Harmonic vibrato amp, 1960 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 6G7 Bandmaster (vibrato channel).

The vibrato channel of a 1960 brownface amp. The signal splits into a low band and a high band, each with its own triode, and a slow oscillator turns one up while it turns the other down, swinging the balance between lows and highs a few times a second.

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. Speed runs the oscillator from about 3.2 to 9.3 times a second. Intensity sets how much of its swing reaches the band tubes. The low band runs through 470 kΩ and .005 µF, the high band through 250 pF, and the two meet near 400 Hz. With the knobs at their default settings, the overall level moves only about 12 % either side while the balance between lows and highs swings back and forth. At full Intensity each band tube’s plate swings about 60 V. The plates move opposite ways, so most of that cancels at the mixing node, and the .001 µF into the inverter cuts what’s left to under a volt. The signal comes off a pad on the second stage, about 16 to 1 down, before the split. With the footswitch down, the oscillator’s ladder is grounded at its middle and the oscillator stops. The normal channel sits idle, there for the cathodes it shares. On Preamp only, the phase inverter is only a load and box B is held steady. The tube models are typical of a 12AX7, not measured.

Download.

Parts and what they do

Stock values for the As drawn version. Tap a part on the schematic to change it.

R1A, 68 kΩ
The vibrato 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.
R1B, 68 kΩ
The vibrato 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.
RIN, 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.
56k 1W, 56 kΩ
Drops the phase inverter’s supply to box A for the gain stages.
20µ, 20 µF electrolytic
Box A’s filter cap.
56k 1W, 56 kΩ
From box B down to the vibrato stage’s supply.
20µ, 20 µF electrolytic
The vibrato stage’s filter cap.
56k 1W, 56 kΩ
On down to the oscillator’s supply.
20µ, 20 µF electrolytic
The oscillator’s filter cap.
V1a, 12AX7 / ECC83 / 7025
The vibrato channel’s first gain stage.
100k, 100 kΩ
First stage plate resistor.
820, 820 Ω
Cathode resistor shared with the normal channel’s first stage, so it carries both triodes’ current.
25µ, 25 µF electrolytic
Bypass cap on the shared cathode.
34k, 34 kΩ
The normal channel’s input resistors, with nothing plugged in. Its grid sits at ground.
V1b, 12AX7 / ECC83 / 7025
The normal channel’s first stage, idle. It’s included for the cathode it shares.
100k, 100 kΩ
Plate resistor for the normal channel’s first stage. The normal channel’s stack after it is left out.
.05µ, 50 nF
Coupling cap into the tone stack.
250p, 250 pF
Treble cap, to the top of the treble pot.
100k, 100 kΩ
The low path, to the bottom of the treble pot and the top of the bass network.
Treble, 250 kΩ pot, linear taper
Treble, 250 kΩ linear. Its wiper is the stack’s output.
Bass, 250 kΩ pot, audio taper
Bass, 250 kΩ audio, from the low path to ground.
.01µ, 10 nF
Bass cap, from the top of the bass pot to its wiper.
10k, 10 kΩ
From the bass pot’s wiper to ground.
Volume, 500 kΩ pot, linear taper
Volume, 500 kΩ, marked linear.
47p, 47 pF
Bright cap across the top of the volume.
V2a, 12AX7 / ECC83 / 7025
Second gain stage.
100k, 100 kΩ
Most of the plate load. The vibrato’s signal comes from its bottom end, where it meets the 6800.
6800, 6.8 kΩ
The rest of the plate load, down to box A. The signal across it is about a sixteenth of the signal at the plate, and that’s what feeds the vibrato.
820, 820 Ω
Cathode resistor shared with the normal channel’s second stage.
25µ, 25 µF electrolytic
Bypass cap on the shared cathode.
.05µ, 50 nF
Couples the pad into the vibrato’s two bands.
V2b, 12AX7 / ECC83 / 7025
The normal channel’s second stage, its volume at 0.
220k, 220 kΩ
Its plate resistor.
.05µ, 50 nF
The normal channel’s coupling cap to the mixing node.
1M, 1 MΩ
The normal channel’s mixing resistor. With that channel’s volume at 0 it only loads the mixing node.
470k, 470 kΩ
Into the low band. With the .005 µF below it, it rolls off the highs.
.005µ, 5 nF
The low band’s cap to ground. With the 470 kΩ it makes a gentle low-pass, 6 dB per octave from about 70 Hz on paper.
.01µ, 10 nF
Couples the low band to V3a’s grid. The LFO reaches the same grid through 1 MΩ.
220k, 220 kΩ
Into the high band. With the 220 kΩ to ground it halves the signal.
220k, 220 kΩ
Bottom half of the divider that feeds the high band.
250p, 250 pF
Only 250 pF into V3b’s grid, so the lows barely get through. The two bands’ grids get equal signal near 400 Hz.
V3a, 12AX7 / ECC83 / 7025
Low-band stage. The LFO swings its grid, which moves its bias and so its gain.
V3b, 12AX7 / ECC83 / 7025
High-band stage. Its grid gets the LFO inverted, so its gain rises while V3a’s gain falls.
4700, 4.7 kΩ
Cathode resistor shared by the two band tubes. As the LFO turns one up and the other down, one current rises as the other falls, so the cathode barely moves, about 0.5 V p-p at full Intensity.
2µ, 2 µF electrolytic
Shared bypass cap, only 2 µF. At 100 Hz it leaves the low band about 15 % short of full gain.
100k, 100 kΩ
V3a’s plate resistor, from the vibrato supply.
100k, 100 kΩ
V3b’s plate resistor, from the vibrato supply.
470k, 470 kΩ
