Two-channel amp with a tone stack on each channel, 1965 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 Vox AC50 (Mk III, 1965).

Two channels, each a triode stage, a volume, a gain stage and a cathode follower driving its own treble and bass. The two meet at the phase inverter through 470 kΩ each. The brilliant channel’s 500 pF coupling cap cuts most of the low end before the volume.

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 triode stages and a cathode follower, the same as the AC30’s top-boost circuit with the volume moved ahead of the second stage. The two channels differ only in the first stage and the coupling cap after it. The normal channel uses half an ECC82 with 47 kΩ and 4.7 kΩ and a .022 µF coupling cap, so it’s clean and full. The brilliant channel uses the other half with 100 kΩ and 1.5 kΩ, and a 500 pF coupling cap that cuts most of the low end before the volume. Each stack is the AC30’s: 47 pF treble cap, 100 kΩ slope, .022 µF bass and mid caps, a fixed 10 kΩ where a Fender has its middle pot. The two treble wipers meet at the inverter’s grid through 470 kΩ each, so the channel not in use still loads the other. There’s no negative feedback and no master.

Download.

Parts and what they do

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

R34, 68 kΩ
The brilliant 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.
R33, 68 kΩ
The brilliant 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.
R35, 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.
R2, 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.
R1, 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.
R3, 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, 12AU7 / ECC82
Half an ECC82 (12AU7), the normal channel’s first stage. A low-gain triode with a small 47 kΩ plate resistor and a large cathode resistor, so this channel starts cleaner than the brilliant one.
R5, 47 kΩ
Plate resistor for the normal channel’s first stage, 47 kΩ from the 300 V node.
R4, 4.7 kΩ
Cathode resistor, 4.7 kΩ. Large for an ECC82, so the stage runs cool and its gain is low.
C2, 25 µF electrolytic
Cathode bypass, 25 µF, so the stage keeps its full gain down past the low E.
C1, 22 nF
Coupling cap, .022 µF, to the volume. It passes the full bass.
Normal vol., 470 kΩ pot, audio taper
Normal channel volume, 470 kΩ log, ahead of the gain stage, so it sets how hard the gain stage and follower are driven.
V2a, 12AX7 / ECC83 / 7025
The normal channel’s gain stage, half an ECC83 with a 100 kΩ plate resistor and its cathode bypassed. Its plate drives the follower directly.
R7, 100 kΩ
Plate resistor for the gain stage. It also sets the follower’s grid voltage, since the two are wired directly.
R8, 1.5 kΩ
Cathode resistor for the gain stage.
C5, 25 µF electrolytic
Cathode bypass for the gain stage, for its full gain.
V2b, 12AX7 / ECC83 / 7025
Cathode follower, DC-coupled from the gain stage’s plate. It drives the tone stack from a low impedance, so the stack loses less level. With 56 kΩ under it, as on the AC30, its grid sits close to its cathode, so a big signal pulls grid current and squashes the top of the wave.
R9, 56 kΩ
The follower’s cathode load, 56 kΩ.
C8, 47 pF
Treble cap, 47 pF, to the top of the treble pot.
R10, 100 kΩ
Slope resistor, 100 kΩ, from the follower to the bass and mid caps.
C7, 22 nF
Bass cap, .022 µF, to the bottom of the treble pot and the top of the bass pot.
C6, 22 nF
Mid cap, .022 µF, to the bass pot’s wiper and the 10 kΩ.
Normal treble, 1 MΩ pot, linear taper
Treble, 1 MΩ linear (the drawing marks it). Up brings the wiper toward the 47 pF, for more treble. Its wiper is the channel’s output.
Normal bass, 1 MΩ pot, audio taper
Bass, 1 MΩ log. Top on the treble pot’s bottom, wiper on the mid cap and the 10 kΩ, far end on ground, as on the AC30. Up puts more track between the treble pot and the wiper, for more bass. At full it shorts the 10 kΩ and the mids drop.
R11, 10 kΩ
Fixed 10 kΩ where a Fender stack has its middle pot, so the mids are always scooped slightly.
R12, 470 kΩ
Mixing resistor, 470 kΩ, from the normal channel’s treble wiper to the inverter’s grid. The other channel’s 470 kΩ and its tone stack load the grid too.
V1b, 12AU7 / ECC82
The other half of the ECC82, the brilliant channel’s first stage, with a 100 kΩ plate resistor and 1.5 kΩ on its cathode.
R32, 100 kΩ
Plate resistor for the brilliant channel’s first stage, 100 kΩ from the 300 V node. The drawing’s label is partly lost ("R36 00K"). 100 kΩ fits the 100 V written on the plate.
R36, 1.5 kΩ
Cathode resistor, 1.5 kΩ (3.5 V on the drawing).
C14, 25 µF electrolytic
Cathode bypass, 25 µF, so the stage keeps its full gain down past the low E.
