Six-stage high-gain lead amp, early 1990s 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 Peavey 5150 (lead channel).

Six triode gain stages in a row, one a cold clipper and the last with plate-to-grid feedback. A 470 kΩ feeds the tone stack, then the post gain pot, a cathode follower, and a recovery stage before 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. The front of the amp has more gain than it can use, and much of the design is about bringing it back down. Pre gain sits between the first two stages. V2A has a 39 kΩ cathode resistor, so it runs almost cut off and clips one side of the wave hard. The sixth stage has 1 MΩ from its output back to its grid against 1 MΩ in series at its input, which holds its gain near 1 and its output steady whatever comes in. The 470 kΩ into the tone stack, against the stack’s 33 kΩ and slope network, drops the level about 30 dB before the tone controls. Probe V2A’s plate with Pre gain up to see the cold clipper, and compare it with V5A’s plate.

Download.

Parts and what they do

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

R61, 10 kΩ
Dropping resistor from V2+ to V1+, the supply for the first three stages.
C33, 22 µF electrolytic
Filter cap on V1+. With R61 it keeps ripple and the later stages’ current swings off the first three stages.
R27, 22 kΩ
High input series resistor.
C20, 39 pF
Radio filter with R27.
C15, 100 nF
Input coupling cap, so no DC from the guitar reaches the grid.
R25, 1 MΩ
Grid leak. It sets the input impedance near 1 MΩ.
R22, 68 kΩ
Grid stopper.
V1A, 12AX7 / ECC83 / 7025
First stage, shared with the rhythm channel.
R2, 220 kΩ
Plate load.
R12, 1.82 kΩ
Cathode resistor.
C3, 1 µF
Cathode bypass, 1 µF. The stage has full gain above about 100 Hz and less below.
C6, 22 nF
Coupling cap to the channel networks.
R29, 470 kΩ
Series resistor into the lead pre gain pot. With the pot and R41 it drops the level by about 10 dB.
C17, 2.2 nF
Across R29. It lets the treble past, above about 150 Hz.
R41, 1 MΩ
Load at the top of the pre gain pot.
Pre gain, 1 MΩ pot, audio taper
Lead pre gain (ULTRA PRE on the schematic).
C19, 1 nF
A bright cap from the top of the pot to its wiper. At low settings it passes treble around the pot.
R40, 1 MΩ
Across C19. It keeps some signal on the wiper even at zero.
R82, 470 kΩ
Grid stopper for V1B, large enough to cut the treble with the tube’s input capacitance.
V1B, 12AX7 / ECC83 / 7025
Second stage.
R3, 100 kΩ
Plate load.
R17, 1.82 kΩ
Cathode resistor.
C4, 1 µF
Cathode bypass, 1 µF.
C26, 22 µF electrolytic
A second, large bypass cap. With K1 energized it goes straight to ground and bypasses the cathode down to the lowest notes. Otherwise it returns through R18 and does little.
R18, 47 kΩ
C26’s return while K1 is off.
C2, 22 nF
Coupling cap.
R6, 470 kΩ
Top of a divider into V2A, and its grid stopper.
R9, 1 MΩ
Bottom of the divider, and V2A’s grid leak.
V2A, 12AX7 / ECC83 / 7025
Third stage, a cold clipper. Its 39 kΩ cathode resistor biases it close to cutoff.
R1, 100 kΩ
Plate load.
R15, 39 kΩ
Unbypassed cathode resistor. The stage has a gain near 2 and cuts off on the negative half of a large signal.
C1, 22 nF
Coupling cap.
C48, 1 nF
Treble cut at the next grid.
R11, 330 kΩ
Grid leak side of the divider.
R7, 220 kΩ
Grid stopper for V2B.
V2B, 12AX7 / ECC83 / 7025
Fourth stage.
R4, 220 kΩ
Plate load.
R13, 1.82 kΩ
Cathode resistor.
C5, 1 µF
Cathode bypass, 1 µF.
R32, 1 MΩ
Series resistor ahead of the coupling cap. Against R101 it divides the signal by 11.
C57, 22 nF
Coupling cap.
R101, 100 kΩ
Grid leak, only 100 kΩ.
V5B, 12AX7 / ECC83 / 7025
Fifth stage.
R96, 220 kΩ
Plate load.
R102, 2.2 kΩ
Unbypassed cathode resistor.
R87, 1 MΩ
Input resistor of the feedback stage.
C56, 22 nF
Coupling cap.
R91, 1 MΩ
Grid leak.
V5A, 12AX7 / ECC83 / 7025
Sixth stage, with shunt feedback. Its gain is set near R88 / R87, about 1.
R86, 100 kΩ
Plate load.
R93, 2.2 kΩ
Unbypassed cathode resistor.
C58, 22 nF
Output coupling cap.
R88, 1 MΩ
Feedback from the output back to the grid.
R89, 470 kΩ
Series resistor into the tone stack.
R24, 33 kΩ
Tone stack input to ground. With R89 it’s a divider of about 1 in 15.
C59, 10 nF
With R94, a treble shunt at the stack input.
R94, 47 kΩ
In series with C59.
C14, C60, 570 pF
Treble cap, 470 pF and 100 pF drawn in parallel on the schematic.
High, 250 kΩ pot, linear taper
Treble, 250 kΩ linear. Its wiper is the stack’s output.
R28, 47 kΩ
Slope resistor.
C18, 22 nF
Bass cap.
Low, 1 MΩ pot, audio taper
Bass, 1 MΩ audio, wired as a variable resistor from the High pot’s bottom to the Mid pot.
C21, 22 nF
Mid cap, to the Mid pot’s wiper.
Mid, 50 kΩ pot, linear taper
Mid, 50 kΩ linear.
Post gain, 1 MΩ pot, audio taper
Lead post gain (ULTRA POST), a master volume for the lead channel.
C52, 22 nF
Coupling cap into the follower.
R79, 1 MΩ
Grid leak returned to the cathode resistor’s tap, so the follower’s own output bootstraps it.
V3B, 12AX7 / ECC83 / 7025
Cathode follower driving the effects send.
R80, 1 kΩ
Bias resistor between the cathode and the grid-leak tap.
R16, 100 kΩ
Cathode load.
C53, 330 nF
Send coupling cap. With no loop cable the send is normalled straight to the return.
R103, 100 kΩ
Send load.
C11, 47 nF
Return coupling cap.
