Master-volume tube amp, late 1970s 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 Marshall JCM800 2203.

Four triode stages from the high input to the master volume: a gain stage, a cold-biased clipper, a hot stage, and a cathode follower driving the treble-mid-bass tone stack. The master volume feeds the phase inverter.

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. Most of the character is in the middle two stages. The second triode idles at a few tenths of a milliamp, with 10 kΩ in its cathode. The negative half of the signal cuts it off early while the positive half stays clean: lopsided clipping with a lot of second harmonic. Put the probe on its plate and turn the preamp volume up. The third stage is biased hot and clips the other half. Ahead of each of those grids sits 470 kΩ with 470 pF across it. The cap lets the treble around the divider, and the resistor limits grid current so the stage doesn’t block and the sound keeps its edge. The cathode follower is the first stage to draw grid current. The tone stack after it is passive and can only cut, so the three knobs interact and the mid knob is a scoop control.

Download.

Parts and what they do

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

CF2, 47 µF electrolytic
Filter cap on the V2 supply node.
RD2, 10 kΩ
Second dropping resistor, to the V1 node. Some amps had one 10 k here and some two. With two, V1 sits about 15 V lower.
CF1, 47 µF electrolytic
Filter cap on the V1 node. With the dropping resistor it decouples V1 from V2, so the stages don’t interact through the supply.
R2, 1 MΩ
The high input’s 1 MΩ to ground. It’s the grid leak for V1a and the input impedance the guitar sees.
R1, 68 kΩ
Grid stopper. With the tube’s input capacitance it rolls off radio frequencies, and it limits grid current when a hot pickup drives the grid positive.
V1a, 12AX7 / ECC83 / 7025
First gain stage, a common-cathode triode with a 100 kΩ plate load. Gain about 60 with the cathode bypassed, from the mids up.
RP1, 100 kΩ
V1a plate load resistor.
RK1, 2.7 kΩ
V1a cathode resistor. 2.7 kΩ biases the stage slightly cool. The tweed-style 1.5 kΩ runs warmer with more gain.
CK1, 680 nF
V1a cathode bypass cap. 0.68 µF against 2.7 kΩ bypasses only above about 90 Hz, so the bass gets less gain than the mids and treble. With a bigger cap the stage has full gain all the way down.
C1, 22 nF
V1a coupling cap. Marshall used 22 nF and, in many amps, 2.2 nF, which thins the bass going into the divider.
R3, 470 kΩ
Top of the first divider, with the 1 MΩ preamp volume as the bottom. It drops about a third of the signal at low and mid frequencies.
C3a, 470 pF
Treble peaker across R3. It lets the highs around the divider, so the divider cuts less as the frequency goes up.
Preamp vol., 1 MΩ pot, audio taper
Preamp volume. It sets how hard the cold clipper and everything after it are driven.
CB1, 1 nF
Bright cap across the top of the preamp volume. With the pot turned down it lets the treble around it. At full it does nothing.
V1b, 12AX7 / ECC83 / 7025
The cold clipper. With 10 kΩ in its cathode and no bypass cap, it idles at a few tenths of a milliamp, and the negative half of the signal cuts it off early.
RP2, 100 kΩ
V1b plate load.
RK2, 10 kΩ
The cold clipper’s cathode resistor, unbypassed. 10 kΩ is the factory value. Later high-gain amps used 39 kΩ, colder still.
C2, 22 nF
Coupling cap out of the cold clipper.
R5, 470 kΩ
To ground after the coupling cap. It’s the DC return for the cap and, through R6, for V2a’s grid.
R6, 470 kΩ
Grid stopper for V2a. At idle the grid pulls no current, so only the grid’s capacitance shaves the treble, about 1 dB at 1 kHz. Driven positive, the grid pulls current and this resistor limits it, so the coupling cap doesn’t charge up and block.
C3, 470 pF
Treble peaker across R6, doing the same job as C3a.
V2a, 12AX7 / ECC83 / 7025
Third gain stage, biased hot with 820 Ω. It clips the half of the signal the cold clipper leaves alone, and its plate drives the cathode follower directly.
RP3, 100 kΩ
V2a plate load. 100 kΩ in most amps, 82 kΩ in some.
RK3, 820 Ω
V2a cathode resistor, a warm bias. Unbypassed in the factory circuit.
V2b, 12AX7 / ECC83 / 7025
Cathode follower, DC-coupled from V2a’s plate. Unity gain and a low output impedance to drive the tone stack. Its grid sits about 200 V above ground, and it’s the first stage to draw grid current when pushed.
RK4, 100 kΩ
Cathode follower load resistor.
CT, 470 pF
Treble cap, the high-pass path into the treble pot.
R7, 33 kΩ
