Two-channel 6L6 amp with bass, treble and loudness, mid 1960s

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-75 (1960s).

Two 6EU7 channels, each a gain stage, a Loudness pot, a passive bass and treble network and a second stage, mixed through 100 kΩ each into a 6C4 cathodyne. A 6FQ7 pair drives two 6L6GCs on fixed bias.

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. Each channel has two 6EU7 stages with full cathode bypass and a passive bass and treble network between them, so there’s plenty of gain: about 60 times in the first stage and over 30 in the second, before the network’s loss. Loudness is the channel’s volume. The two channels mix through 100 kΩ each into a 6C4 cathodyne with no driver ahead of it, then a 6FQ7 pair on one cathode resistor drives two 6L6GCs on about -50 V of fixed bias, at about 470 V. There’s no feedback in this model (the drawing doesn’t settle it). The tube models are typical of the types, the transformers typical figures, not measured.

Download.

Parts and what they do

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

R2, 68 kΩ
The channel i channel’s J2 (the high jack) resistor into the grid. With nothing in J1 (the low jack) it sits in parallel with J1 (the low jack)’s resistor, so a guitar in J2 (the high jack) sees 34 kΩ to the grid: enough to stop radio pickup, not enough to cut treble.
R1, 68 kΩ
The channel i channel’s J1 (the low jack) resistor into the grid. With nothing in J2 (the high jack), J2 (the high jack)’s tip is grounded, so the two 68 kΩ make a divider: J1 (the low jack) is about 6 dB quieter and loads the guitar with 136 kΩ.
R3, 1 MΩ
J2’s 1 MΩ to ground. It sets the load a guitar in J2 (the high jack) sees, and with a patch lead in J2 (the high jack) it is the end of the lead’s path.
R33, 68 kΩ
The channel ii channel’s J2 (the high jack) resistor into the grid. With nothing in J1 (the low jack) it sits in parallel with J1 (the low jack)’s resistor, so a guitar in J2 (the high jack) sees 34 kΩ to the grid: enough to stop radio pickup, not enough to cut treble.
R32, 68 kΩ
The channel ii channel’s J1 (the low jack) resistor into the grid. With nothing in J2 (the high jack), J2 (the high jack)’s tip is grounded, so the two 68 kΩ make a divider: J1 (the low jack) is about 6 dB quieter and loads the guitar with 136 kΩ.
R34, 1 MΩ
J2’s 1 MΩ to ground. It sets the load a guitar in J2 (the high jack) sees, and with a patch lead in J2 (the high jack) it is the end of the lead’s path.
V1b, 6EU7
One half of the first 6EU7, channel I’s input stage.
100k, 100 kΩ
Plate resistor for V1b. The drawing reads +223 V on the plate.
1.2k, 1.2 kΩ
Cathode resistor for V1b, +1.55 V on the drawing.
20µ, 20 µF electrolytic
Bypass for V1b’s cathode, full gain.
.047µ, 47 nF
Coupling cap to channel I’s Loudness pot.
Loudness, 500 kΩ pot, audio taper
Channel I’s Loudness, 500 kΩ audio, the channel’s volume. Its wiper feeds the bass and treble network.
47k, 47 kΩ
From the network’s input to the top of the Bass pot.
Bass, 1 MΩ pot, linear taper
Channel I’s Bass, 1 MΩ linear. Turned up, its wiper moves toward the 47 kΩ, shorting the .0022 µF, and more bass gets through. Turned down, it moves toward the .022 µF and the 10 kΩ, and the bass is cut.
.0022µ, 2.2 nF
From the Bass pot’s top to its wiper. It lets the mids and treble past the pot.
.022µ, 22 nF
From the Bass pot’s wiper to its bottom.
10k, 10 kΩ
From the bottom of the Bass pot to ground. It sets how far the bass and mids come down.
100k, 100 kΩ
From the Bass wiper to the Treble wiper, where the two halves of the network meet. It keeps the two controls from loading each other much.
250p, 250 pF
Into the top of the Treble pot. Only the treble gets through it.
Treble, 1 MΩ pot, linear taper
Channel I’s Treble, 1 MΩ linear. Its wiper is V2a’s grid. Turned up, the wiper moves toward the 250 pF and the treble comes up. Turned down, it moves toward the .0022 µF to ground and the treble is cut.
.0022µ, 2.2 nF
From the bottom of the Treble pot to ground.
V2a, 6EU7
One half of the second 6EU7, channel I’s second stage, after the tone controls.
