Pitch vibrato amp with a 6CG7 varistor bridge, one channel, late 1950s 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 Magnatone 280-A.

Channel 1 of a stereo amp: a 12AX7 stage, a bass and treble stack, the volume and a second stage, then two 6CG7 split loads whose varistors, swung by the LFO, bend the pitch. A 6CG7 stage and cathodyne drive two 6CZ5s.

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 first stage, the stack and the volume are the M10A’s, a few years earlier. The vibrato moves the pitch. Each 6CG7 split load sends its plate’s signal through 820 pF and its cathode’s, the other way round, through two varistors to one node. The LFO swings the varistors’ resistance, so the phase at that node swings, and a moving phase is a moving pitch. The amp is stereo, with a second channel and power amp of its own. The bench has Channel 1 only.

Download.

Parts and what they do

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

47k, 47 kΩ
Jack 1A’s 47 kΩ into the grid.
4.7M, 4.7 MΩ
From jack 1A’s tip to ground, 4.7 MΩ, the load a guitar in that jack sees.
V1a, 12AX7 / ECC83 / 7025
First stage, half a 12AX7.
270k, 270 kΩ
First stage plate resistor, 270 kΩ from A. The drawing reads +106 V on the plate.
470, 470 Ω
First stage cathode resistor, 470 Ω, not bypassed. The drawing reads +.8 V.
220k, 220 kΩ
220 kΩ from A to the cathode, as on the drawing. Its current through the 470 Ω lifts the cathode, so the stage runs colder.
.047µ, 47 nF
Coupling cap from the first plate into the stack, .047 µF.
330k, 330 kΩ
The bass side’s feed, 330 kΩ to the top of the bass pot.
Bass, 1 MΩ pot, audio taper
Bass, 1 MΩ. Its wiper feeds the output through 120 kΩ. Turned up, the lows come up. Audio taper and its direction are typical, not on the drawing.
47k, 47 kΩ
From the bottom of the bass pot to ground, 47 kΩ.
470p, 470 pF
470 pF from the top of the bass pot to its wiper.
.0047µ, 4.7 nF
.0047 µF from the wiper to the bottom of the bass pot.
120k, 120 kΩ
120 kΩ from the bass wiper to the treble wiper.
470p, 470 pF
Treble cap, 470 pF to the top of the treble pot.
Treble, 1 MΩ pot, audio taper
Treble, 1 MΩ. Its wiper is the stack’s output and the volume’s top. Turned up, more treble. Audio taper and its direction are typical, not on the drawing.
.0047µ, 4.7 nF
.0047 µF from the bottom of the treble pot to ground.
.0047µ, 4.7 nF
The Mellow switch’s cap, .0047 µF to ground.
Volume, 1 MΩ pot, audio taper
Volume, 1 MΩ, after the stack. Its wiper is V1b’s grid. Audio taper is typical, not on the drawing.
V1b, 12AX7 / ECC83 / 7025
Second stage, the other half of the 12AX7. Its plate drives the first vibrato stage’s grid with no cap.
470k, 470 kΩ
Second stage plate resistor, 470 kΩ from A. It also sets V3b’s grid voltage. The drawing reads +85 V.
470, 470 Ω
Second stage cathode resistor, 470 Ω, not bypassed.
220k, 220 kΩ
220 kΩ from A to the cathode, as on the first stage.
V3b, 6CG7
The first vibrato stage, half a 6CG7, a split load: its plate and cathode swing opposite ways.
8.2k, 8.2 kΩ
Plate resistor, 8.2 kΩ from B. The drawing reads +250 V.
8.2k, 8.2 kΩ
Cathode resistor, 8.2 kΩ, the same as the plate’s. The drawing reads +90 V.
820p, 820 pF
820 pF from the plate to the stage’s output node. With the varistors’ resistance it sets the phase there.
.1µ, 100 nF
.1 µF from the cathode to the first varistor.
.1µ, 100 nF
.1 µF from the cathode to the second varistor.
VR1, SiC varistor, one direction (fit), one each way
Varistor, from the cathode’s cap to the output node. Its resistance falls steeply as the voltage across it rises, so the LFO through it sets how much of the cathode’s signal reaches the node against the plate’s.
11.2k, 11.2 kΩ
Part of the varistor’s model, not a part of its own: the 11.2 kΩ in series that, with the pair beside it, follows a measured SiC curve.
VR2, SiC varistor, one direction (fit), one each way
The second varistor, the same, on the other LFO line.
11.2k, 11.2 kΩ
Part of the varistor’s model, not a part of its own: the 11.2 kΩ in series that, with the pair beside it, follows a measured SiC curve.
47k, 47 kΩ
47 kΩ from LFO line 1 to the first varistor.
47k, 47 kΩ
47 kΩ from LFO line 2 to the second varistor.
