Two-channel KT66 combo with tremolo, 1964 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 1962 Bluesbreaker combo.

The JTM45’s preamp with a tremolo on one channel: a triode oscillator turns a germanium transistor on and off, and the transistor shunts that channel’s volume wiper to ground through the Intensity pot.

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. Two channels, each one triode with its own volume, mixed through 270 kΩ into a third stage, a cathode follower and the treble, middle and bass stack, as on the JTM45. The tremolo works only on the tremolo channel. The oscillator, one half of an ECC83, drives a germanium transistor through 680 kΩ and 0.1 µF. Each time it swings down, the transistor turns on and pulls the pot’s wiper to ground, and the channel’s volume wiper is loaded down through the 0.1 µF and 10 kΩ. Intensity sets how much of the 50 kΩ is between them. The footswitch is in the transistor’s collector.

Download.

Parts and what they do

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

R1B, 68 kΩ
The bright 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.
R3B, 68 kΩ
The bright 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.
R2, 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.
R1T, 68 kΩ
The tremolo 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.
R3T, 68 kΩ
The tremolo 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.
R2T, 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.
10k, 10 kΩ
Dropping resistor, 10 kΩ 1 W, from the inverter’s 380 V node to the 310 V node.
16µ, 16 µF electrolytic
Filter cap on the 310 V node.
V1b, 12AX7 / ECC83 / 7025
Bright channel gain stage. Its cathode is shared with the tremolo channel’s.
100k, 100 kΩ
Bright stage plate resistor. The chart reads 220 V on the plate.
.02µ, 20 nF
Bright channel coupling cap.
Volume, 1 MΩ pot, audio taper
Bright channel volume, 1 MΩ.
100p, 100 pF
Bright cap from the volume’s top to its wiper. It lifts the treble with the volume low and does nothing at full.
270k, 270 kΩ
Bright channel mixing resistor to V3a’s grid.
500p, 500 pF
Cap across the bright mixing resistor, to put back treble. Its value can’t be read on the drawing, so a typical 500 pF is assumed.
V1a, 12AX7 / ECC83 / 7025
Tremolo channel gain stage.
100k, 100 kΩ
Tremolo stage plate resistor.
820, 820 Ω
Cathode resistor for both V1 halves, 820 Ω. The chart reads 1.6 V on the cathodes.
250µ, 250 µF electrolytic
Bypass cap on the shared cathode. The drawing reads 250 µF, enough for full gain through the bass.
.02µ, 20 nF
Tremolo channel coupling cap.
Volume, 1 MΩ pot, audio taper
Tremolo channel volume, 1 MΩ. The tremolo works on its wiper.
270k, 270 kΩ
Tremolo channel mixing resistor.
V3a, 12AX7 / ECC83 / 7025
Third stage, shared by both channels. Its plate drives the cathode follower directly.
100k, 100 kΩ
V3a plate resistor. The chart reads 190 V.
820, 820 Ω
V3a cathode resistor, no bypass cap, so this stage has less gain than on later Marshalls.
V3b, 12AX7 / ECC83 / 7025
Cathode follower driving the tone stack, DC-coupled from V3a’s plate.
100k, 100 kΩ
Cathode follower load.
270p, 270 pF
Treble cap.
56k, 56 kΩ
Slope resistor, 56 kΩ, where later Marshalls have 33 kΩ.
.02µ, 20 nF
Bass cap.
.02µ, 20 nF
Mid cap.
Treble, 250 kΩ pot, linear taper
Treble pot, 250 kΩ. Its wiper is the preamp’s output. The drawing marks no taper, so linear is assumed, as on Marshall’s later drawings.
Bass, 1 MΩ pot, audio taper
Bass pot, 1 MΩ, a variable resistor.
Middle, 25 kΩ pot, linear taper
Mid pot, 25 kΩ, the mid cap on its wiper and its track from the bass pot to ground.
.02µ, 20 nF
Coupling cap into the inverter’s grid.
.1µ, 100 nF
Cap from the tremolo channel’s volume wiper into the shunt. The drawing marks it “.1M”, read as 0.1 µF.
10k, 10 kΩ
The least the shunt can be, 10 kΩ, with Intensity on full and the transistor on.
Intensity, 50 kΩ pot, linear taper
Intensity, 50 kΩ. The transistor grounds its wiper, so turned up less of the track sits between the 10 kΩ and ground and the dips go deeper. The taper isn’t marked, so linear is assumed.
33k, 33 kΩ
Bottom of the shunt, 33 kΩ to ground. With the transistor off, the 10 kΩ, the pot and this load the wiper all the time.
Q1, 2G371
The 2G371, a germanium PNP used as a switch. Its emitter is grounded. When the oscillator swings its base below about −0.2 V it turns on and grounds the Intensity pot’s wiper through the footswitch.
4.3k, 4.3 kΩ
