Four-input amp with a cathode-follower tone stack, 1953 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 Gibson GA-77 Vanguard.

Two channels on one 12AY7, the lower one with its own tone pot, mixed through 270 kΩ into a 12AX7 gain stage and a cathode follower. The follower drives a bass and treble network unlike Fender’s or Marshall’s.

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. Both channels have linear volume pots and share one 820 Ω cathode resistor, so turning one channel up nudges the other. The lower channel’s 2 MΩ tone pot swings between a 500 pF bright cap around its volume and a 5 nF cut to ground. The tone stack is fed from a cathode follower, so the pots don’t load the gain stage. The treble pot’s wiper feeds the driver directly, and the third knob, in the power amp, sets how much of the feedback is bypassed in the treble.

Download.

Parts and what they do

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

R1U, 68 kΩ
The upper 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.
R3U, 68 kΩ
The upper 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.
R2U, 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.
R1L, 68 kΩ
The lower 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.
R3L, 68 kΩ
The lower 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.
R2L, 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.
V1a, 12AY7 / 6072
Upper channel input stage, half of the 12AY7. With a mu of about 45 it gains about 25, less than a 12AX7 stage.
100k, 100 kΩ
Plate resistor for the upper channel.
V1b, 12AY7 / 6072
Lower channel input stage, the other half of the 12AY7. With nothing plugged in it still pulls current through the shared cathode resistor.
100k, 100 kΩ
Plate resistor for the lower channel.
820, 820 Ω
Cathode resistor shared by both 12AY7 halves. It carries both currents, so each half sees about twice its value.
20µ, 20 µF electrolytic
Bypass for the shared cathode, 20 µF, full gain down to about 20 Hz.
.02µ, 20 nF
Coupling cap from the upper channel’s plate to the top of its volume.
Vol 1M, 1 MΩ pot, linear taper
Upper channel volume, 1 MΩ linear as drawn. Its wiper feeds the mixer.
270k, 270 kΩ
Upper channel mixer resistor. With the lower one it mixes the two channels into V2a, and each channel’s volume loads the other through them.
.02µ, 20 nF
Coupling cap from the lower channel’s plate to the top of its volume.
Vol 1M, 1 MΩ pot, linear taper
Lower channel volume, 1 MΩ linear. Turned up with nothing plugged in, it hangs the lower channel’s plate on the mixer through less track.
500p, 500 pF
Bright cap, 500 pF, from the top of the lower volume to the tone pot. Toward this end the pot carries treble around the volume.
Tone 2M, 2 MΩ pot, linear taper
The lower channel’s tone, 2 MΩ linear, its wiper on the volume’s wiper. Up moves it toward the 500 pF, brighter. Down puts the 5 nF to ground on the wiper, which cuts treble. The drawing doesn’t mark which way is up, so up is assumed to be brighter.
.005µ, 5 nF
Treble cut cap, 5 nF to ground from the tone pot’s other end.
270k, 270 kΩ
Lower channel mixer resistor.
V2a, 12AX7 / ECC83 / 7025
Gain stage after the mixer, half of the 12AX7. Its cathode is bypassed, so it gains about 60. Its grid’s only path to ground is through the mixers and the volumes.
100k, 100 kΩ
Plate resistor for V2a.
1.5k, 1.5 kΩ
V2a cathode resistor.
20µ, 20 µF electrolytic
V2a cathode bypass, 20 µF.
V2b, 12AX7 / ECC83 / 7025
Cathode follower, its grid straight on V2a’s plate. Its gain is slightly under 1, and its low output impedance drives the tone stack.
56k, 56 kΩ
Cathode follower load, 56 kΩ.
50p, 50 pF
Treble cap, 50 pF (.00005 µF on the drawing), into the top of the treble pot.
100k, 100 kΩ
From the follower to the junction of the two .02 µF. With them it carries the mids and lows to the bottom of the treble pot.
.02µ, 20 nF
From the bottom of the treble pot to the 100 kΩ.
.02µ, 20 nF
From the 100 kΩ down to the bass pot’s wiper and the 10 kΩ.
10k, 10 kΩ
The bottom of the stack, 10 kΩ to ground.
Treble 1M, 1 MΩ pot, audio taper
Treble, 1 MΩ audio. Its wiper feeds the driver’s grid. Up moves it toward the 50 pF, so more treble.