Low band to the mixing node. At full Intensity each band plate swings about 60 V at the LFO’s rate, opposite ways, so most of it cancels at the mixing node and about 5 V is left.
470k, 470 kΩ
High band to the mixing node, where the two bands add back together.
.001µ, 1 nF
Into the phase inverter. Only .001 µF into the inverter’s 1 MΩ, so it trims the deep bass and cuts the LFO left at the mixing node to about 0.7 V p-p at full Intensity.
V4a, 12AX7 / ECC83 / 7025
The oscillator. The three caps after its plate turn its signal upside down at one frequency, and it oscillates at that frequency.
470k, 470 kΩ
Oscillator plate resistor.
4700, 4.7 kΩ
Oscillator cathode resistor.
25µ, 25 µF electrolytic
Oscillator bypass cap.
.03µ, 30 nF
Plate cap to ground.
.02µ, 20 nF
First ladder cap.
Speed, 4 MΩ pot, reverse audio taper
Speed, 4 MΩ reverse audio, used as a variable resistor. About 3.2 to 9.3 Hz across the knob.
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.
.01µ, 10 nF
Third ladder cap.
1M, 1 MΩ
Oscillator grid leak.
4.7M, 4.7 MΩ
Couples the LFO from the oscillator’s plate. At 4.7 MΩ the vibrato network barely loads it.
.005µ, 5 nF
Blocks the plate’s DC.
.1µ, 100 nF
To ground, with the 4.7 MΩ above it. It cuts the LFO well down and smooths it.
Intensity, 10 MΩ pot, linear taper
Intensity, 10 MΩ linear. It sets how much LFO reaches the band tubes. At 0 their grids barely move.
.05µ, 50 nF
Smooths the wiper.
1M, 1 MΩ
Carries the LFO to V3a’s grid.
1M, 1 MΩ
From V3a’s grid to the divider on V4b’s grid.
1M, 1 MΩ
The divider’s resistor to ground.
1M, 1 MΩ
From the divider to V3b’s grid.
.05µ, 50 nF
On V4b’s grid, to ground. It shunts the audio that leaks into the divider from V3a’s grid.
V4b, 12AX7 / ECC83 / 7025
Inverts the LFO for V3b.
.001µ, 1 nF
From V4b’s plate back to its grid, which cuts the stage’s gain at audio frequencies.
100k, 100 kΩ
V4b’s plate resistor, from the oscillator’s supply.
1500, 1.5 kΩ
V4b’s cathode resistor.
25µ, 25 µF electrolytic
V4b’s bypass cap.
.25µ, 250 nF
On V4b’s plate, to ground. It shunts audio and passes the slow LFO.
.05µ, 50 nF
Couples the inverted LFO toward V3b’s grid.
1M, 1 MΩ
Inverted LFO to V3b’s grid.
V5a, 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.
V5b, 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.
47p, 47 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.
10k, 10 kΩ
The feedback resistor from the speaker. With the 1.5k Ω below it, it sends about 13 % of the speaker voltage back to the inverter, which lowers the gain, flattens the response and tightens the bass.
1.5k, 1.5 kΩ
The bottom of the feedback loop, 1.5k Ω to ground. The fed-back speaker voltage arrives here and moves the tail.
.1µ, 100 nF
The presence cap, in series with the pot across the leg. With the pot it sets where the lift starts.
Presence, 5 kΩ pot, linear taper
Presence, a 5k linear pot as a variable resistor, its resistance falling as it’s turned up, in series with the cap across the leg. Turned up, the cap shunts more of the leg’s treble, so less treble is fed back and the top end rises.
.05µ, 50 nF
Coupling cap to tube A’s grid.
.05µ, 50 nF
Coupling cap to tube B’s grid.
V6, 6L6GC
6L6GC, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply.
V7, 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+.
20µ, 20 µF electrolytic
Filter cap on the screen supply.
4.7k, 4.7 kΩ
Dropping resistor from the screen supply node to the node for the inverter and the preamp, as on the Fender 6G7 Bandmaster 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.
20µ, 20 µF electrolytic
Filter cap at the node for the inverter and 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 4 Ω, 40 H primary, 10 mH leakage
The output transformer, 4 kΩ plate to plate into 4 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, 343 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 343 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, 343 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.
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 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.
20µ, 20 µF electrolytic
Another reservoir cap, in parallel with the first.
Bias tap, 48 V ac, 60 Hz
The power transformer’s bias tap, 47.8 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.
8µ, 8 µF electrolytic
Filter cap on the bias supply, its plus side to ground.
10k, 10 kΩ
Second resistor of the bias supply, 10k Ω to the bias line. With the cap after it, it filters the ripple a second time.
8µ, 8 µF electrolytic
Second filter cap, on the bias line, its plus side to ground.
56k, 56 kΩ
From the bias line to ground, 56k Ω. With the 10k 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, 4.34 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 4 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, 128 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, 232 µH
With Loss R across it, the treble rises and then levels off, like the losses in a real coil.
Loss R, 7.15 Ω
Limits what Loss L adds to the treble rise.
Loss L2, 1.06 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, 1.09 Ω
Limits what Loss L2 adds.
Tank R, 41.9 Ω
Height of the bass resonance, about 46 Ω in all at its peak. The feedback holds the output steadier against that than it would be without feedback.
Tank L, 10.6 mH
With Tank C, sets the resonance: 74 Hz.
Tank C, 440 µF
With Tank L, sets the resonance. With Tank R, it sets how narrow the peak is.

