High-gain amp with a cold-biased third stage, 1993 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 Mesa/Boogie Dual Rectifier (Red channel).

Four gain stages and a cathode follower. The second stage feeds a third that runs almost cut off on a 39 kΩ cathode resistor, so it clips one side of the wave hard. A fourth gain stage follows, then the cathode follower and the treble, bass and middle stack.

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 first stage is fully bypassed. Its output passes through 2.2 MΩ and 680 kΩ, each bridged by a small cap, before the Gain pot, so the treble reaches the second stage with less loss than the bass. The third stage idles at about 0.15 mA with 6 V on its cathode. Small signals pass, but its grid has only a few volts of room before the tube cuts off. Probe its plate to see the flattened half of the wave, then V3a’s plate to see the fourth stage clip the other side. Presence is a treble cut to ground after the tone stack. Turning it up cuts less.

Download.

Parts and what they do

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

R261, 15 kΩ
Dropping resistor from node C (the HT here) to node D, which feeds V2. The sheet marks D at 406 V.
C62, 30 µF electrolytic
Filter cap at node D.
R252, 22 kΩ
Dropping resistor from node C to node E, which feeds V1a. The sheet marks E at 402 V.
C61, 30 µF electrolytic
Filter cap at node E.
R211, 1 MΩ
Grid leak at the input jack.
V1a, 12AX7 / ECC83 / 7025
First stage. The sheet marks 200 V at the plate and 1.6 V at the cathode.
R221, 220 kΩ
Plate load.
R291, 1.8 kΩ
Cathode resistor.
C31, 1 µF electrolytic
Cathode bypass, 1 µF: full gain above about 90 Hz. On the sheet a 47 kΩ sits in series with it, shorted by LDR3 in this mode.
C21, 20 nF
Coupling cap.
R321, 2.2 MΩ
To ground after the coupling cap.
R322, 2.2 MΩ
Series resistor to the Gain pot.
C10, 82 pF
Across R322, to pass the treble around it.
R110, 680 kΩ
Switched in parallel with R322 by LDR4 in this mode, so less of the signal is lost before the Gain pot.
C9, 2 nF
Across R110. With R110 and R322 in parallel (about 520 kΩ) and the Gain pot, the treble shelf starts near 150 Hz.
Gain, 1 MΩ pot, audio taper
Red Gain, 1 MΩ (taper assumed audio). Its wiper feeds V2a.
.001, 1 nF
Bright cap from the top of the Gain pot to its wiper: the lower the Gain, the more treble it passes relative to the rest.
R241, 470 kΩ
Series resistor into V2a. With V2a’s input capacitance and C12 it rolls off the extreme treble.
C12, 20 pF
Grid to ground. With R241 it makes a low-pass near 17 kHz, before the tube’s own capacitance is added.
V2a, 12AX7 / ECC83 / 7025
Second stage. The sheet marks 280 V at the plate and 2 V at the cathode.
R201, 100 kΩ
Plate load.
R292, 1.8 kΩ
Cathode resistor.
C32, 1 µF electrolytic
Cathode bypass, 1 µF (the 47 kΩ in series on the sheet is shorted by LDR3 in this mode).
C22, 20 nF
Coupling cap.
R242, 470 kΩ
Series resistor into the cold stage. When V2b’s grid is driven positive it draws current, and this resistor limits it.
R212, 1 MΩ
Grid leak. With R242 in front it also divides the signal to about two thirds.
V2b, 12AX7 / ECC83 / 7025
The cold stage. The sheet marks 384 V at the plate and 6 V at the cathode: about 0.15 mA, near cut-off.
R202, 100 kΩ
Plate load.
C6, 1 nF
Across the plate load: a treble cut above about 1.6 kHz, since node D is an AC ground.
R103, 39 kΩ
Unbypassed 39 kΩ cathode resistor: biases the stage cold and adds heavy local feedback, so the stage gains little.
C27, 20 nF
Coupling cap.
R106, 330 kΩ
To ground after the coupling cap.
R225, 220 kΩ
Grid stopper into V3a. It also limits grid current when V3a clips.
V3a, 12AX7 / ECC83 / 7025
Fourth stage. The sheet marks 213 V at the plate and 1.6 V at the cathode.
R224, 220 kΩ
Plate load, from node C.
R293, 1.8 kΩ
Cathode resistor.
C33, 1 µF electrolytic
Cathode bypass, 1 µF (the 47 kΩ in series on the sheet is shorted by LDR10 in this mode).
V3b, 12AX7 / ECC83 / 7025
Cathode follower, direct-coupled from V3a’s plate, driving the tone stack. The sheet marks 216 V at the cathode.
R203, 100 kΩ
Cathode load.
C4, 680 pF
Treble cap (reached through LDR8 in this mode).
Treble, 250 kΩ pot, linear taper
Treble, 250 kΩ (taper assumed linear). Its wiper is the stack’s output.
R273, 47 kΩ
Slope resistor.
C23, 20 nF
Bass cap.
Bass, 1 MΩ pot, audio taper
Bass, 1 MΩ as a variable resistor (taper assumed audio).
C24, 20 nF
Mid cap, to the Mid pot’s wiper.
Middle, 25 kΩ pot, linear taper
Middle, 25 kΩ (taper assumed linear).