Slope resistor. 33 kΩ is the usual Marshall value. 56 kΩ, the tweed value, moves the mid scoop lower.
CB, 22 nF
Bass cap.
CM, 22 nF
Mid cap. Marshall used 22 nF for both bass and mid. Fender used 100 nF for the bass and 47 nF for the mid, which lets more bass through.
Treble, 220 kΩ pot, linear taper
Treble pot, 220 kΩ linear, as on Marshall’s drawing. Its wiper is the tone stack’s output.
Bass, 1 MΩ pot, audio taper
Bass pot, wired as a variable resistor between the treble pot’s bottom and the mid pot.
Middle, 22 kΩ pot, linear taper
Mid pot, 22 kΩ linear, with the mid cap on the wiper and the bass pot returning to ground through the whole track. That’s the 5F6-A wiring, which Marshall kept. Blackface Fenders wire it as a variable resistor instead. The two match at full mid. Turned down, this wiring leaves slightly more bass.
Master, 1 MΩ pot, audio taper
Master volume, ahead of the phase inverter. It sets how hard the inverter and power tubes are driven and leaves the preamp alone.
C7, 22 nF
Coupling cap into the phase inverter grid.
V3a, 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.
V3b, 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.
470, 470 Ω
Bias resistor for both halves. The cathodes sit this far above the grids’ return, a volt or so.
10k, 10 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.
100k, 100 kΩ
The feedback resistor from the speaker. With the 4.7k Ω below it, it sends about 4 % of the speaker voltage back to the inverter, which lowers the gain, flattens the response and tightens the bass.
4.7k, 4.7 kΩ
The bottom of the feedback loop, 4.7k 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, 22 kΩ pot, linear taper
Presence, a 22k 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.
.022µ, 22 nF
Coupling cap to tube A’s grid.
.022µ, 22 nF
Coupling cap to tube B’s grid.
2.8k, 2.8 kΩ
Grid stopper for tube A. It keeps the tube from oscillating and softens grid current. Each of the two tubes has its own 5.6k. The resistors are drawn as one part, the two in parallel.
2.8k, 2.8 kΩ
Grid stopper for tube B. Each of the two tubes has its own 5.6k. The resistors are drawn as one part, the two in parallel.
V4, V6, 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, V7, 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.
5 H, 5 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+.
50µ, 50 µF electrolytic
Filter cap on the screen supply.
50µ, 50 µF electrolytic
Second filter cap on the screen supply, beside the first.
10k, 10 kΩ
Dropping resistor from the screen supply node to the node for the inverter and the preamp, as on the Marshall JCM800 2203 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.
OT, 1.7 kΩ to 4 Ω, 20 H primary, 5 mH leakage
The output transformer, 1.7 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, 360 V ac, 60 Hz
The power transformer’s high-voltage winding, 360 V ac end to end, into a bridge of four silicon diodes with the center tap unused. It’s drawn as two sources of 360 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, not measured.
PT, 360 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, not measured.
PT R, 40 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.
50µ, 50 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.
50µ, 50 µF electrolytic
Another reservoir cap, in parallel with the first.
Bias tap, 96 V ac, 60 Hz
The power transformer’s bias tap, 96 V ac, as a sine source at the mains frequency. Typical of the part, not measured.
27k, 27 kΩ
Series resistor into the bias rectifier, 27k 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 to the bias line. With the cap after it, it filters the ripple a second time.
10µ, 10 µF electrolytic
Second filter cap, on the bias line, its plus side to ground.
47k, 47 kΩ
From the bias line toward ground, 47k Ω, in series with the bias pot. These two and the 15k Ω above them divide the supply down to the bias.
Bias 22k, 22 kΩ pot, linear taper
Bias adjust, a 22k Ω trimmer used as a variable resistor, the bottom of the divider. More resistance puts more of the supply on the grids, so turned up the tubes idle cooler. Which way it turns isn’t marked on the drawing.
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.

About this version

12AX7 throughout
Two 12AX7s, the factory tubes. The tube models are typical of the type, not measured. Real tubes spread 20 % or more in current at the same bias.

Sources

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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