Two 6SJ7 channels with bass and treble, late 1940s

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 Gibson GA-50.

A mic channel and an instrument channel, one 6SJ7 pentode each, meet on one bus through their volume pots. A bass and treble network feeds a 6J5 paraphase and two cathode-biased 6L6s.

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 47 kΩ onto a 100 kΩ grid resistor, so about a third of its level is gone before the instrument channel’s 6SJ7. The mic channel’s jack goes straight to the 6SJ7’s grid. Each channel’s plate drives the wiper of its volume pot, and the pots’ top ends join on one bus with no mixing resistors, so each pot loads the other. From the bus, 470 kΩ feeds the inverter’s grid. Treble is a pot between two .001 µF caps: up, they carry treble around the 470 kΩ. Down, they bleed it from the grid. Bass is a pot under 120 kΩ with .001 µF across it. Two 6J5s make the paraphase inverter. From each 6L6’s plate, 330 kΩ and 250 pF feed back to the 6J5 that drives it, which cuts treble in the output stage. The 6L6s run near the tube manuals’ class A1 point, about 270 V on 125 Ω, and deliver about 18 W before they clip. The tube models are typical of the types, and the transformers are typical figures, not measured.

Download.

Parts and what they do

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

47k, 47 kΩ
The instrument jack’s 47 kΩ into the channel. Each of the three jacks has one, so guitars in two jacks mix without shorting each other.
.05µ, 50 nF
Coupling cap into V2. With the 100 kΩ grid resistor it passes everything above about 30 Hz.
100k, 100 kΩ
V2’s grid resistor, only 100 kΩ. With the 47 kΩ ahead of it, it cuts the guitar’s level by about a third before the tube.
V2, 6SJ7
6SJ7 pentode, the instrument channel’s one stage. Its suppressor is tied to the cathode.
2200, 2.2 kΩ
Cathode resistor for V2.
20µ, 20 µF electrolytic
Bypass for V2’s cathode resistor, full gain down to a few hertz.
1M, 1 MΩ
Screen resistor for V2. 1 MΩ holds the screen low, so the plate current is small.
.25µ, 250 nF
Screen bypass cap for V2.
100k, 100 kΩ
Plate resistor for V2.
.1µ, 100 nF
Coupling cap to the instrument volume’s wiper.
Vol .5M, 500 kΩ pot, audio taper
Instrument volume, .5 MΩ. The signal comes in on the wiper and leaves from the top end, which is the bus. Turned down, the wiper shorts the signal to ground. The drawing marks no taper, so audio is assumed.
250p, 250 pF
Bright cap, 250 pF from the wiper to the bus. Below full volume it lets the treble around the pot.
1M, 1 MΩ
The mic stage’s grid resistor, 1 MΩ. With nothing in the jack, the jack’s switch grounds the grid.
V1, 6SJ7
6SJ7 pentode, the mic channel’s one stage. Its grid is the jack’s tip, with no resistor in series, so a guitar here gets a lot more gain than in an instrument jack.
1000, 1 kΩ
Cathode resistor for V1.
20µ, 20 µF electrolytic
Bypass for V1’s cathode resistor.
1M, 1 MΩ
Screen resistor for V1.
.05µ, 50 nF
Screen bypass cap for V1.
270k, 270 kΩ
Plate resistor for V1, 270 kΩ. With the 1 MΩ screen resistor the mic stage has much more gain than the instrument stage.
.02µ, 20 nF
Coupling cap to the mic volume’s wiper.
Vol .5M, 500 kΩ pot, audio taper
Mic volume, .5 MΩ, wired like the instrument volume. The instruction sheet says to turn it all the way off when the mic jack isn’t in use. The drawing marks no taper, so audio is assumed.
470k, 470 kΩ
From the bus to the inverter’s grid. With the bass network below the grid it makes a divider, so most of the level is lost here.
.001µ, 1 nF
Treble cap, .001 µF from the bus to the treble pot.
Treble 1M, 1 MΩ pot, audio taper
Treble, 1 MΩ. Its wiper feeds the grid through .001 µF. Turned up, the wiper moves to the cap from the bus, so treble goes around the 470 kΩ. Turned down, the wiper moves to ground and the second cap bleeds the grid’s treble away. The drawing marks no taper or direction, so audio is assumed, up brighter.
.001µ, 1 nF
Treble cap from the pot’s wiper to the grid.
120k, 120 kΩ
From the grid down to the bass pot. It’s the bottom of the divider when the bass cap shorts the pot.
Bass 1M, 1 MΩ pot, audio taper
Bass, 1 MΩ as a variable resistor to ground. The cap across it shorts it for treble, so its resistance only adds to the divider for bass. Turned up, its resistance rises and more bass reaches the grid. The drawing marks no taper or direction, so audio is assumed, up more bass.
