Push-pull 6L6 reverb amp for four 10s, 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 Fender AB763 Super Reverb.

Two 7025 channels, Normal and Vibrato, each with the blackface tone stack and a Bright switch. The Vibrato channel adds a Middle pot and the reverb send and mixer. A 12AT7 long-tail pair drives two 6L6GCs into four 10 in speakers.

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. Each channel is a gain stage, the tone stack, the volume and a second stage. The Normal channel reaches the inverter through .047 µF and 220 kΩ. The Vibrato channel’s second stage splits: 500 pF into the reverb driver, and the dry signal through 3.3 MΩ to the mixer, dropping to about a twentieth, which the mixer tube brings back up. The Intensity pot loads the mixer with 50 kΩ, and the two channels’ 220 kΩ resistors meet at the inverter’s input, so each channel loses about half its level into the other. The vibrato is the Deluxe Reverb’s: a 12AX7 phase-shift oscillator, about 4 to 12 times a second, flashes a neon lamp on a photocell from the Intensity pot’s wiper to ground. The cell’s speed is typical of data sheets, not measured. The springs are a measured tank, played from its impulse response.

Download.

Parts and what they do

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

R1V, 68 kΩ
The vibrato 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.
R3V, 68 kΩ
The vibrato 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.
R2V, 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.
R1N, 68 kΩ
The normal 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.
R3N, 68 kΩ
The normal 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.
R2N, 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.
V2a, 12AX7 / ECC83 / 7025
Vibrato channel’s first stage, half a 7025 (a low-noise 12AX7). Its plate drives the tone stack directly.
100k, 100 kΩ
First stage plate resistor, from box D.
1500, 1.5 kΩ
First stage cathode resistor. The drawing shows 2.1 V across it.
25µ, 25 µF electrolytic
Bypass on the first stage’s cathode, for full gain.
250p, 250 pF
Treble cap, 250 pF, from the plate to the top of the treble pot.
100k, 100 kΩ
Slope resistor, 100 kΩ. With the treble cap it sets where the treble control starts to work and how deep the mid scoop goes.
.1µ, 100 nF
Bass cap, .1 µF, to the bottom of the treble pot and the top of the bass pot.
.022µ, 22 nF
Mid cap, .022 µF on this drawing, half the Deluxe Reverb’s .047 µF, so the mids drop slightly more.
Treble, 250 kΩ pot, audio taper
Vibrato treble, 250 kΩ audio. Its wiper is the stack’s output, and it reaches well down into the mids.
Bass, 250 kΩ pot, audio taper
Vibrato bass, 250 kΩ audio, a variable resistor between the bass cap and the mid cap.
Middle, 10 kΩ pot, audio taper
Middle, 10 kΩ audio, a variable resistor from the bottom of the stack to ground. Turned down, it shorts the mid cap and the bass pot to ground and the scoop gets deeper.
Volume, 1 MΩ pot, audio taper
Vibrato volume, 1 MΩ audio. The treble wiper drives its top and the wiper feeds the second stage’s grid.
V2b, 12AX7 / ECC83 / 7025
Vibrato channel’s second stage, the other half of its 7025. Its grid returns to ground through the volume pot, and its cathode is shared with the other channel’s second stage.
100k, 100 kΩ
Second stage plate resistor, from box D.
V1a, 12AX7 / ECC83 / 7025
Normal channel’s first stage, half a 7025 (a low-noise 12AX7). Its plate drives the tone stack directly.
100k, 100 kΩ
First stage plate resistor, from box D.
1500, 1.5 kΩ
First stage cathode resistor. The drawing shows 2.1 V across it.
25µ, 25 µF electrolytic
Bypass on the first stage’s cathode, for full gain.
250p, 250 pF
Treble cap, 250 pF, from the plate to the top of the treble pot.
100k, 100 kΩ
Slope resistor, 100 kΩ. With the treble cap it sets where the treble control starts to work and how deep the mid scoop goes.
.1µ, 100 nF
Bass cap, .1 µF, to the bottom of the treble pot and the top of the bass pot.
.022µ, 22 nF
Mid cap, .022 µF on this drawing, half the Deluxe Reverb’s .047 µF, so the mids drop slightly more.
Treble, 250 kΩ pot, audio taper
Normal treble, 250 kΩ audio. Its wiper is the stack’s output, and it reaches well down into the mids.
Bass, 250 kΩ pot, audio taper
Normal bass, 250 kΩ audio, a variable resistor between the bass cap and the mid cap.
6800, 6.8 kΩ
A fixed 6.8 kΩ where a middle pot would go. It works like the other channel’s 10 kΩ audio Middle set at about 9.
Volume, 1 MΩ pot, audio taper
Normal volume, 1 MΩ audio. The treble wiper drives its top and the wiper feeds the second stage’s grid.
V1b, 12AX7 / ECC83 / 7025
