Brownface two-channel amp with bias tremolo, 1962 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 6G11-A Vibrolux.

Two 7025 channels, Normal and Bright, each with Fender’s tapped-treble tone network and a second stage, mixed straight off their plates. A long-tail pair drives two 6L6GCs, and the tremolo swings their 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 is a gain stage, the tone network, the volume and a second stage. Both second stages reach the mixing node through 220 kΩ straight off their plates, then .005 µF into the inverter. The Normal channel has .003 µF across its second plate resistor, which trims its top end. The tremolo moves the output tubes’ bias. The 6G9-B build is the Tremolux of the same time, with brighter treble caps.

Download.

Parts and what they do

Stock values for the 6G11-A version. Tap a part on the schematic to change it.

R1B, 68 kΩ
The bright channel’s input 1 resistor into the grid. With nothing in input 2 it sits in parallel with input 2’s resistor, so a guitar in input 1 sees 34 kΩ to the grid: enough to stop radio pickup, not enough to cut treble.
R3B, 68 kΩ
The bright channel’s input 2 resistor into the grid. With nothing in input 1, input 1’s tip is grounded, so the two 68 kΩ make a divider: input 2 is about 6 dB quieter and loads the guitar with 136 kΩ.
R2B, 1 MΩ
Input 1’s 1 MΩ to ground. It sets the load a guitar in input 1 sees, and with a patch lead in input 1 it is the end of the lead’s path.
R1N, 68 kΩ
The normal channel’s input 1 resistor into the grid. With nothing in input 2 it sits in parallel with input 2’s resistor, so a guitar in input 1 sees 34 kΩ to the grid: enough to stop radio pickup, not enough to cut treble.
R3N, 68 kΩ
The normal channel’s input 2 resistor into the grid. With nothing in input 1, input 1’s tip is grounded, so the two 68 kΩ make a divider: input 2 is about 6 dB quieter and loads the guitar with 136 kΩ.
R2N, 1 MΩ
Input 1’s 1 MΩ to ground. It sets the load a guitar in input 1 sees, and with a patch lead in input 1 it is the end of the lead’s path.
V1b, 12AX7 / ECC83 / 7025
Bright channel’s first stage, half a 7025 (a low-noise 12AX7). Its plate drives the tone network directly.
100k, 100 kΩ
First stage plate resistor, from box D.
1500, 1.5 kΩ
First stage cathode resistor, about 1.5 V of bias.
25µ, 25 µF electrolytic
Bypass on the first stage’s cathode, for full gain.
250p, 250 pF
Treble cap, 250 pF, from the plate to the top of the treble pot. No coupling cap comes first: every path from the plate to ground goes through a cap.
100k, 100 kΩ
From the plate down to the two caps below. The lows and mids go this way.
.1µ, 100 nF
Carries the lows and mids on to the treble pot’s tap.
.05µ, 50 nF
Bass cap, to the bottom of the bass pot and the 6800 Ω.
Bass, 250 kΩ pot, audio taper
Bass, 250 kΩ audio, a variable resistor from the treble pot’s tap to the bass cap. Turned down, it ties the tap to the 6800 Ω through the bass cap and the lows drain away.
6800, 6.8 kΩ
From the bottom of the bass pot to ground.
Treble, 350 kΩ pot, audio taper
Treble, 350 kΩ audio, tapped 70 kΩ from its bottom lug. The highs reach the top and the lows the tap, and the wiper mixes the two. Below the tap the wiper slides toward the .005 µF and the highs drop away.
.005µ, 5 nF
From the treble pot’s bottom lug to ground. With the wiper below the tap it cuts the highs.
Volume, 1 MΩ pot, audio taper
Bright volume, 1 MΩ audio. The treble wiper drives its top.
47p, 47 pF
Bright cap, 47 pF from the volume’s top to its wiper. With the volume down, treble skips around the track. At full it does nothing.
V2b, 12AX7 / ECC83 / 7025
Bright channel’s second stage, half the second 7025. Its cathode is shared with the other channel’s second stage.
V1a, 12AX7 / ECC83 / 7025
Normal channel’s first stage, half a 7025 (a low-noise 12AX7). Its plate drives the tone network directly.
100k, 100 kΩ
First stage plate resistor, from box D.
1500, 1.5 kΩ
First stage cathode resistor, about 1.5 V of bias.
25µ, 25 µF electrolytic
Bypass on the first stage’s cathode, for full gain.
250p, 250 pF
Treble cap, 250 pF, from the plate to the top of the treble pot. No coupling cap comes first: every path from the plate to ground goes through a cap.
100k, 100 kΩ
From the plate down to the two caps below. The lows and mids go this way.
.1µ, 100 nF