R254, 22 kΩ
Series resistor in the presence network.
C8, 3 nF
The cap that makes the presence network act on the treble.
Presence, 25 kΩ pot, linear taper
Red Presence, 25 kΩ, a variable resistor to ground. Turning it up adds resistance, so the network cuts less treble. Direction and taper are assumed, since the sheet shows neither.
Master, 1 MΩ pot, audio taper
Red Master, 1 MΩ (taper assumed audio), reached through LDR14. In the amp its wiper goes to the effects-loop relay and on to the phase inverter. On the bench it drives a standard phase-inverter load.
.001µ, 1 nF
Coupling cap into the phase inverter. With the grid resistor it sets how low the inverter goes, around 106 Hz.
V4a, 12AT7 / ECC81
12AT7, the driven half of the long-tail pair phase inverter. Its plate swings opposite the input and drives tube A.
V4b, 12AT7 / ECC81
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.
22k, 22 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.
820, 820 Ω
The feedback resistor from the speaker. With the 100 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.
100, 100 Ω
The bottom of the feedback loop, 100 Ω to ground. The fed-back speaker voltage arrives here and moves the tail.
.1µ, 100 nF
Coupling cap to tube A’s grid.
.1µ, 100 nF
Coupling cap to tube B’s grid.
750, 750 Ω
Grid stopper for tube A. It keeps the tube from oscillating and softens grid current. Each of the two tubes has its own 1.5k. The resistors are drawn as one part, the two in parallel.
750, 750 Ω
Grid stopper for tube B. Each of the two tubes has its own 1.5k. The resistors are drawn as one part, the two in parallel.
V5, V7, 6L6GC
6L6GC, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply. Two tubes in parallel, drawn as one.
V6, V8, 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.
235, 235 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 470. The resistors are drawn as one part, the two in parallel.
235, 235 Ω
Screen resistor for tube B. Each of the two tubes has its own 470. The resistors are drawn as one part, the two in parallel.
7.75 H, 7.75 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.
112, 112 Ω
The choke’s winding resistance, typical, not measured. It’s all the DC drop the screens see from B+.
20µ, 20 µF electrolytic
Filter cap on the screen supply.
1k, 1 kΩ
Dropping resistor from the screen supply node to the node for the inverter, as on the Fender AB763 Twin Reverb 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.
20µ, 20 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.
4.7k, 4.7 kΩ
Dropping resistor from the node for the inverter to the node for the preamp, as on the Fender AB763 Twin Reverb 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.
20µ, 20 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.
Reverb, vibrato, 85 kΩ
Stands for the reverb driver and the vibrato’s oscillator on the screen supply node. It pulls about the current worked out from the voltages on the drawing. That part of the amp isn’t modeled here. Typical, not measured.
OT, 2 kΩ to 4 Ω, 20 H primary, 5 mH leakage
The output transformer, 2 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, 340 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 340 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, 340 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, 12 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, 12 Ω
Resistance of the other half. 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.
35µ, 35 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, 48 V ac, 60 Hz
The power transformer’s bias tap, 48.4 V ac, as a sine source at the mains frequency. Typical of the part, not measured.
470, 470 Ω
Series resistor into the bias rectifier, 470 Ω. It limits the charging current.
Bias diode, 1N4001
The bias rectifier, a silicon diode turned so the supply comes out negative.
50µ, 50 µF electrolytic
Filter cap on the bias supply, its plus side to ground.
Bias 10k, 10 kΩ pot, linear taper
Bias adjust, 10k Ω linear. Its wiper feeds both output grids through their grid leaks. Toward the diode end the grids go more negative and the tubes idle cooler.
27k, 27 kΩ
The bottom of the bias divider, 27k Ω to ground.
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

ECC83 throughout
Five ECC83 triodes, typical of the type, not measured. Real tubes spread 20 % or more. The HT is set to the 422 V the sheet marks at node C.

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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