Pentode-input 6L6 amp with a 6N7 paraphase and a field-coil speaker, 1946 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 Pro (1946).

The first Fender Pro, before the tweeds. A 6SJ7 pentode for the mic and one for the two instrument jacks, straight into the volumes, a 6N7 paraphase with a treble cut on its first plate, two 6L6s on an unbypassed cathode resistor, and a 15 in speaker whose field coil hangs across B+.

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. Both input tubes are 6SJ7 pentodes, with their plates on 150 kΩ from a low node, so each gains a lot at low voltage. The instrument jacks go in through 100 kΩ each. The signal comes off the volumes’ wipers with no mixing resistors, so both channels meet on the 6N7’s grid. The 6N7 is a gain stage and a paraphase inverter, with a 0.5 MΩ and .003 µF treble cut on its first plate. The 6L6s sit on 250 Ω with no bypass cap, which lowers the output stage’s gain, and their screens on a node near 200 V. The speaker’s field coil pulls a steady current across B+. The transformers are typical figures, not measured.

Download.

Parts and what they do

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

2M, 2 MΩ
The mic grid’s resistor to ground, 2 MΩ. The jack goes straight on the grid with no cap.
V1, 6SJ7
The mic channel’s 6SJ7, a small pentode. With its screen held low and a 150 kΩ plate resistor it gains well over 100.
150k, 150 kΩ
The mic tube’s plate resistor, 150 kΩ from node C.
.02µ, 20 nF
Coupling cap from the mic tube’s plate into its volume’s wiper, .02 µF 600 V.
Mic 500k, 500 kΩ pot, audio taper
Mic volume, 0.5 MΩ. The signal comes in on the wiper. One end goes to ground and the other to the 6N7’s grid, so up moves the wiper toward the grid. Down, the wiper grounds the mic channel. The drawing marks no taper, so audio is assumed.
250p, 250 pF
A cap across the mic volume’s upper track, from its wiper to the 6N7’s grid, so treble gets around the pot when it’s turned down. The drawing shows no value, so a typical 250 pF is assumed.
100k, 100 kΩ
From the first instrument jack to the grid, 100 kΩ. With the 2 MΩ to ground on each jack, it makes a light divider.
2M, 2 MΩ
From the first instrument jack’s tip to ground, 2 MΩ.
100k, 100 kΩ
From the second instrument jack to the grid, 100 kΩ. That jack is empty, so this and its 2 MΩ load the grid with 2.1 MΩ to ground.
2M, 2 MΩ
From the second instrument jack’s tip to ground, 2 MΩ.
V2, 6SJ7
The instrument channel’s 6SJ7. Its cathode and screen are shared with the mic tube.
150k, 150 kΩ
The instrument tube’s plate resistor, 150 kΩ from node C.
1.5k, 1.5 kΩ
Cathode resistor shared by both 6SJ7s, 1.5 kΩ 1 W. It carries both tubes’ plate and screen current.
25µ, 25 µF electrolytic
Bypass across the shared cathode resistor, 25 µF 25 V.
500k, 500 kΩ
Screen resistor for both 6SJ7s, 500 kΩ from node C. It holds the screens low, so the tubes run at a small current and gain a lot.
.05µ, 50 nF
Screen bypass, .05 µF 600 V to ground, so the screens hold still at signal frequencies.
.02µ, 20 nF
Coupling cap from the instrument tube’s plate into its volume’s wiper, .02 µF 600 V.
Inst 500k, 500 kΩ pot, audio taper
Instrument volume, 0.5 MΩ, wired as the mic volume. The drawing marks no taper, so audio is assumed.
V3a, 6N7
6N7, 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.
150k, 150 kΩ
Plate resistor for the first half, the output to tube A.
150, 150 Ω
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.
10k, 10 kΩ
The tap, 10k 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, 6N7
The second half. It turns the divided signal over again, so its plate swings with the input. It drives tube B.
150k, 150 kΩ
Plate resistor for the second half, the output to tube B.
.003µ, 3 nF
Tone cap, .003µF from the first half’s plate to the tone pot. With the pot it bleeds treble to ground.
Tone, 500 kΩ pot, audio taper
Tone, a 500k pot as a variable resistor from the cap to ground. Turned down, its resistance drops and the cap bleeds more treble off the first half’s plate. Tube B’s drive comes from tube A’s grid, so the cut reaches both sides. Turned all the way up, the pot barely loads the plate. On the 1946 Pro the pot is 0.5 MΩ and the cap .003 µF. The drawing marks no taper or direction, so audio is assumed, clockwise brighter, as on every tone control on the bench except a Vox-style Cut.
.02µ, 20 nF
Coupling cap to tube A’s grid.
.02µ, 20 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.
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.
15k, 15 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.
40µ, 40 µF electrolytic
Filter cap on the screen supply.
30k, 30 kΩ
Dropping resistor from the screen supply node to the node for the preamp, as on the Fender 1946 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.
40µ, 40 µF electrolytic
Filter cap at the node for the 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, 420 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 420 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, 420 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.
40µ, 40 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.
Field 7k, 7 kΩ
The speaker’s field coil, 7k Ω from B+ to ground. It’s the speaker’s magnet, an electromagnet fed from the supply, so it pulls a steady 66 mA or so on top of the tubes, and B+ sits lower and sags more. Only its resistance is here. Typical of the part, not measured.
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
Jeff Bober’s redraw, with the inverter’s tap moved under tube A’s grid resistor where the redraw flags an error. The 6SJ7 model is typical of the type. The 6L6s run on the 5881 model. The transformers, rectifier and field coil are typical figures, not measured.

Sources

  • Jeff Bober, hand redraw of a 1946 Pro

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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