C15, 500 pF
Coupling cap, 500 pF. Into the 470 kΩ volume it’s a high-pass near 680 Hz, so most of the low end never reaches the gain stage. This is what makes the channel brilliant.
Brilliant vol., 470 kΩ pot, audio taper
Brilliant channel volume, 470 kΩ log, ahead of the gain stage.
V6a, 12AX7 / ECC83 / 7025
The brilliant channel’s gain stage, half an ECC83 with a 100 kΩ plate resistor and its cathode bypassed. Its plate drives the follower directly.
R37, 100 kΩ
Plate resistor for the gain stage. It also sets the follower’s grid voltage, since the two are wired directly.
R38, 1.5 kΩ
Cathode resistor for the gain stage.
C18, 25 µF electrolytic
Cathode bypass for the gain stage, for its full gain.
V6b, 12AX7 / ECC83 / 7025
Cathode follower, DC-coupled from the gain stage’s plate. It drives the tone stack from a low impedance, so the stack loses less level. With 56 kΩ under it, as on the AC30, its grid sits close to its cathode, so a big signal pulls grid current and squashes the top of the wave.
R39, 56 kΩ
The follower’s cathode load, 56 kΩ.
C21, 47 pF
Treble cap, 47 pF, to the top of the treble pot.
R40, 100 kΩ
Slope resistor, 100 kΩ, from the follower to the bass and mid caps.
C20, 22 nF
Bass cap, .022 µF, to the bottom of the treble pot and the top of the bass pot.
C19, 22 nF
Mid cap, .022 µF, to the bass pot’s wiper and the 10 kΩ.
Brilliant treble, 1 MΩ pot, linear taper
Treble, 1 MΩ linear (the drawing marks it). Up brings the wiper toward the 47 pF, for more treble. Its wiper is the channel’s output.
Brilliant bass, 1 MΩ pot, audio taper
Bass, 1 MΩ log. Top on the treble pot’s bottom, wiper on the mid cap and the 10 kΩ, far end on ground, as on the AC30. Up puts more track between the treble pot and the wiper, for more bass. At full it shorts the 10 kΩ and the mids drop.
R41, 10 kΩ
Fixed 10 kΩ where a Fender stack has its middle pot, so the mids are always scooped slightly.
R42, 470 kΩ
Mixing resistor, 470 kΩ, from the brilliant channel’s treble wiper to the inverter’s grid. The other channel’s 470 kΩ and its tone stack load the grid too.
C17, 150 pF
Bright cap, 150 pF, from the top of the volume to its wiper through the Brilliant switch. With the volume below full it lets treble skip around the pot. Issue 3 of the drawing added the switch. Before that the cap was 100 pF.
R6, 22 kΩ
Dropping resistor, 22 kΩ, from the normal channel’s 390 V node to the 300 V node, where both input stages hang.
C3, 16 µF electrolytic
Filter cap on the 300 V node, 16 µF 450 V.
V3a, 12AX7 / ECC83 / 7025
12AX7, the driven half of the floating paraphase 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. Its grid is fed from the middle of the divider, so its plate swings the other way and drives tube B.
1M, 1 MΩ
Grid resistor for the driven half, to ground.
1M, 1 MΩ
Grid resistor for the other half, to ground.
820, 820 Ω
Cathode resistor for both halves. It sets their bias, a couple of volts.
25µ, 25 µF electrolytic
Bypass cap across the shared cathode resistor. The cathodes hold still, so the second half gets its signal only from the divider.
100k, 100 kΩ
Plate resistor for the driven half.
100k, 100 kΩ
Plate resistor for the other half.
470k, 470 kΩ
Top of the divider, from tube A’s plate. With the resistor below it, from tube B’s, the middle sits at the average of the two sides. With the two swinging equal and opposite, the middle holds still. Whatever is left goes to the second half’s grid, which corrects it, so the inverter keeps itself even as the tube ages.
470k, 470 kΩ
Bottom of the divider, to tube B’s plate.
.1µ, 100 nF
From the middle of the divider to the second half’s grid. It keeps the plates’ DC off that grid.
.1µ, 100 nF
Coupling cap to tube A’s grid.
.1µ, 100 nF
Coupling cap to tube B’s grid.
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.
V4, EL34 / 6CA7
EL34, tube A of the push-pull pair, biased from the fixed bias supply.
V5, EL34 / 6CA7
EL34, tube B of the pair.
47, 47 Ω
Cathode resistor for tube A, 47 Ω to ground. The bias is fixed, so it’s there to set the bias by: the voltage across it is the cathode current times 47 Ω. It isn’t bypassed, so it also lowers the tube’s gain slightly.
47, 47 Ω
Cathode resistor for tube B, the same.
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, returned to its own bias pot.
100, 100 Ω
Screen resistor for tube A. It limits the screen current when the tube is driven hard.
100, 100 Ω
Screen resistor for tube B.
750, 750 Ω
One screen resistor shared by both tubes, ahead of the resistor on each screen, with no cap after it. It carries both screens’ current. The two screens’ signal currents are opposite and cancel in it, so its drop follows the even harmonics and grows as the stage is driven harder.