Base resistor to ground. With the 680 kΩ above it, it divides the oscillator’s swing down to under a volt at the base. Read as 4.3 kΩ.
0.1µ, 100 nF
Coupling cap from the oscillator into the base.
680k, 680 kΩ
From the oscillator’s plate toward the transistor’s base.
V2, 12AX7 / ECC83 / 7025
The tremolo oscillator, one half of an ECC83 (the other half is unused). The chart reads 140 V on the plate and 1.2 V on the cathode.
220k, 220 kΩ
Oscillator plate resistor, from the screen node.
150k, 150 kΩ
From the oscillator’s plate to ground. With the 220 kΩ it divides the screen node down for the plate.
2.7k, 2.7 kΩ
Oscillator cathode resistor. The drawing’s figure reads like “37K”, but 2.7 kΩ fits the chart’s 1.5 V.
50µ, 50 µF electrolytic
Oscillator cathode bypass cap.
.022µ, 22 nF
First cap of the phase-shift ladder.
Speed, 2 MΩ pot, linear taper
Speed, 2 MΩ as a variable resistor in series with 470 kΩ, the ladder’s first leg to ground. Turned up, the resistance drops and the oscillator runs faster. The taper isn’t marked, so linear is assumed.
470k, 470 kΩ
In series with the Speed pot. It sets the fastest speed.
.01µ, 10 nF
Second ladder cap.
1M, 1 MΩ
Second ladder leg, 1 MΩ to the oscillator’s cathode.
.01µ, 10 nF
Third ladder cap, into the grid.
1M, 1 MΩ
Oscillator grid leak and the ladder’s last leg.
V4a, 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.
V4b, 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.
27k, 27 kΩ
The feedback resistor, 27 kΩ from the speaker tap. With the presence pot it sends about 16 % of the speaker voltage back to the inverter, which lowers the gain, flattens the response and tightens the bass.
Presence, 5 kΩ pot, linear taper
Presence, 5 kΩ linear, the bottom of the feedback loop. Its wiper goes to ground through the cap. Turned up, the cap shunts more of the fed-back treble to ground, so less treble is fed back and the top end rises, a few dB around 5 kHz.
.1µ, 100 nF
The presence cap. With the pot it sets where the lift starts.
.1µ, 100 nF
Coupling cap to tube A’s grid.
.1µ, 100 nF
Coupling cap to tube B’s grid.
V5, KT66
KT66, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply.
V6, KT66
KT66, 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.
470, 470 Ω
Screen resistor for tube A. It limits the screen current when the tube is driven hard.
470, 470 Ω
Screen resistor for tube B.
1k, 1 kΩ
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.
20 H, 20 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+.
32µ, 32 µF electrolytic
Filter cap on the screen supply.
8.2k, 8.2 kΩ
Dropping resistor from the screen supply node to the node for the inverter and the preamp, as on the Marshall JTM45 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 inverter 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, 7.4 kΩ to 16 Ω, 40 H primary, 10 mH leakage
The output transformer, 7.4 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, not measured.
PT, 355 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 355 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, 355 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, 40 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, 40 Ω
Resistance of the other half. Typical of the part, not measured.
V7, GZ34
One plate of the GZ34 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.
V7, GZ34
The rectifier’s other plate, 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.
PT, 355 V ac, 60 Hz
The end of the high-voltage winding the bias comes off, 355 V ac, the same end as the rectifier’s first plate. It’s drawn as its own source so the bias supply’s average can be found. Keep it equal to the winding beside the rectifier.
150k, 150 kΩ
Series resistor into the bias rectifier, 150k Ω. There’s no bias tap, so the bias comes straight off the high-voltage winding, and this resistor drops most of it.
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.
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.
8µ, 8 µF electrolytic
Second filter cap, on the bias line, its plus side to ground.
56k, 56 kΩ
From the bias line to ground, 56k Ω. With the 15k Ω and 150k Ω above it, it divides the supply down to the bias. There’s no bias pot, so change this resistor to change the bias. Bigger runs the tubes colder.
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.

About this version

As drawn
ECC83 (12AX7) tubes, as drawn, and a germanium PNP for the 2G371. The models are typical of the parts, 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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