Four-input amp with master volume, early 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 Hiwatt DR103 Custom 100.

Two input stages, one bright and one full-range, mix through 470 kΩ into a second stage. Then a passive treble-middle-bass network, laid out unlike the Fender-derived stack, a master volume and a third stage into the phase inverter. Six builds, from redrawn schematics of the late 1960s to the late 1970s.

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 passes a 100 mV note with under 1 % distortion, and the stand-in power amp adds some of its own, about 6 % in all. With both volumes and the master full up, V2 and then V3a start to clip from about 100 mV at the input. In the late 1970s build each volume drives its own half of V2, biased at about 1 V with no mixing loss, so clipping starts nearer 30 mV. The Brilliant channel gets its treble from a 1 nF coupling cap and a 47 nF cathode cap that bypasses only above about 1.5 kHz. The Normal channel passes the full range. The tone network drops about 20 dB in the bass and less toward the treble. Middle down cuts a notch about 10 dB deep near 500 to 600 Hz, and Middle up removes it. Compare the builds: through the early 1970s the third stage drives the inverter through a DC-coupled cathode follower. From the mid 1970s it drives the inverter through a coupling cap, the cathode follower only sets the inverter’s bias, and the third stage sees a 100 kΩ load. The phase inverter and power amp add distortion of their own. This amp’s own power amp isn’t on the bench yet, so it opens on a stand-in, the JCM800 2203’s.

Download.

Parts and what they do

Stock values for the Early 1970s four-input version. Tap a part on the schematic to change it.

R1, 22 kΩ
Dropping resistor, 2 W, from the phase inverter’s supply (HT3 on the drawings) to the V2 node. 22 kΩ in the 1970s amps, 47 kΩ in the 1960s ones.
C1, 32 µF electrolytic
Filter cap on the V2 node, 450 V.
R2, 10 kΩ
Dropping resistor, 2 W, to the V1 node. 10 kΩ in the 1970s amps, 22 kΩ in the 1960s ones.
C2, 16 µF electrolytic
Filter cap on the V1 node, 350 V. In the 1970s amps V3 pulls its current from here as well.
R4, 1 MΩ
The Brilliant channel’s 1 MΩ input resistor. Jumpered, the guitar sees it in parallel with the Normal channel’s, 500 kΩ in all.
R5, 68 kΩ
Grid resistor from the high-gain jack. The low-gain jack’s 68 kΩ is not drawn.
R6, 1 MΩ
The Normal channel’s 1 MΩ input resistor. With the jumper out it holds the unused grid at ground.
R7, 68 kΩ
Normal channel grid resistor.
V1a, 12AX7 / ECC83 / 7025
Brilliant channel input stage.
R8, 220 kΩ
Brilliant stage plate load.
R9, 2.2 kΩ
Brilliant stage cathode resistor.
C4, 47 nF
Brilliant stage bypass cap. Against 2.2 kΩ it bypasses only above about 1.5 kHz, so the treble gets up to about 6 dB more gain. Try 25 µF to hear the channel without the treble lift.
V1b, 12AX7 / ECC83 / 7025
Normal channel input stage.
R10, 220 kΩ
Normal stage plate load.
R11, 2.2 kΩ
Normal stage cathode resistor. Unbypassed in the early and mid 1970s amps. In the others, both input stages share it.
C6, 1 nF
Brilliant coupling cap. 1 nF into the 470 kΩ volume cuts below about 340 Hz. The earliest drawing has 22 nF, which passes the bass.
Brilliant, 470 kΩ pot, audio taper
Brilliant volume, 470 kΩ log.
R12, 470 kΩ
Brilliant mixing resistor. With the other 470 kΩ it halves each channel’s signal, and with V2a’s input capacitance it rolls off some treble.
C8, 22 nF
Normal coupling cap, full range.
Normal, 470 kΩ pot, audio taper
Normal volume, 470 kΩ log.
R13, 470 kΩ
Normal mixing resistor.
V2a, 12AX7 / ECC83 / 7025
Second stage, driving the tone network from its plate. In the late 1970s build only the Brilliant volume feeds it.
R14, 220 kΩ
V2 plate load: 220 kΩ or 100 kΩ depending on the year. It is also the source resistance the tone network sees.
R15, 2.2 kΩ
V2a cathode resistor.
C10, 1 nF
Feeds the treble and middle legs of the tone network. It passes little below a few hundred hertz.
C11, 220 pF
Treble cap, to the top of the Treble pot.
R17, 220 kΩ
From the same point to the bottom of the Treble pot, the path for the middle frequencies.
Treble, 220 kΩ pot, linear taper
Treble, 220 kΩ linear. Its wiper is the network’s output.
R18, 22 kΩ
From the bottom of the Treble pot to the Middle pot.
Middle, 100 kΩ pot, linear taper
Middle, 100 kΩ linear, wired as a variable resistor between the treble leg and the bass leg.
C12, 1 nF
Across the Middle pot. Above a few kilohertz it shorts the pot, so Middle leaves the top end alone.
R19, 100 kΩ
Feeds the bass leg straight from V2’s plate.
C13, 47 nF
With the 100 kΩ a low-pass at about 34 Hz, so only the bass gets through this leg with any strength.
C14, 47 nF
Couples the bass leg to the Bass and Middle pots.
Bass, 470 kΩ pot, audio taper
Bass, 470 kΩ log, wired as a variable resistor to ground. Turned down it shorts the bass leg.
R20, 22 kΩ
From the Treble wiper to the master. 22 kΩ in the 1970s amps. The 1960s drawings show 470 kΩ and 220 kΩ, which lose more level.
Master, 220 kΩ pot, audio taper
Master volume, 220 kΩ log (470 kΩ in the 1960s amps).
V3a, 12AX7 / ECC83 / 7025
Third stage, after the master.
R21, 100 kΩ
V3a plate load.
R22, 2.2 kΩ
V3a cathode resistor: 2.2 kΩ, 1.6 kΩ or 1.5 kΩ depending on the year.
C15, 47 nF
V3a bypass cap, treble only again, above about 1.5 to 2.3 kHz depending on the year. Drawn dashed on the mid and late 1970s drawings, so the switch starts off there.
R23, 1.8 MΩ
DC coupling to the cathode follower. With the resistor to ground below it, it sets V3b’s grid at about a third of V3a’s plate voltage.
C18, 22 nF
Across the coupling resistor, so the signal reaches V3b’s grid without the divider’s loss. 10 nF in the earliest drawing.
R24, 1 MΩ
Lower resistor of V3b’s grid divider.
V3b, 12AX7 / ECC83 / 7025
Cathode follower. Until the mid 1970s it drives the phase inverter’s grid directly. Later it only supplies that grid’s bias.
R26, 100 kΩ
Cathode follower load: 100 kΩ or 220 kΩ.
.022µ, 22 nF
Coupling cap into the phase inverter. With the grid resistor it sets how low the inverter goes, around 5 Hz.
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.
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.
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.
V5, V7, 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.
V6, V8, 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.