100k, 100 kΩ
Plate resistor for V2a, +228 V on the plate on the drawing.
680, 680 Ω
Cathode resistor for V2a, +1.57 V on the drawing.
20µ, 20 µF electrolytic
Bypass for V2a’s cathode, full gain.
100k, 100 kΩ
From channel I’s second stage to the mixing node.
V1a, 6EU7
One half of the first 6EU7, channel II’s input stage.
100k, 100 kΩ
Plate resistor for V1a. The drawing reads +223 V on the plate.
1.2k, 1.2 kΩ
Cathode resistor for V1a, +1.55 V on the drawing.
20µ, 20 µF electrolytic
Bypass for V1a’s cathode, full gain.
.047µ, 47 nF
Coupling cap to channel II’s Loudness pot.
Loudness, 500 kΩ pot, audio taper
Channel II’s Loudness, 500 kΩ audio, the channel’s volume. Its wiper feeds the bass and treble network.
47k, 47 kΩ
From the network’s input to the top of the Bass pot.
Bass, 1 MΩ pot, linear taper
Channel II’s Bass, 1 MΩ linear. Turned up, its wiper moves toward the 47 kΩ, shorting the .0022 µF, and more bass gets through. Turned down, it moves toward the .022 µF and the 10 kΩ, and the bass is cut.
.0022µ, 2.2 nF
From the Bass pot’s top to its wiper. It lets the mids and treble past the pot.
.022µ, 22 nF
From the Bass pot’s wiper to its bottom.
10k, 10 kΩ
From the bottom of the Bass pot to ground. It sets how far the bass and mids come down.
100k, 100 kΩ
From the Bass wiper to the Treble wiper, where the two halves of the network meet. It keeps the two controls from loading each other much.
250p, 250 pF
Into the top of the Treble pot. Only the treble gets through it.
Treble, 1 MΩ pot, linear taper
Channel II’s Treble, 1 MΩ linear. Its wiper is V2b’s grid. Turned up, the wiper moves toward the 250 pF and the treble comes up. Turned down, it moves toward the .0022 µF to ground and the treble is cut.
.0022µ, 2.2 nF
From the bottom of the Treble pot to ground.
V2b, 6EU7
One half of the second 6EU7, channel II’s second stage, after the tone controls.
100k, 100 kΩ
Plate resistor for V2b, +228 V on the plate on the drawing.
680, 680 Ω
Cathode resistor for V2b, +1.57 V on the drawing.
20µ, 20 µF electrolytic
Bypass for V2b’s cathode, full gain.
100k, 100 kΩ
From channel II’s second stage to the mixing node.
150p, 150 pF
From the mixing node to ground. With the two 100 kΩ it rolls off the top end slightly, above about 20 kHz.
.0047µ, 4.7 nF
Coupling cap into the cathodyne’s grid. The cathodyne’s 470 kΩ is bootstrapped, so it looks much bigger and the bass still gets through.
V3, 12AU7 / ECC82
12AU7 (ECC82), a cathodyne, or split-load inverter, with no driver ahead of it. The same current runs through its plate resistor and its cathode resistors, which are about equal, so the plate swings one way and the cathode the other by about the same amount. The cathode drives tube A, the plate tube B. Its gain is slightly under 1 on each side, so the preamp has to bring the whole swing the output tubes need.
100k, 100 kΩ
The cathodyne’s plate resistor, the output to tube B.
2.2k, 2.2 kΩ
The cathodyne’s bias resistor. The grid returns below it, so the cathode sits this far above the grid.
100k, 100 kΩ
The cathodyne’s cathode load, the same value as its plate resistor. The cathode above the bias resistor is the output to tube A.
470k, 470 kΩ
Grid resistor for the cathodyne, returned to the junction of its cathode resistors so the grid sits near the cathode’s high DC voltage. It’s the grid’s only path to ground, and the cathode follows the grid, so the preamp sees much more than 470k Ω.
.022µ, 22 nF
Coupling cap from the inverter to side A’s driver.
.022µ, 22 nF
Coupling cap from the inverter to side B’s driver.
2.2k, 2.2 kΩ
The drivers’ cathode resistor, 2.2k Ω to ground for both halves, with no bypass cap. The two cathodes share it, so the pair works somewhat like a long-tail pair: what one half gains in current the other loses, which evens out the two sides’ swings.
220k, 220 kΩ
Grid leak for side A’s driver, to ground.
V4a, 6SN7
6SN7, side A’s driver: a gain stage that raises the inverter’s signal about 9 times on its way to tube A’s grids. The other half drives side B, and the two share one cathode resistor.