R26, 1 MΩ
Grid leak.
V3A, 12AX7 / ECC83 / 7025
Recovery stage after the loop.
R5, 68 kΩ
Plate load.
R14, 3.3 kΩ
Cathode resistor.
C61, 4.7 nF
A small cathode bypass. It raises the gain only above about 10 kHz.
C49, 22 nF
Coupling cap to the phase inverter.
V6a, 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.
V6b, 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.
100k, 100 kΩ
Grid stopper into the driven half, 100k Ω. With the tube’s input capacitance it rolls off the far treble. Driven hard, the grid rises past the cathode and pulls current, and this resistor holds that current down.
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.
.047µ, 47 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.
39k, 39 kΩ
The feedback resistor from the speaker. With the 4.7k Ω below it, it sends about 11 % 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.
.13µ, 133 nF
The presence cap, in series with the pot across the leg. With the pot it sets where the lift starts.
Presence, 10 kΩ pot, audio taper
Presence, a 10k audio 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.
Resonance, 1 MΩ pot, audio taper
Resonance, a 1M audio pot wired as a variable resistor in the feedback path, its resistance rising as it’s turned up. The .0068µF cap across it lets the mids and treble past, so only the lows have to go through the pot. Turned up, less bass is fed back, the loop fights the speaker’s bass resonance less, and the low end comes up. It’s in series with the 39k Ω feedback resistor, as on the drawing.
.0068µ, 6.8 nF
Across the resonance pot. It passes the mids and treble around the pot, so the pot only cuts bass from the feedback.
22µ, 22 µF
In series with the feedback resistor. It blocks DC and cuts the deepest bass from the loop at any pot setting.
.047µ, 47 nF
Coupling cap to tube A’s grid.
.047µ, 47 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.
V7, V9, 6L6GC
6L6GC, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply. Two tubes in parallel, drawn as one.
V8, V10, 6L6GC
6L6GC, 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.
50, 50 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 100. The resistors are drawn as one part, the two in parallel.
50, 50 Ω
Screen resistor for tube B. Each of the two tubes has its own 100. The resistors are drawn as one part, the two in parallel.
400, 400 Ω
Dropping resistor from B+ to the screen supply. The screens sit slightly below the plates.
50µ, 50 µF electrolytic
Filter cap on the screen supply.
4.7k, 4.7 kΩ
Dropping resistor from the screen supply node to the node for the inverter, as on the Peavey 5150 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.
47µ, 47 µF electrolytic
Filter cap at the node for the inverter. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
22k, 22 kΩ
Dropping resistor from the node for the inverter to the node for the preamp, as on the Peavey 5150 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.
22µ, 22 µF electrolytic
Filter cap at the node for 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, 2 kΩ to 8 Ω, 20 H primary, 5 mH leakage
The output transformer, 2 kΩ plate to plate into 8 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, 345 V ac, 60 Hz
The power transformer’s high-voltage winding, 345 V ac end to end, into a bridge of four silicon diodes with the center tap unused. It’s drawn as two sources of 345 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, 345 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, 15 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, 15 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.
165µ, 165 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, 51 V ac, 60 Hz
The power transformer’s bias tap, 51 V ac, as a sine source at the mains frequency. Typical of the part.
tap R, 50 Ω
The bias tap’s own resistance, 50 Ω, typical. The drawing has no resistor between the tap and the diode.
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.
3.7k, 3.7 kΩ
Second resistor of the bias supply, 3.7k 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.
15k, 15 kΩ
From the bias line to ground, 15k Ω. With the 3.7k 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, 8.69 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 8 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, 256 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, 464 µH
With Loss R across it, the treble rises and then levels off, like the losses in a real coil.
Loss R, 14.3 Ω
Limits what Loss L adds to the treble rise.
Loss L2, 2.13 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, 2.19 Ω
Limits what Loss L2 adds.
Tank R, 83.7 Ω
Height of the bass resonance, about 92 Ω in all at its peak. The feedback holds the output steadier against that than it would be without feedback.
Tank L, 21.1 mH
With Tank C, sets the resonance: 74 Hz.
Tank C, 220 µF
With Tank L, sets the resonance. With Tank R, it sets how narrow the peak is.

About this version

12AX7 throughout
Eight 12AX7 triodes in the lead path. Real tubes spread 20 % or more. The supply voltage is assumed, since the schematic set has no power-supply page.

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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