8 H, 8 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.
300, 300 Ω
The choke’s winding resistance, typical, not measured. It’s all the DC drop the screens see from B+.
32µ, 32 µF electrolytic
Filter cap on the screen supply.
22k, 22 kΩ
Dropping resistor from the screen supply node to the node for the inverter, as on the Vox AC50 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 screen supply node to the node for the normal channel, as on the Vox AC50 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 normal channel. 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 screen supply node to the node for the brilliant channel, as on the Vox AC50 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 brilliant channel. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
OT, 3.5 kΩ to 15 Ω, 40 H primary, 10 mH leakage
The output transformer, 3.5 kΩ plate to plate into 15 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, 380 V ac, 60 Hz
The power transformer’s high-voltage winding, 380 V ac end to end, into a bridge of four silicon diodes. It’s drawn as two sources of 380 V ac a half cycle apart into two diodes, which charges the reservoir the same way at one diode drop (under a volt) higher. The winding’s marked value, on the drawing.
PT, 380 V ac, 60 Hz
The same winding on the other half cycle. It runs opposite the first source, so the bridge charges the reservoir on every half cycle.
PT R, 110 Ω
Resistance of the whole winding, plus the primary’s share. In a bridge every charging pulse runs through the whole winding. With the diodes’ drop it sets how far B+ sags on a hard note. Typical of the part, not measured.
PT R, 110 Ω
The same winding’s resistance, on the other half cycle. 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.
32µ, 32 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.
Pump, 216 V ac, 60 Hz
Stands in for the bias pump with the part below it. On the drawing, two 4 µF caps in series from one leg of the bridge, with 220 k across each, and a diode to ground make the bias. The operating point can’t be solved through a cap in series, so this 216.08 V ac source and its resistor replace it. They were fitted to a simulation of the real pump on this winding: the same bias, and the same shift when the grids pull current on a hard note. It moves with the wall voltage, as the pump does.
pump R, 18.7 kΩ
Stands in for the pump with the source beside it, 18.7k Ω, fitted. It sets how far the bias moves under grid current. The amp has two 4 µF caps in series from the bridge’s leg here, 220 k across each, a diode to ground at their far end, and 33 k from there into the first filter cap.
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.
33k, 33 kΩ
Second resistor of the bias supply, 33k Ω. With the cap after it, it filters the ripple a second time.
8µ, 8 µF electrolytic
Second filter cap, its plus side to ground. The bias pot hangs from here.
Bias A, 25 kΩ pot, linear taper
Bias adjust for tube A, 25k Ω linear, from the second cap down to the resistor below it. Its wiper feeds tube A’s grid through its grid leak. Toward the cap end the grid goes more negative and the tube idles cooler.
Bias B, 25 kΩ pot, linear taper
Bias adjust for tube B, the same, beside the first. With one for each tube, the two can be set to idle alike.
2k, 2 kΩ
The bottom of the bias divider, 2k Ω to ground, under both pots. It sets the least bias the pots can set.
Coil R, 16.3 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 15 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, 480 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, 870 µH
With Loss R across it, the treble rises and then levels off, like the losses in a real coil.
Loss R, 26.8 Ω
Limits what Loss L adds to the treble rise.
Loss L2, 3.99 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, 4.11 Ω
Limits what Loss L2 adds.
Tank R, 157 Ω
Height of the bass resonance, about 173 Ω in all at its peak. With no feedback, the voltage at the speaker rises and falls with its impedance.
Tank L, 39.6 mH
With Tank C, sets the resonance: 74 Hz.
Tank C, 117 µF
With Tank L, sets the resonance. With Tank R, it sets how narrow the peak is.

About this version

As drawn, Mk III 1965
An ECC82 (12AU7) for the two input stages and an ECC83 (12AX7) per channel for the gain stage and follower, as drawn. The tube models are typical of the type, not measured. Real tubes spread 20 % or more.

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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