 has a 1 MΩ from the speaker. The transformers are typical figures, not measured.

Download.

Parts and what they do

Stock values for the 5B3 version. Tap a part on the schematic to change it.

50k, 50 kΩ
The 50 kΩ behind the instrument jack the guitar is in (75 kΩ on the 5A3). With the other jack’s resistor to ground it halves the signal, and the pickup sees 100 kΩ, so some treble is lost right at the input. With the guitar in the mic jack, this jack is empty and its tip grounded.
50k, 50 kΩ
The other instrument jack’s 50 kΩ (75 kΩ on the 5A3). With nothing plugged in, that jack grounds its tip, so this goes to ground. Plug a second guitar in there and the two mix.
.05µ, 50 nF
Input cap, .05 µF 600 V. The grid has no cathode resistor to sit against, so this cap keeps the input jack’s DC off it.
5M, 5 MΩ
Grid leak, 5 MΩ. The cathode is grounded, so the grid biases itself: a trickle of grid current through 5 MΩ holds it near −1 V. Big peaks pull more grid current, and the bias walks colder for a moment.
V1a, 6SC7
First half of the 6SC7, the instrument channel. The 6SC7 has one cathode for both triodes, tied straight to ground. A mu of about 70.
250k, 250 kΩ
Plate resistor for the instrument channel. A large value for a low-current tube, so the stage gains about 50.
.05µ, 50 nF
The mic jack’s input cap, .05 µF 600 V, from the jack’s tip to the grid. With nothing plugged in, the tip is grounded. A guitar here sees only the 5 MΩ grid leak, so it keeps all its treble and the full signal reaches the grid.
5M, 5 MΩ
The mic channel’s 5 MΩ grid leak.
V1b, 6SC7
Second half of the first 6SC7, the mic channel. With nothing plugged in, its plate still hangs on the mixing node through the mic volume. With the guitar in the mic jack it gets the whole signal, about 6 dB more than the instrument channel gets through its divider.
250k, 250 kΩ
Plate resistor for the mic channel.
.05µ, 50 nF
Coupling cap from the instrument channel’s plate to its volume pot’s wiper, .05 µF 600 V.
Vol 1M, 1 MΩ pot, audio taper
Instrument channel volume, 1 MΩ audio. The signal comes in on the wiper. The track toward the mixing node is in series, and the track to ground shunts it.
500p, 500 pF
Treble cap, 500 pF, 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 mixing node. Up puts the node near the 500 pF. Down puts the .01 µF to ground on it, which cuts treble. Clockwise is brighter, as on every amp on the bench.
.01µ, 10 nF
Treble cut cap, .01 µF to ground from the other end of the tone pot, twice the 5C3’s, so Tone down cuts deeper. The 5A3 has .005 µF, as the 5C3 does.
Vol 1M, 1 MΩ pot, audio taper
Mic channel volume. At 0 it grounds the mic channel’s output, leaving its whole 1 MΩ from the mixing node to ground. Turned up, there’s less track between the mic channel’s plate and the mixing node, and it loads the instrument channel more.
.05µ, 50 nF
The mic channel’s coupling cap.
V2a, 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.
250k, 250 kΩ
Plate resistor for the first half, the output to tube A.
2.5k, 2.5 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.
250k, 250 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.
7k, 7 kΩ
The tap, 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.
V2b, 6SC7
The second half. It turns the divided signal over again, so its plate swings with the input. It drives tube B.
250k, 250 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.
V3, 6V6GT
6V6GT, tube A of the push-pull pair, its cathode on the shared cathode resistor, so it biases itself.
V4, 6V6GT
6V6GT, tube B of the pair.
250k, 250 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.
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 Fender 5B3 Deluxe 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, 8 kΩ to 8 Ω, 10 H primary, 10 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, 325 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 325 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, 325 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.
V5, 5Y3GT
One plate of the 5Y3GT 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.
V5, 5Y3GT
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, 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.

Versions

5B3
The 6SC7 model is fitted only to the type’s published operating point, with curves borrowed from the 6SL7 away from it. The 6V6GT and 5Y3GT models are fitted to their data sheets. The transformers are typical figures, not measured.
5A3, modeled after the Fender 5A3 Deluxe
The 5A3 drawing, the Deluxe before the 5B3: the same circuit, with 75 kΩ input resistors in place of 50 kΩ and a .005 µF tone cap in place of .01 µF (both as on the 5C3), and 16 µF filter caps in place of 20 µF. The copies in hand carry typed notes where the scan was unreadable. They disagree on the tap under tube A’s grid resistor (7 k or 7.5 k, and 7 k is used) and on the second tube (6SC7 is used, as Fender lettered it). 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.

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