V4a, 6CG7
The second vibrato stage, half a 6CG7, the same kind of split load. Its grid is the first stage’s output node, with no cap, so the varistors set its DC level.
18k, 18 kΩ
Plate resistor, 18 kΩ from B. The drawing reads +258 V.
82k, 82 kΩ
Cathode resistor, 82 kΩ. The drawing reads +145 V.
820p, 820 pF
820 pF from the plate to the output node.
.1µ, 100 nF
.1 µF from the cathode to the third varistor.
.1µ, 100 nF
.1 µF from the cathode to the fourth varistor.
VR3, SiC varistor, one direction (fit), one each way
The third varistor, from the cathode’s cap to the output node.
11.2k, 11.2 kΩ
Part of the varistor’s model, not a part of its own: the 11.2 kΩ in series that, with the pair beside it, follows a measured SiC curve.
VR4, SiC varistor, one direction (fit), one each way
The fourth varistor, the same, on the other LFO line.
11.2k, 11.2 kΩ
Part of the varistor’s model, not a part of its own: the 11.2 kΩ in series that, with the pair beside it, follows a measured SiC curve.
47k, 47 kΩ
47 kΩ from LFO line 1.
47k, 47 kΩ
47 kΩ from LFO line 2.
.0022µ, 2.2 nF
.0022 µF from the vibrato’s output toward V3a.
2.2M, 2.2 MΩ
2.2 MΩ in series into V3a’s grid. With the 2.2 MΩ to ground it halves the signal.
2.2M, 2.2 MΩ
V3a’s grid resistor, 2.2 MΩ.
V3a, 6CG7
Half a 6CG7, the last gain stage. Its plate drives the cathodyne’s grid with no cap, and the feedback from the speaker comes in at its cathode.
470k, 470 kΩ
Plate resistor, 470 kΩ from B. It sets the cathodyne’s grid voltage. The drawing reads +78 V.
V6a, 12AX7 / ECC83 / 7025
The oscillator, half a 12AX7. The three-cap ladder from its plate back to its grid turns the phase round at a few hertz.
270k, 270 kΩ
Oscillator plate resistor, 270 kΩ from B. The drawing reads +180 V.
1k, 1 kΩ
Oscillator cathode resistor, 1 kΩ, not bypassed.
.022µ, 22 nF
The ladder’s first cap, .022 µF.
220k, 220 kΩ
220 kΩ in series with the Speed pot, the fastest end’s limit.
Speed, 500 kΩ pot, linear taper
Speed, 500 kΩ as a variable resistor to ground. Turned up, less resistance, so the oscillator runs faster. Linear taper and its direction are typical, not on the drawing.
.01µ, 10 nF
The ladder’s second cap, .01 µF.
820k, 820 kΩ
820 kΩ to ground at the ladder’s middle.
.01µ, 10 nF
The ladder’s third cap, .01 µF, into the grid.
3.3M, 3.3 MΩ
The oscillator’s grid leak, 3.3 MΩ.
.068µ, 68 nF
.068 µF from the oscillator’s plate to the Intensity pot.
Intensity, 1 MΩ pot, audio taper
Vibrato intensity, 1 MΩ. Its wiper sets how much of the oscillator’s swing reaches the LFO split load. Audio taper and its direction are typical, not on the drawing.
47k, 47 kΩ
47 kΩ from the wiper to the vibrato selector. With the selector’s line grounded, the oscillator’s swing goes to ground through it.
330k, 330 kΩ
The first of two 330 kΩ and .0047 µF sections. Together they round off the oscillator’s wave.
.0047µ, 4.7 nF
.0047 µF to ground.
330k, 330 kΩ
The second 330 kΩ.
.0047µ, 4.7 nF
.0047 µF to ground.
.047µ, 47 nF
.047 µF into the LFO split load’s grid.
10M, 10 MΩ
10 MΩ from B to the LFO split load’s grid. With the 3.3 MΩ to ground it sets the grid near 70 V.
3.3M, 3.3 MΩ
3.3 MΩ from the grid to ground.
V7a, 12BH7 (GE, JJ)
The LFO split load, half a 12BH7. Its plate side and its cathode carry the oscillator’s swing to the varistors, one line each, opposite ways.
2.7k, 2.7 kΩ
2.7 kΩ from the plate to LFO line 1. The drawing reads +198 V on the plate.
22k, 22 kΩ
22 kΩ from B to LFO line 1, the rest of the plate load.
.25µ, 250 nF
.25 µF across the 22 kΩ. With the 2.7 kΩ it keeps the audio off LFO line 1.
22k, 22 kΩ
Cathode resistor, 22 kΩ. The drawing reads +78 V.
.25µ, 250 nF
.25 µF from LFO line 2 to ground. It smooths the LFO and keeps the audio off the line.
V8, 6CG7
6CG7, a cathodyne, or split-load inverter, its grid wired straight to the plate of the stage before it, with no coupling cap. That plate’s voltage sets the grid’s, and the cathode sits a few volts above it. The same current runs through its plate resistor and its cathode resistor, which are 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. On the drawing it’s V5b, half a 6CG7, its grid wired to V3a’s plate.