Normal channel’s second stage, the other half of its 7025. Its grid returns to ground through the volume pot, and its cathode is shared with the other channel’s second stage.
100k, 100 kΩ
Second stage plate resistor, from box D.
820, 820 Ω
Cathode resistor shared by both second stages (box A). It carries both currents, so each half sees about twice its value.
25µ, 25 µF electrolytic
Bypass on the shared cathode.
.047µ, 47 nF
The Normal channel’s coupling cap toward the inverter.
220k, 220 kΩ
The Normal channel’s mixing resistor into the inverter’s input. With the Vibrato channel’s 220 kΩ it makes a divider, so each channel loses about half its signal into the other.
.02µ, 20 nF
Coupling cap out of the Vibrato channel’s second stage. From here the signal splits: one path to the reverb driver, the other on to the mixer.
500p, 500 pF
Only 500 pF into the reverb driver. It sends mostly upper mids and treble to the tank and keeps the driver from loading the dry path in the bass.
3.3M, 3.3 MΩ
The dry signal’s mixing resistor, 3.3 MΩ. With 220 kΩ and 470 kΩ below it at the mixer’s grid, it cuts the dry signal about 20 times.
10p, 10 pF
A 10 pF cap across the 3.3 MΩ. It lets the treble past the divider, so the dry path rises in the highs.
220k, 220 kΩ
The mixer’s grid resistor, the bottom of the dry divider.
1M, 1 MΩ
The reverb driver’s grid resistor.
V3, 12AT7 / ECC81
The reverb driver, a 12AT7 with both halves in parallel, drawn as one tube. It drives the tank’s low-impedance input coil through a transformer.
2200, 2.2 kΩ
Cathode resistor for the reverb driver. The drawing shows 8.4 V here, about 3.8 mA for the pair.
25µ, 25 µF electrolytic
Bypass for the driver’s cathode.
TR4, 22.5 kΩ to 8 Ω, 20 H primary, 50 mH leakage
The reverb transformer, from box B to the driver’s plates, stepping down to the tank’s input coil. It loads the driver and the supply. Every figure is typical of the part, not measured.
Tank in R, 800 mΩ
Resistance of the tank’s input coil, 0.8 Ω, measured on a long tank. The current through it drives the springs, which play from a measured tank’s impulse response.
Tank in L, 1.1 mH
Inductance of the tank’s input coil, 1.1 mH, measured on a long tank.
Tank out L, 350 mH
The tank’s output coil, 0.35 H, from the recovery grid to ground. The springs come back in series with it. Measured on a long tank.
Tank out R, 181 Ω
Resistance of the tank’s output coil, 181 Ω, measured on a long tank.
220k, 220 kΩ
Grid resistor for the recovery stage.
V4a, 12AX7 / ECC83 / 7025
The reverb recovery stage, half the fourth 7025. It brings the springs back up, and its current runs through the cathode resistor it shares with the mixer.
820, 820 Ω
Cathode resistor shared by the recovery stage and the mixer (box E). The drawing shows 2.0 V here.
25µ, 25 µF electrolytic
Bypass on the shared cathode.
100k, 100 kΩ
Recovery stage plate resistor, from box D.
.003µ, 3 nF
From the recovery plate to the reverb pot. At .003 µF it cuts the lows out of the reverb.
Reverb, 100 kΩ pot, linear taper
Reverb, 100 kΩ linear. Turn it up for more of the springs. At 0 the wiper grounds the 470 kΩ.
470k, 470 kΩ
Reverb mixing resistor, from the pot’s wiper to the mixer’s grid.
V4b, 12AX7 / ECC83 / 7025
The mixer, the other half of the fourth 7025. It makes up the level lost in the dry divider, and its plate feeds the inverter.
100k, 100 kΩ
Mixer plate resistor, from box D.
.1µ, 100 nF
Coupling cap from the mixer toward the inverter.
Intensity, 50 kΩ pot, reverse audio taper
The vibrato’s Intensity pot, 50 kΩ reverse audio, to ground. It loads the Vibrato channel with about 50 kΩ whether the vibrato runs or not. Turned up, its wiper moves toward the signal, so the LDR on the wiper pulls more of the signal down.
LDR, Fender roach cell
The photocell in the light cell (the roach), from the Intensity pot’s wiper to ground. The neon lights it about 4 to 12 times a second and it drops from megohms to a few hundred ohms, pulling the signal down in pulses. Dark about 4 MΩ and 500 Ω at the neon’s 0.3 mA. Its rise and fall times, 25 and 35 ms, are typical of data sheets, not measured. A real one can tick at the edges of each flash. That isn’t modeled.
220k, 220 kΩ
The Vibrato channel’s mixing resistor into the inverter’s input. With the LDR it makes the vibrato’s divider.
V5a, 12AX7 / ECC83 / 7025
First half of the tremolo’s 12AX7, the oscillator. Its plate feeds a ladder of three caps back to its own grid, which turns the phase round at a few hertz, so it runs on its own.
220k, 220 kΩ
The oscillator’s plate resistor, from box B.
2700, 2.7 kΩ