Carries the lows and mids on to the treble pot’s tap.
.05µ, 50 nF
Bass cap, to the bottom of the bass pot and the 6800 Ω.
Bass, 250 kΩ pot, audio taper
Bass, 250 kΩ audio, a variable resistor from the treble pot’s tap to the bass cap. Turned down, it ties the tap to the 6800 Ω through the bass cap and the lows drain away.
6800, 6.8 kΩ
From the bottom of the bass pot to ground.
Treble, 350 kΩ pot, audio taper
Treble, 350 kΩ audio, tapped 70 kΩ from its bottom lug. The highs reach the top and the lows the tap, and the wiper mixes the two. Below the tap the wiper slides toward the .005 µF and the highs drop away.
.005µ, 5 nF
From the treble pot’s bottom lug to ground. With the wiper below the tap it cuts the highs.
Volume, 1 MΩ pot, audio taper
Normal volume, 1 MΩ audio. The treble wiper drives its top.
V2a, 12AX7 / ECC83 / 7025
Normal channel’s second stage, half the second 7025. Its cathode is shared with the other channel’s second stage.
820, 820 Ω
Cathode resistor shared by both second stages (box B). It carries both currents, so each half sees about twice its value.
25µ, 25 µF electrolytic
Bypass on the shared cathode.
100k, 100 kΩ
Normal second stage plate resistor, from the +275 V node.
.003µ, 3 nF
Across the Normal second stage’s plate resistor. It shunts the plate load in the treble, so this channel’s top end rolls off above a few kilohertz.
220k, 220 kΩ
Normal channel’s mixing resistor, straight off the plate with no cap. The cap after the mixing node blocks the DC.
100k, 100 kΩ
Bright second stage plate resistor, from the +275 V node.
220k, 220 kΩ
Bright channel’s mixing resistor. With the other channel’s 220 kΩ to its plate, about half the signal is lost at the mixing node.
.005µ, 5 nF
Into the inverter’s grid, .005 µF.
V3a, 12AX7 / ECC83 / 7025
12AX7, the driven half of the long-tail pair phase inverter. Its plate swings opposite the input and drives tube A. Its grid is fed through the preamp’s coupling cap, so there’s no second one here.
V3b, 12AX7 / ECC83 / 7025
The other half of the inverter. Its grid sits at the feedback point for signal, so it’s driven through the shared cathodes, and its plate swings with the input. It drives tube B.
1M, 1 MΩ
Grid resistor for the driven half, returned to the bias point above the tail, not to ground.
1M, 1 MΩ
Grid resistor for the other half.
820, 820 Ω
Bias resistor for both halves. The cathodes sit this far above the grids’ return, a volt or so.
6.8k, 6.8 kΩ
The tail resistor. It’s large, so the pair shares a nearly fixed current. When one half conducts more, the other conducts less, which splits the phase.
.1µ, 100 nF
Ties the second grid to the feedback point for signal, so the feedback acts across the pair.
82k, 82 kΩ
Plate resistor for the driven half. It’s smaller than the other so the two outputs come out about even.
100k, 100 kΩ
Plate resistor for the other half.
100p, 100 pF
A small cap from plate to plate. It rolls off the far top end the inverter passes on, which keeps the feedback loop steady.
100k, 100 kΩ
The feedback resistor from the speaker. With the 4.7k Ω below it, it sends about 4 % of the speaker voltage back to the inverter, which lowers the gain, flattens the response and tightens the bass.
4.7k, 4.7 kΩ
The bottom of the feedback loop, 4.7k Ω to ground. The fed-back speaker voltage arrives here and moves the tail.
.1µ, 100 nF
Coupling cap to tube A’s grid.
.1µ, 100 nF
Coupling cap to tube B’s grid.
V4, 6L6GC
6L6GC, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply.
V5, 6L6GC
6L6GC, tube B of the pair.
220k, 220 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.
220k, 220 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
A choke from B+ to the screen supply, as in the amp this style is based on. It passes DC with little drop, so the screens sit close to B+. With the cap after it, it would filter ripple, but ripple isn’t modeled here. Typical of the part, not measured.
100, 100 Ω
The choke’s winding resistance, typical, not measured. It’s all the DC drop the screens see from B+.
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, as on the Fender 6G11-A Vibrolux 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.
16µ, 16 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.
10k, 10 kΩ
