Amp with passive Baxandall tone, a six-way coupling switch and a cathodyne, 1974 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 Orange Graphic MkII (120 W).

A gain stage feeds a passive bass-and-treble network of the kind usually built around an op-amp. Then a volume, a second stage, a rotary switch that changes the coupling cap into the driver stage, and a treble boost in that stage’s cathode, where the speaker’s feedback comes in. Its plate drives a cathodyne directly, and four EL34s follow.

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 Baxandall network drops about 20 dB with both controls centered. Each knob moves its band up or down around that level without doing much to the other. The six-way switch, FAC on the front panel, changes only the coupling cap into the third stage. The first position, 68 nF, passes everything. Each one after puts more small caps in series, down to about 200 pF, so the low end is cut higher each time, up to about 2.4 kHz. Tap that cap on the schematic and click through its common values while watching the frequency response. The third stage, V2a, runs on a 390 kΩ plate load, so its plate sits near 100 V, and that plate drives the cathodyne’s grid with no coupling cap between them. The speaker’s feedback comes back from the 16 Ω tap into V2a’s cathode through 15 kΩ. The Boost pot bypasses that cathode through 0.47 µF, which also cuts the feedback above the bass, so it works as a large treble lift. Some amps have a 2 mH choke in series with the cap. The Boost choke switch puts it in.

Download.

Parts and what they do

Stock values for the Musonic sheet, 1974 version. Tap a part on the schematic to change it.