The oscillator’s cathode resistor. The drawing shows 2.5 V here.
25µ, 25 µF electrolytic
Bypass for the oscillator’s cathode. At a few hertz it isn’t a full short, and its phase moves the speed slightly.
.02µ, 20 nF
First cap of the ladder, from the oscillator’s plate.
Speed 3M, 3 MΩ pot, reverse audio taper
Speed, 3 MΩ reverse audio, used as a variable resistor from the ladder’s first node toward ground. Turned up, there’s less track in the circuit and the oscillator runs faster.
100k, 100 kΩ
The fixed 100 kΩ under the Speed pot. It sets the top speed.
.01µ, 10 nF
Second cap of the ladder.
1M, 1 MΩ
The ladder’s second leg, from its middle node to the footswitch line.
.01µ, 10 nF
Last cap of the ladder, into the oscillator’s grid.
1M, 1 MΩ
The oscillator’s grid leak, to the footswitch line.
2.2M, 2.2 MΩ
From the footswitch line to the bias supply. With the footswitch open, it pulls the oscillator’s grid down to about -60 V and the tremolo stops. With it closed, the line sits at ground and the 2.2 MΩ only loads the bias supply slightly.
V5b, 12AX7 / ECC83 / 7025
Second half of the 12AX7, the lamp driver. Its grid hangs on the ladder’s middle node, where the swing is tens of volts, so it runs from cut off to full current each cycle and flashes the neon in its plate.
100k, 100 kΩ
The lamp driver’s cathode resistor, 100 kΩ. The drawing shows 17 V here.
25µ, 25 µF electrolytic
Bypass for the lamp driver’s cathode, so the grid’s whole swing works on the tube.
Neon, NE-2 neon
The neon lamp in the light cell, between the lamp driver’s plate and its 100 kΩ from box B. It strikes at about 76 V and once lit holds about 58 V. Its light follows its current, so it flashes as the driver swings. NE-2 class figures from its data sheet. When it lights and goes out isn’t modeled to the microsecond, and the tick the real one puts in the signal isn’t either.
100k, 100 kΩ
The neon’s series resistor from box B. It sets the lamp’s current once lit.
10M, 10 MΩ
A 10 MΩ across the neon and its 100 kΩ. It keeps the driver’s plate up near box B while the lamp is dark.
.001µ, 1 nF
Coupling cap into the phase inverter. With the grid resistor it sets how low the inverter goes, around 106 Hz.
V6a, 12AT7 / ECC81
12AT7, the driven half of the long-tail pair phase inverter. Its plate swings opposite the input and drives tube A.
V6b, 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.
1.5k, 1.5 kΩ
Grid stopper for tube A. It keeps the tube from oscillating and softens grid current.
1.5k, 1.5 kΩ
Grid stopper for tube B.
V7, 6L6GC
6L6GC, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply.
V8, 6L6GC
6L6GC, 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.
4 H, 4 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+.
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 Super 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 Super 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.
OT, 4 kΩ to 2 Ω, 40 H primary, 10 mH leakage
The output transformer, 4 kΩ plate to plate into 2 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
One half of the power transformer’s high-voltage winding, 360 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, 360 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.
V9, 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.
V9, GZ34
The rectifier’s other plate, 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 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.
25µ, 25 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, 2.17 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 2 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, 64 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, 116 µH
With Loss R across it, the treble rises and then levels off, like the losses in a real coil.
Loss R, 3.58 Ω
Limits what Loss L adds to the treble rise.
Loss L2, 533 µH
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, 548 mΩ
Limits what Loss L2 adds.
Tank R, 20.9 Ω
Height of the bass resonance, about 23 Ω in all at its peak. The feedback holds the output steadier against that than it would be without feedback.
Tank L, 5.28 mH
With Tank C, sets the resonance: 74 Hz.
Tank C, 881 µF
With Tank L, sets the resonance. With Tank R, it sets how narrow the peak is.

About this version

As drawn
Every 7025 is modeled as a 12AX7 (a low-noise version of it), fitted to the Brimar data sheet, and the reverb driver as a GE 12AT7. Typical of the type, not measured. The tank’s coils are measured on a long tank. The reverb transformer is typical, 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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