Dropping resistor from the node for the inverter to the node for the preamp, as on the Fender 6G11-A Vibrolux 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 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, 298 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 298 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, 298 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, 100 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, 100 Ω
Resistance of the other half. Typical of the part, not measured.
V6, GZ34
One plate of the GZ34 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, GZ34
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.
PT, 298 V ac, 60 Hz
The end of the high-voltage winding the bias comes off, 298 V ac, the same end as the rectifier’s first plate. It’s drawn as its own source so the bias supply’s average can be found. Keep it equal to the winding beside the rectifier.
100k, 100 kΩ
Series resistor into the bias rectifier, 100k Ω. There’s no bias tap, so the bias comes straight off the high-voltage winding, and this resistor drops most of it.
Bias diode, 1N4001
The bias rectifier, a silicon diode turned so the supply comes out negative.
25µ, 25 µF electrolytic
Filter cap on the bias supply, its plus side to ground. It also holds the Intensity pot’s bias end still, so the LFO swings the wiper.
33k, 33 kΩ
The bias supply’s load, 33k Ω to ground. With the 100k Ω above it, it sets the bias. There’s no bias pot, so change this resistor to change the bias. Bigger runs the tubes colder.
V7a, 12AX7 / ECC83 / 7025
The tremolo’s oscillator, half a 12AX7. The three caps after its plate turn the signal around at one frequency, and the oscillator runs at that frequency, a few times a second. Its other half is the follower beside it.
V7b, 12AX7 / ECC83 / 7025
The other half of the oscillator’s tube, a cathode follower straight off the oscillator’s plate. It copies the plate’s swing to the bias network, so the network barely loads the oscillator.
470k, 470 kΩ
The follower’s cathode resistor. Its cathode sits a volt or two above its grid, near the oscillator’s plate voltage.
470k, 470 kΩ
Oscillator plate resistor, from the screen supply.
4.7k, 4.7 kΩ
Oscillator cathode resistor.
25µ, 25 µF electrolytic
Oscillator bypass cap. At a few hertz it isn’t a full short, and it adds some of the phase shift that keeps the loop oscillating.
.02µ, 20 nF
First ladder cap.
Speed, 3.5 MΩ pot, reverse audio taper
Speed, 3.5M Ω reverse audio, used as a variable resistor. Less resistance runs the oscillator faster.
100k, 100 kΩ
Sets the top speed. It’s all that’s left with Speed on 10.
.01µ, 10 nF
Second ladder cap.
1M, 1 MΩ
Middle resistor of the ladder, returned to the cathode. The footswitch grounds this point.
.01µ, 10 nF
Third ladder cap.
1M, 1 MΩ
Oscillator grid leak.
220k, 220 kΩ
Couples the LFO from the follower’s cathode. It keeps the bias network from loading the oscillator much.
.1µ, 100 nF
Blocks the oscillator’s DC, so only its swing reaches the Intensity pot.
Intensity, 250 kΩ pot, linear taper
Intensity, 250k Ω linear. One end gets the LFO, the other sits on the bias supply, and the wiper is the bias line for both output tubes. At 0 the wiper is on the bias supply and the bias holds still. Turned up, more of the LFO rides on the bias, so both grids move together and the output stage’s gain rises and falls. Which way it turns isn’t marked on the drawing. Up is assumed to be more.
.05µ, 50 nF
On the bias line, to ground. With the Intensity pot it rounds off the LFO slightly.
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. The feedback holds the output steadier against that than it would be without feedback.
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. The feedback holds the output steadier against that than it would be without feedback.
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.

Versions

6G11-A
The 6G11-A Vibrolux drawing. Every 7025 is modeled as a 12AX7 (a low-noise version of it), fitted to the Brimar data sheet. Typical of the type, not measured.
6G9-B, modeled after the Fender 6G9-B Tremolux
The 6G9-B Tremolux drawing: 500 pF treble caps for more treble, .001 µF across the Normal second plate resistor and .001 µF into the inverter. Its power amp is the 6G11-A’s. The real one has two speaker jacks. The model has one 8 Ω speaker.

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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