100k, 100 kΩ
The cathodyne’s plate resistor, the output to tube B.
100k, 100 kΩ
The cathodyne’s cathode resistor, the same value as its plate resistor, straight to ground, and the output to tube A. There’s no separate bias resistor.
18k, 18 kΩ
The feedback resistor from the speaker. With the 12k Ω below it, it sends about 40 % of the speaker voltage back to the driver’s cathode, which lowers the gain, flattens the response and tightens the bass.
12k, 12 kΩ
The driver’s cathode resistor, 12k Ω to ground. It sets the driver’s bias, and it’s also the bottom of the feedback loop: the feedback resistor from the speaker connects at the top of it, so the fed-back speaker voltage moves the cathode. It isn’t bypassed.
.047µ, 47 nF
Coupling cap to tube A’s grid.
.047µ, 47 nF
Coupling cap to tube B’s grid.
V9, 6CZ5
6CZ5, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply.
V10, 6CZ5
6CZ5, 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.
3.3k, 3.3 kΩ
Dropping resistor from B+ to the screens. It’s large, so the screens sit about 30 V below the plates and drop further when they draw more current on a hard note.
10µ, 10 µF electrolytic
Filter cap on the screen supply.
1.2k, 1.2 kΩ
Dropping resistor from B+ to the node for the next step, as on the Magnatone 280-A 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 next step. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
1.2k, 1.2 kΩ
Dropping resistor from the node for the next step to the node for the next step, as on the Magnatone 280-A 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 next step. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
1.2k, 1.2 kΩ
Dropping resistor from the node for the next step to the node for the vibrato, the LFO and the inverter, as on the Magnatone 280-A 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 vibrato, the LFO and 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 vibrato, the LFO and the inverter to the node for the preamp, as on the Magnatone 280-A 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.
Ch. 2 pair, 3.5 kΩ
Stands for Channel 2’s pair of 6CZ5s on B+. It pulls about the current worked out from the voltages on the drawing. That part of the amp isn’t modeled here. A typical value.
Ch. 2 vibrato, 27 kΩ
Stands for Channel 2’s vibrato, inverter and LFO split load on the node for the vibrato, the LFO and the inverter. It pulls about the current worked out from the voltages on the drawing. That part of the amp isn’t modeled here. A typical value.
Ch. 2 preamp, 600 kΩ
Stands for Channel 2’s preamp on the node for the preamp. It pulls about the current worked out from the voltages on the drawing. That part of the amp isn’t modeled here. A typical value.
OT, 6.6 kΩ to 8 Ω, 15 H primary, 10 mH leakage
The output transformer, 6.6 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.
PT, 365 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 365 V ac from the grounded center tap, as a sine source at the mains frequency. The value is typical of the part.
PT, 365 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.
PT R, 50 Ω
Resistance of the other half. Typical of the part.
V11, 5U4GB
One plate of the 5U4GB 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.
V11, 5U4GB
The rectifier’s other plate, 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.
Bias tap, 48 V ac, 60 Hz
The power transformer’s bias tap, 47.5 V ac, as a sine source at the mains frequency. Typical of the part.
12k, 12 kΩ
Series resistor into the bias rectifier, 12k Ω. It limits the charging current.
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.
47k, 47 kΩ
The bias supply’s load, 47k Ω to ground. With the 12k 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.
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.

About this version

As drawn
Channel 1: a 12AX7, three 6CG7 halves, the oscillator’s 12AX7 half and the LFO’s 12BH7 half. The tube models are typical of the types. The varistors follow a measured SiC curve.

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. Caps are ideal, and the battery or HT is a voltage behind a resistance.

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