One-stage 6L6GC bass amp, 1966

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 Supro Thunderbolt S6420.

One 12AX7 stage, a Volume and a bridged Tone network, then a paraphase inverter and two cathode-biased 6L6GCs off a silicon bridge.

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. One 12AX7 stage with its cathode bypassed, then the Volume, then a Tone pot with .003 µF on one end and 150 kΩ on the other, the 150 kΩ end going to ground through .01 µF and 110 kΩ. Turned up, the wiper passes more treble through the .003 µF. Turned down, the 150 kΩ and .01 µF roll the top off. The .001 µF on the plate trims the far top end. The Tone’s wiper drives the inverter directly. The tube model is typical of the type, not measured.

Download.

Parts and what they do

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

47k, 47 kΩ
Input 1’s resistor. With nothing in input 2, that jack’s 47 kΩ hangs open.
47k, 47 kΩ
Input 2’s resistor, the same as input 1’s.
.005µ, 5 nF
Input cap. Into the 270 kΩ grid resistor it rolls off below about 120 Hz, so a bass’s lowest notes come in slightly low.
270k, 270 kΩ
Grid resistor for V1a.
V1a, 12AX7 / ECC83 / 7025
Half of a 12AX7, the only gain stage ahead of the inverter. The other half isn’t used.
3.3k, 3.3 kΩ
Cathode resistor for V1a, 3.3 kΩ on two of the redraws (3.9 kΩ on the third). It biases the stage slightly cold for a 12AX7.
35µ, 35 µF electrolytic
Bypass for V1a’s cathode, full gain down to a couple of hertz.
100k, 100 kΩ
Plate resistor for V1a, from node D.
.001µ, 1 nF
From the plate to ground. With the plate’s own resistance it trims the top above about 4 kHz.
.05µ, 50 nF
Coupling cap to the Volume.
Vol 500k, 500 kΩ pot, audio taper
Volume, 500 kΩ audio, as marked. There’s no master, so it sets how hard the inverter and the 6L6GCs are pushed.
.003µ, 3 nF
From the Volume’s wiper to the Tone pot’s bright end. It passes the treble.
150k, 150 kΩ
From the Volume’s wiper to the Tone pot’s dark end. With the .01 µF below it, a low-pass.
.01µ, 10 nF
At the Tone pot’s dark end, to ground. With the 150 kΩ it rolls the top off above about 100 Hz.
110k, 110 kΩ
At the Tone pot’s dark end, to ground: 110 kΩ on two of the redraws (100 kΩ on the third). With the 150 kΩ it’s a divider, so the dark end is slightly quieter. It’s the inverter grid’s DC path to ground.
Tone 500k, 500 kΩ pot, audio taper
Tone, 500 kΩ audio, as marked. Its wiper is the inverter’s grid. Turned up, the wiper moves toward the .003 µF and the top end comes up. Which way it turns isn’t marked, so brighter clockwise is assumed.
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.
270k, 270 kΩ
Plate resistor for the first half, the output to tube A.
3.9k, 3.9 kΩ
Cathode resistor for the first half. It sets its bias, a volt or so.
270k, 270 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.
12k, 12 kΩ
The tap, 12k 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.
270k, 270 kΩ
Plate resistor for the second half, the output to tube B.
2.2k, 2.2 kΩ
Cathode resistor for the second half, straight to ground.
35µ, 35 µF electrolytic
Bypass cap across the second half’s cathode resistor.
.05µ, 50 nF
Coupling cap to tube A’s grid.
.05µ, 50 nF
Coupling cap to tube B’s grid.
47k, 47 kΩ
Grid stopper for tube A. It keeps the tube from oscillating and softens grid current.
47k, 47 kΩ
Grid stopper for tube B.
V3, 6L6GC
6L6GC, tube A of the push-pull pair, its cathode on the shared cathode resistor, so it biases itself.
V4, 6L6GC
6L6GC, tube B of the pair.
470k, 470 kΩ
Grid leak for tube B.
200, 200 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.
1k, 1 kΩ
Dropping resistor from B+ to the screen supply. The screens sit slightly below the plates.
30µ, 30 µF electrolytic
Filter cap on the screen supply.
27k, 27 kΩ
Dropping resistor from the screen supply node to the node for the inverter, as on the Supro Thunderbolt S6420 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.
10µ, 10 µF electrolytic
Filter cap at the node for the inverter. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
100k, 100 kΩ
Dropping resistor from the node for the inverter to the node for the preamp, as on the Supro Thunderbolt S6420 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.
4µ, 4 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, 4.3 kΩ to 8 Ω, 30 H primary, 10 mH leakage
The output transformer, 4.3 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, 293 V ac, 60 Hz
The power transformer’s high-voltage winding, 293 V ac end to end, into a bridge of four silicon diodes with the center tap unused. It’s drawn as two sources of 293 V ac a half cycle apart into two diodes, which charges the reservoir the same way at one diode drop (under a volt) higher. The value is typical of the part, not measured.
PT, 293 V ac, 60 Hz
The same winding on the other half cycle. It runs opposite the first source, so the bridge charges the reservoir on every half cycle.
PT R, 30 Ω
Resistance of the whole winding, plus the primary’s share. In a bridge every charging pulse runs through the whole winding. With the diodes’ drop it sets how far B+ sags on a hard note. Typical of the part, not measured.
PT R, 30 Ω
The same winding’s resistance, on the other half cycle. Typical of the part, not measured.
D1, 1N4001
One of the silicon rectifier diodes. It drops under a volt, so B+ holds up better under load than with a tube rectifier.
D2, 1N4001
The other rectifier diode, 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.

About this version

As drawn
The 12AX7 model is fitted to its data sheet (Brimar). Values as two of the three redraws have them.

Sources

  • C.J. Poulos, hand-drawn schematic, 2004, and corrections, 2006
  • Sean Weatherford, redrawn schematic, 2003
  • Rik Shoup, redrawn schematic

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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