Four-channel amp with separately voiced inputs, late 1960s 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 Sound City Hiwatt Lead 105.

Four inputs, each with a triode stage and volume. The channels differ only in the coupling cap into the volume: 22 nF with a 4.7 nF treble cut, 68 nF, and two at 470 pF. They mix through 470 kΩ into a gain stage, then a two-knob treble and bass network, a master volume, a third gain stage and a DC-coupled cathode follower into 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. At the default settings the preamp keeps a 100 mV note under 1 % distortion. Through the stand-in power amp it reads about 17 %. With a channel volume and the master full up, V4a after the master clips first, from about 100 mV at the input, then V3. Channel 1 passes the full range, its top end rolled off by 4.7 nF on the volume. Channel 2 is full range. Channels 3 and 4 pass only the upper mids and treble, since 470 pF into a 500 kΩ pot cuts below about 680 Hz. Patch more than one input to the guitar to blend them, as players did with jumper leads. Each input adds 470 kΩ of load. There’s no middle control. With Bass down the tone network is close to flat, about 20 dB below its input. Turning Bass up lifts the bass and also cuts a dip near 1 kHz, up to about 15 dB deep. The stage after the master and the cathode follower follow the late 1960s builds. This amp’s own power amp isn’t on the bench yet, so it opens on the JCM800 2203’s power amp as a stand-in.

Download.

Parts and what they do

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

C1, 8 µF electrolytic
Filter cap on the phase inverter’s node, which V3 also pulls its current from.
R1, 47 kΩ
Dropping resistor to the input stages’ node.
C2, 8 µF electrolytic
Filter cap for V1 and V2.
R2, 100 kΩ
Dropping resistor to V4’s node. Both halves of V4 pull their current through it.
C3, 8 µF electrolytic
Filter cap for V4, 500 V.
R3, 470 kΩ
Input 1’s resistor to ground. There is no grid resistor: the jack goes straight to the grid.
R4, 470 kΩ
Input 2’s resistor to ground.
R5, 470 kΩ
Input 3’s resistor to ground.
R6, 470 kΩ
Input 4’s resistor to ground.
V1a, 12AX7 / ECC83 / 7025
Input 1 stage.
R7, 220 kΩ
Plate load.
V1b, 12AX7 / ECC83 / 7025
Input 2 stage, sharing V1a’s cathode.
R8, 220 kΩ
Plate load.
R9, 1.5 kΩ
V1’s shared cathode resistor. With both halves pulling current through it, each sits at about 1.5 V bias.
C4, 64 µF electrolytic
Shared bypass, full gain through the bass for both stages.
V2a, 12AX7 / ECC83 / 7025
Input 3 stage.
R10, 220 kΩ
Plate load.
V2b, 12AX7 / ECC83 / 7025
Input 4 stage, sharing V2a’s cathode.
R11, 220 kΩ
Plate load.
R12, 1.5 kΩ
V2’s shared cathode resistor.
C5, 64 µF electrolytic
Shared bypass.
C6, 22 nF
Channel 1 coupling cap, full range.
Volume 1, 500 kΩ pot, audio taper
Volume 1, 500 kΩ log.
C7, 4.7 nF
From volume 1’s wiper to ground, marked uncertain on the drawing. With the pot’s resistance and the 470 kΩ mixer it rolls off the treble, most with the volume near halfway.
R13, 470 kΩ
Channel 1 mixing resistor.
C8, 68 nF
Channel 2 coupling cap, full range.
Volume 2, 500 kΩ pot, audio taper
Volume 2, 500 kΩ log.
R14, 470 kΩ
Channel 2 mixing resistor.
C9, 470 pF
Channel 3 coupling cap. 470 pF into the 500 kΩ volume cuts below about 680 Hz.
Volume 3, 500 kΩ pot, audio taper
Volume 3, 500 kΩ log.
R15, 470 kΩ
Channel 3 mixing resistor.
C10, 470 pF
Channel 4 coupling cap, the same as channel 3.
Volume 4, 500 kΩ pot, audio taper
Volume 4, 500 kΩ log.
R16, 470 kΩ
Channel 4 mixing resistor. With four 470 kΩ on the bus, each channel reaches V3 at about a quarter of its wiper voltage.
V3, 12AX7 / ECC83 / 7025
Gain stage after the mixer. The tube’s other half is unused.
R17, 220 kΩ
Plate load, from the phase inverter’s node.
R18, 2.2 kΩ
Cathode resistor.
C11, 47 nF
Bypasses the cathode only above about 1.5 kHz, a treble lift.
C12, 47 nF
Coupling cap into the tone network.
C13, 220 pF
Treble cap, to the top of the Treble pot.
Treble, 220 kΩ pot, audio taper
Treble, 220 kΩ. The taper isn’t marked, so log is assumed. Its wiper feeds the master.
R19, 100 kΩ
The path for the lows and mids, to the bottom of the Treble pot.
Bass, 500 kΩ pot, audio taper
Bass, 500 kΩ log, from the bottom of the Treble pot to ground, its wiper on the 47 nF.
R20, 10 kΩ
From the top of the Bass pot to its wiper.
C14, 47 nF
From the Bass wiper to ground: with the wiper high it shorts the mids and treble at the bottom of the Treble pot, leaving the bass.
Master, 220 kΩ pot, audio taper
Master volume, 220 kΩ. The taper isn’t marked, so log is assumed.
V4a, 12AX7 / ECC83 / 7025
Gain stage after the master.
R21, 100 kΩ
Plate load.
R22, 2.2 kΩ
Cathode resistor.
C15, 47 nF
Treble-only bypass again, above about 1.5 kHz.
R23, 1.8 MΩ
DC coupling to the cathode follower. With the 1 MΩ below it, V4b’s grid sits at about a third of V4a’s plate voltage.
C16, 22 nF
Across the 1.8 MΩ so the signal skips the divider’s loss. Marked uncertain on the drawing.
R24, 1 MΩ
Lower resistor of the divider.
V4b, 12AX7 / ECC83 / 7025
Cathode follower, its cathode wired straight to the phase inverter’s grid.
R25, 100 kΩ
Cathode follower load.
.022µ, 22 nF
Coupling cap into the phase inverter. With the grid resistor it sets how low the inverter goes, around 5 Hz.
V5a, 12AX7 / ECC83 / 7025
12AX7, the driven half of the long-tail pair phase inverter. Its plate swings opposite the input and drives tube A.
V5b, 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.
V6, V8, 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.
V7, V9, 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.
50µ, 50 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, 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

S/N 223
From Mark Huss’s drawing of serial 223 (information from Chris M.). Four ECC83s, typical of the type, not measured. The drawing marks three values uncertain: the 4.7 nF on volume 1, the 22 nF across the 1.8 MΩ, and a 47 nF in the feedback network, which isn’t modeled.

Sources

  • Mark Huss, redrawn Sound City Lead 105 schematic

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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