.001µ, 1 nF
Bass cap across the bass pot. Its value isn’t marked on this drawing. The GA-50T drawing marks .001 µF in the same place, so that value is used.
V3, 6J5
6J5, 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.
2.2k, 2.2 kΩ
Cathode resistor for both halves. The tube has one cathode for its two triodes, so this carries both currents and sets both biases.
330k, 330 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.
27k, 27 kΩ
The tap, 27k 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.
V4, 6J5
The second half. It turns the divided signal over again, so its plate swings with the input. It drives tube B.
100k, 100 kΩ
Plate resistor for the second half, the output to tube B.
330k, 330 kΩ
Feedback, 330k Ω from tube A’s plate back to the first half’s plate, which drives tube A, through the cap beside it. The output plate swings the opposite way to the drive, so this lowers the output stage’s gain, more as the pitch rises. It doesn’t reach around the output transformer.
250p, 250 pF
In series with the feedback resistor, 250pF. It blocks the plates’ DC and passes less the lower the pitch, so the feedback works mostly on the treble: above about 1900 Hz the resistor sets it, below that the cap holds it back.
330k, 330 kΩ
The same from tube B’s plate back to the second half’s plate.
250p, 250 pF
The same cap on tube B’s side.
.05µ, 50 nF
Coupling cap to tube A’s grid.
.05µ, 50 nF
Coupling cap to tube B’s grid.
V5, 6L6GC
6L6GC, tube A of the push-pull pair, its cathode on the shared cathode resistor, so it biases itself.
V6, 6L6GC
6L6GC, tube B of the pair.
330k, 330 kΩ
Grid leak for tube B.
125, 125 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.
20µ, 20 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.
10 H, 10 H
The choke from the reservoir cap to B+, as on the amp’s drawing. The output transformer’s center tap, the screens and everything after them hang past it, so all of their current runs through it. It passes DC with little drop. With the cap after it, it would filter ripple, but ripple isn’t modeled here. Typical of the part, not measured.
150, 150 Ω
The choke’s winding resistance, typical, not measured. With the output tubes’ whole current through it, it’s most of the DC drop between the reservoir and B+.
20µ, 20 µF electrolytic
Filter cap on B+ after the choke, where the output transformer’s center tap and the screens hang.
30 H, 30 H
The second choke, 30 H, from B+ after the first choke to the inverter’s node. The drawing marks it #1898. The 30 H is marked on the GA-50T drawing. It passes DC with little drop. With the 10 µF after it, it removes most of the 120 Hz ripple from the inverter’s supply.
600, 600 Ω
The second choke’s winding resistance, 600 Ω, typical, not on the drawing. It’s all the DC drop from B+ to the inverter’s node.
10µ, 10 µF electrolytic
Filter cap at the node for the inverter. With the choke it filters the supply and keeps the stages on this node from coupling to each other through it.
47k, 47 kΩ
Dropping resistor from the node for the inverter to the node for the preamp, as on the Gibson GA-50 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.
10µ, 10 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, 5 kΩ to 8 Ω, 40 H primary, 10 mH leakage
The output transformer, 5 kΩ plate to plate into 8 Ω, center tap on B+ after the choke. 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, 265 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 265 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, 265 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, 40 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, 40 Ω
Resistance of the other half. Typical of the part, not measured.
V7, 5V4G
One plate of the 5V4G 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, 5V4G
The rectifier’s other plate, on the other half cycle.
10µ, 10 µF electrolytic
The reservoir cap. It feeds the choke, and past it the output transformer’s center tap and everything else. 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 6SJ7 model is fitted to the GE sheet. The 6J5 uses the 6SN7 model, and the 6L6 is the bench’s 6L6GC model. The transformers are typical, not on the drawing.

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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