Bass 1M, 1 MΩ pot, audio taper
Bass, 1 MΩ audio, from the bottom of the treble pot to ground, its wiper on the 10 kΩ. Up grounds the bottom of the stack. Down ties it to the treble pot, and the two .02 µF with the 10 kΩ cut the bass.
V3a, 12AU7 / ECC82
12AU7 (ECC82), the driver. A cathodyne has no gain of its own, so this stage raises the preamp’s signal enough to drive the output tubes. The feedback from the speaker goes to its cathode. 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.
100k, 100 kΩ
Plate resistor for the driver.
.02µ, 20 nF
Coupling cap from the driver to the cathodyne.
V3b, 12AU7 / ECC82
The cathodyne, or split-load inverter. 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.
56k, 56 kΩ
The cathodyne’s plate resistor, the output to tube B.
1.5k, 1.5 kΩ
The cathodyne’s bias resistor. The grid returns below it, so the cathode sits this far above the grid.
56k, 56 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.
1M, 1 MΩ
Grid resistor for the cathodyne, returned to the junction of its cathode resistors so the grid sits near the cathode’s high DC voltage.
100k, 100 kΩ
The feedback resistor from the speaker. With the 1.5k Ω below it, it sends about 1 % of the speaker voltage back to the driver’s cathode, which lowers the gain, flattens the response and tightens the bass.
1.5k, 1.5 kΩ
The driver’s cathode resistor, 1.5k 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. The presence cap and pot across it bypass part of it in the treble.
.1µ, 100 nF
The presence cap, in series with the pot across the leg. With the pot it sets where the lift starts.
Presence, 50 kΩ pot, linear taper
Presence, a 50k 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.
.1µ, 100 nF
Coupling cap to tube A’s grid.
.1µ, 100 nF
Coupling cap to tube B’s grid.
1.5k, 1.5 kΩ
Grid stopper for tube A. It keeps the tube from oscillating and softens grid current.
1.5k, 1.5 kΩ
Grid stopper for tube B.
V4, 5881
n5881, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply.
V5, 5881
n5881, 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.
3 H, 3 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.
60, 60 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+.
10µ, 10 µF electrolytic
Filter cap on B+ after the choke, where the output transformer’s center tap and the screens hang.
10k, 10 kΩ
Dropping resistor from the screen supply node to the node for the driver and inverter, as on the Gibson GA-77 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 driver and 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 driver and inverter to the node for the preamp, as on the Gibson GA-77 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 8 Ω, 40 H primary, 10 mH leakage
The output transformer, 6.6 kΩ plate to plate into 8 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, 360 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 360 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, 360 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, 75 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, 75 Ω
Resistance of the other half. Typical of the part, not measured.
V6, 5V4G
One plate of the 5V4G 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, 5V4G
The rectifier’s other plate, 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, 50 V ac, 60 Hz
The power transformer’s bias tap, 50.1 V ac, as a sine source at the mains frequency. Typical of the part, not measured.
5.6k, 5.6 kΩ
Series resistor into the bias rectifier, 5.6k Ω. It limits the charging current.
Bias diode, 1N4001
The bias rectifier, a silicon diode turned so the supply comes out negative.
100µ, 100 µF electrolytic
Filter cap on the bias supply, its plus side to ground.
56k, 56 kΩ
The bias supply’s load, 56k Ω to ground. With the 5.6k 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
The tubes on the drawing: a 12AY7 for both input channels and a 12AX7 for the gain stage and cathode follower. The tube models are fitted to their data sheets, 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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