Versions

Late 1960s (I)
From Mark Huss’s drawing “Late 60s Four-Input Preamp I” (information from Stuart C.). Shared V1 cathode (1.5 kΩ, 100 µF), 22 nF coupling on both channels, 220 pF across the Brilliant mixing resistor, 470 kΩ into a 470 kΩ master, and a presence control in the third stage: a 100 kΩ pot with its wiper grounded, its ends to 220 nF from the cathode and 1 nF from the plate.
Late 1960s (II)
From Mark Huss’s drawing “Late 60s Four-Input Preamp II”, reverse engineered by R. F. Dettorre. Shared V1 cathode (2.2 kΩ, 16 µF), 1 nF and 10 nF coupling, 220 kΩ into a 470 kΩ master, a bypassed third stage and a 220 kΩ cathode-follower load. Its presence control is in the power amp and isn’t modeled.
Early 1970s linked
From Mark Huss’s drawing “Early 70s Linked-Input Preamp”. Like the four-input build, but with a 100 kΩ / 1 kΩ second stage and 1 MΩ and 560 kΩ into the cathode follower. The third jack, Linked, feeds both channels: on the bench, the Channels jumpered switch.
Early 1970s four-input
From Mark Huss’s drawing “Early 70s Four-Input Preamp”, as built in serial 2130, mid-1971. Separate V1 cathodes, a 220 kΩ / 2.2 kΩ unbypassed second stage, a bypassed third stage, and a DC-coupled cathode follower into the phase inverter. Three ECC83s, typical of the type, not measured. Its presence control is in the power amp and isn’t modeled.
Mid 1970s four-input
From Mark Huss’s drawing “Mid-70s Four-Input Preamp”. A bypassed 100 kΩ / 1 kΩ second stage, a 1.6 kΩ third-stage cathode, and 22 nF from the third stage straight to the phase inverter. The 47 nF bypass on that cathode is drawn dashed, so its switch starts off. The phase inverter’s grid is biased from V3b’s cathode through 100 kΩ.
Late 1970s two-input
From Mark Huss’s drawing “Late 70s Two-Input Preamp”. One jack per channel with no 68 kΩ, a shared V1 cathode (1.5 kΩ, 150 µF), and no mixing resistors. Each volume drives its own half of V2, the two plates tied onto one 100 kΩ load. The Brilliant half has 1 kΩ and 47 nF on its cathode, the Normal half 1 kΩ. 47 nF couples V3a’s plate to the phase inverter, biased from V3b as in the mid-1970s build.

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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