Versions

As drawn
Every tube is a 7025, a low-noise 12AX7, and the model is fitted to the Brimar 12AX7 data sheet. Typical of the type, not measured.
6G5 Pro, modeled after the Fender 6G5 Pro (vibrato channel)
The 6G5 Pro drawing: the same preamp and vibrato as the 6G7, part for part and volt for volt. Its power amp is the 6G7’s too, two 6L6GCs on the same supply, with 22 kΩ of feedback in place of 10 kΩ, so slightly less feedback. The Pro drives one 15 in speaker. Tubes typical of the type, not measured.
6G4 Super, modeled after the Fender 6G4 Super (vibrato channel)
The 6G4 Super drawing: the same preamp and vibrato as the 6G7, part for part and volt for volt, and the same power amp values, 10 kΩ of feedback included. It differs in the rectifier, a GZ34 in place of the silicon diodes, which sags slightly more on hard notes, and it drives two 10 in speakers. It opens on the 6G7’s silicon supply. Tubes typical of the type, not measured.
5G13 Vibrasonic, modeled after the Fender 5G13 Vibrasonic (vibrato channel)
The 5G13 Vibrasonic drawing, the first brown Fender: the same preamp and vibrato as the 6G7, part for part and volt for volt. Its power amp is the 6G5 Pro’s, the 6G7’s with 22 kΩ of feedback in place of 10 kΩ, into one 15 in speaker. The drawing shows no speaker impedance, so the 6G7’s typical 4 Ω load stays. Tubes typical of the 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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