R15, 33 kΩ
Dropping resistor, 2 W, to V1’s node.
C3, 16 µF electrolytic
Filter cap at V1’s node.
R1, 68 kΩ
Input resistor from the jack. The second input’s 68 kΩ is not drawn.
R3, 1 MΩ
Grid leak.
V1a, 12AX7 / ECC83 / 7025
First stage.
R6, 220 kΩ
Plate load (2 % on the drawing).
R5, 2.2 kΩ
Cathode resistor.
C1, 47 µF electrolytic
Cathode bypass, full gain down to the lowest notes.
C9, 68 nF
Coupling cap into the tone network.
R7, 100 kΩ
Top resistor of the bass leg.
Bass, 1 MΩ pot, audio taper
Bass, 1 MΩ log. Toward the top it boosts, toward the bottom it cuts.
C10, 2.2 nF
Across the upper half of the Bass pot. Above the bass it shorts that half.
C11, 22 nF
Across the lower half of the Bass pot.
R8, 22 kΩ
Bottom resistor of the bass leg.
R9, 100 kΩ
From the Bass wiper to the Treble wiper, where the two legs meet.
C13, 1.5 nF
Top cap of the treble leg.
Treble, 1 MΩ pot, audio taper
Treble, 1 MΩ log. Its wiper is the tone network’s output.
C12, 10 nF
Bottom cap of the treble leg.
Volume, 1 MΩ pot, audio taper
Volume, 1 MΩ log. It loads the tone network’s output.
C14, 330 pF
Bright cap across the top of the Volume pot.
V1b, 12AX7 / ECC83 / 7025
Second stage.
R11, 220 kΩ
Plate load.
R10, 2.2 kΩ
Cathode resistor.
C2, 47 µF electrolytic
Cathode bypass.
C15 + FAC, 68 nF
The 68 nF coupling cap and the FAC ladder in series, shown as one capacitor, its common values the six switch positions. On the first contact it’s 68 nF. After that, 4.7 nF, 4.7 nF, 2.2 nF, 1 nF and 330 pF come into series one at a time, for 4.4, 2.3, 1.1 and 0.53 nF and then 200 pF. Against the 320 kΩ that follows, the bass corner moves from about 7 Hz to about 2.4 kHz.
R14, 100 kΩ
Series resistor into V2a. The echo return joins here through another 100 kΩ.
R17, 220 kΩ
V2a’s grid leak. With the 100 kΩ it makes a divider of about 0.69.
V2a, 12AX7 / ECC83 / 7025
Third stage, V2’s first half. Its plate is wired straight to the cathodyne’s grid, with no coupling cap, so the plate’s DC voltage, about 100 V, sets the cathodyne’s. With the amp’s own power amp on, its cathode is where the speaker’s feedback comes in.
R16, 390 kΩ
R16, the plate load, 390 kΩ. It’s large so the plate sits low, about 100 V, where the cathodyne’s grid needs it.
C21, 1 nF
C21, across the plate load. It lowers the stage’s gain above about 400 Hz.
Boost, 1 kΩ pot, linear taper
VC4, Boost, 1 kΩ linear, from the cathode to ground. Its wiper goes to ground through the 0.47 µF. Turned up, the cap bypasses more of the cathode resistance above the bass, so the stage has more gain there.
C22, 470 nF
C22, the Boost cap, 0.47 µF from the pot’s wiper to ground. With the Boost at full and V2a’s cathode resistance about 600 Ω, it bypasses the cathode above about 600 Hz.
V2b, 12AX7 / ECC83 / 7025
12AX7, 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 the other half of V2, the ECC83 whose first half is the stage before.
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.
15k, 15 kΩ
The feedback resistor from the speaker. With the 1.5k Ω below it, it sends about 9 % 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. It isn’t bypassed.
.068µ, 68 nF
Coupling cap to tube A’s grid.
.068µ, 68 nF
Coupling cap to tube B’s grid.
1.1k, 1.1 kΩ
Grid stopper for tube A. It keeps the tube from oscillating and softens grid current. Each of the two tubes has its own 2.2k. The resistors are drawn as one part, the two in parallel.
1.1k, 1.1 kΩ
Grid stopper for tube B. Each of the two tubes has its own 2.2k. The resistors are drawn as one part, the two in parallel.
V3, V4, EL34 / 6CA7
EL34, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply. Two tubes in parallel, drawn as one.
V5, V6, EL34 / 6CA7
EL34, tube B of the pair. Two tubes in parallel, drawn as one.
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.
500, 500 Ω
Screen resistor for tube A. It limits the screen current when the tube is driven hard. Each of the two tubes has its own 1k. The resistors are drawn as one part, the two in parallel.
500, 500 Ω
Screen resistor for tube B. Each of the two tubes has its own 1k. The resistors are drawn as one part, the two in parallel.
10 H, 10 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.
100, 100 Ω
The choke’s winding resistance, typical. It’s all the DC drop the screens see from B+.
16.5µ, 16.5 µF electrolytic
Filter cap on the screen supply.
33k, 33 kΩ
Dropping resistor from the screen supply node to the node for the cathodyne and the preamp, as on the Orange Graphic MkII 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 cathodyne 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, 1.7 kΩ to 16 Ω, 20 H primary, 5 mH leakage
The output transformer, 1.7 kΩ plate to plate into 16 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, 368 V ac, 60 Hz
The power transformer’s high-voltage winding, 368 V ac end to end, into a bridge of four silicon diodes with the center tap unused. It’s drawn as two sources of 368 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 value is typical of the part.
PT, 368 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, 40 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.
PT R, 40 Ω
The same winding’s resistance, on the other half cycle. Typical of the part.
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.
50µ, 50 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, 100 V ac, 60 Hz
The power transformer’s bias tap, 100 V ac, as a sine source at the mains frequency. Typical of the part.
22k, 22 kΩ
Series resistor into the bias rectifier, 22k Ω. It limits the charging current.
Bias diode, 1N4001
The bias rectifier, a silicon diode turned so the supply comes out negative.
10µ, 10 µF electrolytic
Filter cap on the bias supply, its plus side to ground.
15k, 15 kΩ
Second resistor of the bias supply, 15k Ω. With the cap after it, it filters the ripple a second time.
10µ, 10 µF electrolytic
Second filter cap, its plus side to ground. The bias pot hangs from here.
Bias 50k, 50 kΩ pot, linear taper
Bias adjust, 50k Ω linear, from the second cap down to the resistor below it. Its wiper feeds both output grids through their grid leaks. Toward the cap end the grid goes more negative and the tube idles cooler. On the drawing it’s VR1, a 100 kΩ preset with R29 100 kΩ across it, run here as one 50 kΩ pot (the same divider at DC).
33k, 33 kΩ
The bottom of the bias divider, 33k Ω to ground. It sets the least bias the pot can set.
Coil R, 17.4 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 16 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, 512 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, 928 µH
With Loss R across it, the treble rises and then levels off, like the losses in a real coil.
Loss R, 28.6 Ω
Limits what Loss L adds to the treble rise.
Loss L2, 4.26 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, 4.38 Ω
Limits what Loss L2 adds.
Tank R, 167 Ω
Height of the bass resonance, about 185 Ω in all at its peak. The feedback holds the output steadier against that than it would be without feedback.
Tank L, 42.2 mH
With Tank C, sets the resonance: 74 Hz.
Tank C, 110 µF
With Tank L, sets the resonance. With Tank R, it sets how narrow the peak is.

Versions

Musonic sheet, 1974
Musonic’s 1974 sheet, with Orange’s parts list where the two agree. ECC83 triodes.
Orange parts list
Orange’s own typed sheet and parts list: C13 330 pF, C12 2200 pF and C11 10 nF in the tone stack, where Musonic’s sheet has 1500 pF, 10 nF and 22 nF. Everything else as the Musonic build. With Bass and Treble at 5 the mids fill in: about 3.5 dB more from 220 to 440 Hz, the same at 55 Hz and above 2 kHz. ECC83 triodes.

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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