100k, 100 kΩ
The driver’s plate resistor, 100k Ω from the node for the drivers.
.047µ, 47 nF
Coupling cap from the driver’s plate to tube A’s grids.
220k, 220 kΩ
Grid leak for side B’s driver, to ground.
V4b, 6SN7
The other half of the 6SN7, side B’s driver.
100k, 100 kΩ
The driver’s plate resistor, 100k Ω from the node for the drivers.
.047µ, 47 nF
Coupling cap from the driver’s plate to tube B’s grids.
1k, 1 kΩ
Grid stopper for tube A. It keeps the tube from oscillating and softens grid current.
1k, 1 kΩ
Grid stopper for tube B.
V5, 6L6GC
6L6GC, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply.
V6, 6L6GC
6L6GC, tube B of the pair.
100k, 100 kΩ
Grid leak for tube A, returned to the bias supply. Driven into grid current, the coupling cap charges through it and the bias goes colder for a moment. That’s blocking distortion.
100k, 100 kΩ
Grid leak for tube B.
470, 470 Ω
Screen resistor for tube A. It limits the screen current when the tube is driven hard.
470, 470 Ω
Screen resistor for tube B.
4 H, 4 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.
45, 45 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.
4k, 4 kΩ
Dropping resistor from the screen supply node to the node for the drivers, as on the Gibson GA-75 (1960s) 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.
20µ, 20 µF electrolytic
Filter cap at the node for the drivers. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
4.7k, 4.7 kΩ
Dropping resistor from the node for the drivers to the node for the inverter, as on the Gibson GA-75 (1960s) 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.
4.7k, 4.7 kΩ
Dropping resistor from the node for the inverter to the node for the preamp, as on the Gibson GA-75 (1960s) 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, 6.6 kΩ to 16 Ω, 40 H primary, 10 mH leakage
The output transformer, 6.6 kΩ plate to plate into 16 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, 362 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 362 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, 362 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.
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 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.
Bias tap, 51 V ac, 60 Hz
The power transformer’s bias tap, 50.6 V ac, as a sine source at the mains frequency. Typical of the part, not measured.
tap R, 100 Ω
The bias tap’s own resistance, 100 Ω, typical, not measured. The drawing has no resistor between the tap and the diode.
Bias diode, 1N4001
The bias rectifier, a silicon diode turned so the supply comes out negative.
10µ, 10 µF electrolytic
Filter cap on the bias supply, its plus side to ground.
7.5k, 7.5 kΩ
Second resistor of the bias supply, 7.5k Ω to the bias line. With the cap after it, it filters the ripple a second time.
10µ, 10 µF electrolytic
Second filter cap, on the bias line, its plus side to ground.
22k, 22 kΩ
From the bias line to ground, 22k Ω. With the 7.5k Ω above it, it divides the supply down to the bias. There’s no bias pot, so change this resistor to change the bias. Bigger runs the tubes colder.
Coil R, 17.4 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 16 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, 512 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, 928 µH
With Loss R across it, the treble rises and then levels off, like the losses in a real coil.
Loss R, 28.6 Ω
Limits what Loss L adds to the treble rise.
Loss L2, 4.26 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, 4.38 Ω
Limits what Loss L2 adds.
Tank R, 167 Ω
Height of the bass resonance, about 185 Ω in all at its peak. With no feedback, the voltage at the speaker rises and falls with its impedance.
Tank L, 42.2 mH
With Tank C, sets the resonance: 74 Hz.
Tank C, 110 µF
With Tank L, sets the resonance. With Tank R, it sets how narrow the peak is.

About this version

As drawn
The 6EU7 uses the 12AX7 model. The tube models are typical of each type, 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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