Push-pull 6L6 amp with a paraphase inverter and bass and treble, 1953 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 5C8 Twin.

The first Twin: Instrument and Microphone channels on a 6SC7 each, grid-leak biased, mixed into a 6J5 gain stage and a bass and treble stack, then a 6SC7 paraphase into two cathode-biased 6L6s with a little feedback, a 5U4G and two 12 in speakers.

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 two channels work as on the 5C5 Pro: each jack on its own half of a 6SC7, grids biased by 5 MΩ grid leaks, both halves on one plate resistor. The volumes are wired the usual way and meet through 75 kΩ each at a 6J5, which makes up the level lost in the tone stack. Treble carries the top end around the stack through 250 pF. Bass is a 2 MΩ pot across a .005 µF cap to ground in the middle of a 250 kΩ ladder: more resistance lets the cap shunt the treble and leaves the bass. The inverter is the 5C5’s with a 1 MΩ from the speaker line to tube A’s grid. The transformers are typical figures, not measured.

Download.

Parts and what they do

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

.1µ, 100 nF
Input cap from the instrument channel’s first jack to its grid, .1 µF 200 V. The grid has no cathode resistor to sit against, so the cap keeps the 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
The instrument channel’s first half of the 6SC7, the one the first jack feeds. One cathode serves both halves, tied straight to ground. A mu of about 70.
.1µ, 100 nF
Input cap from the instrument channel’s second jack, .1 µF 200 V. That jack is empty, so its switch grounds the tip and the grid sits at signal ground.
5M, 5 MΩ
The second half’s 5 MΩ grid leak.
V1b, 6SC7
The instrument channel’s second half, its plate tied to the first half’s. With its jack empty it still pulls current through the shared plate resistor and loads the plate.
250k, 250 kΩ
Plate resistor for both halves of the instrument channel’s 6SC7, 250 kΩ from the preamp node.
.05µ, 50 nF
Coupling cap from the instrument channel’s plates to the top of its volume, .05 µF 600 V.
Vol 1M, 1 MΩ pot, audio taper
Instrument volume, 1 MΩ, wired the usual way: signal on top, bottom to ground, the wiper out through 75 kΩ to the 6J5’s grid. The drawing doesn’t mark the taper, so audio is assumed.
75k, 75 kΩ
Mixing resistor, 75 kΩ from the instrument volume’s wiper to the 6J5’s grid. With the other channel’s 75 kΩ it keeps one volume turned down from grounding the other channel.
.1µ, 100 nF
Input cap from the mic channel’s first jack to its grid, .1 µF 200 V. The grid has no cathode resistor to sit against, so the cap keeps the 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.
V2a, 6SC7
The mic channel’s first half of the 6SC7, the one the first jack feeds. One cathode serves both halves, tied straight to ground. A mu of about 70.
.1µ, 100 nF
Input cap from the mic channel’s second jack, .1 µF 200 V. That jack is empty, so its switch grounds the tip and the grid sits at signal ground.
5M, 5 MΩ
The second half’s 5 MΩ grid leak.
V2b, 6SC7
The mic channel’s second half, its plate tied to the first half’s. With its jack empty it still pulls current through the shared plate resistor and loads the plate.
250k, 250 kΩ
Plate resistor for both halves of the mic channel’s 6SC7, 250 kΩ from the preamp node.
.05µ, 50 nF
Coupling cap from the mic channel’s plates to the top of its volume, .05 µF 600 V.
Vol 1M, 1 MΩ pot, audio taper
Mic volume, 1 MΩ, wired the usual way: signal on top, bottom to ground, the wiper out through 75 kΩ to the 6J5’s grid. The drawing doesn’t mark the taper, so audio is assumed.
75k, 75 kΩ
Mixing resistor, 75 kΩ from the mic volume’s wiper to the 6J5’s grid. With the other channel’s 75 kΩ it keeps one volume turned down from grounding the other channel.
100p, 100 pF
Bright cap, 100 pF (.0001 µF), across the top of the instrument volume. Turned down, the treble gets around the pot, so the channel gets brighter as it’s turned down. The mic volume has none.
V3, 6J5
The 6J5, a single triode with a mu of about 20. It makes up the level lost in the tone stack.
1500, 1.5 kΩ
The 6J5’s cathode resistor, 1500 Ω. It sets the bias, about 5 V.
250µ, 250 µF electrolytic
Bypass across the 6J5’s cathode resistor, 250 µF 6 V, for full gain down to the lowest notes.
100k, 100 kΩ
The 6J5’s plate resistor, 100 kΩ from the preamp node.
250p, 250 pF
Treble cap, 250 pF (.00025 µF), from the 6J5’s plate to the treble pot. It carries the top end around the bass ladder.
Treble 1M, 1 MΩ pot, audio taper
Treble, 1 MΩ, its wiper on the inverter’s grid. Clockwise moves the wiper toward the 250 pF from the plate. Counterclockwise toward the .005 µF to ground, which bleeds treble off the grid. The drawing marks no taper or direction. Audio taper, clockwise brighter.
.005µ, 5 nF
From the treble pot’s far end to ground, .005 µF 600 V.
.05µ, 50 nF
Coupling cap from the 6J5’s plate into the bass ladder, .05 µF 600 V.
1M, 1 MΩ
1 MΩ to ground at the ladder’s input. It keeps the coupling cap’s far end at ground for DC.
250k, 250 kΩ
The bass ladder’s first 250 kΩ.
47k, 47 kΩ
From the middle of the ladder down to the bass cap and pot, 47 kΩ. With the pot turned all the way down it’s a plain 47 kΩ to ground, which cuts every frequency about 16 dB.
.005µ, 5 nF
The bass cap, .005 µF 600 V to ground. With the pot’s resistance across it turned up, it shunts the treble and lets the bass through.
Bass 2M, 2 MΩ pot, audio taper
Bass, 2 MΩ as a variable resistor across the bass cap, its wiper on the 47 kΩ’s foot, one end to ground and the other free. Clockwise adds resistance, so the cap does the shunting and the bass comes up against the treble. Counterclockwise shorts the cap and the 47 kΩ lowers everything evenly. The drawing marks no taper or direction. Audio taper, clockwise more bass.
250k, 250 kΩ
The bass ladder’s second 250 kΩ, into the inverter’s grid.
V4a, 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.
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.
V4b, 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.
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.
V5, 5881
n5881, tube A of the push-pull pair, its cathode on the shared cathode resistor, so it biases itself.
V6, 5881
n5881, 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.
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 5C8 Twin 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 4 Ω, 40 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, 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.
V7, 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.
V7, 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, 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. The feedback holds the output steadier against that than it would be without feedback.
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. The feedback holds the output steadier against that than it would be without feedback.
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

5C8 Twin
The 5C8 drawing. The 6SC7 model is fitted only to the type’s published operating point, with curves borrowed from the 6SL7 away from it. The 6J5 uses the 6SN7 model, one section in its own bottle. The 6L6s run on the 5881 model, as the drawing allows. 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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