Tube spring reverb unit with a 6K6GT driver, 1961 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 6G15 Reverb.

A box between the guitar and the amp: a 12AT7 and a 6K6GT drive a spring reverb unit through a transformer, a 7025 half recovers it through a Tone control, and a cathode follower carries the dry signal to the Mixer, a pan from dry to reverb.

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 guitar goes two ways. A 7025 cathode follower carries it, unchanged in level, to one end of Mixer. The other way, a small tap off V1a’s plate load feeds Dwell, V1b gains it up, and the 6K6GT drives the reverb unit’s 8 Ω input coil through a transformer. Dwell sets how hard that chain is pushed, and turned up the 6K6GT clips, which is part of the sound of these units. The springs come back through the other 7025 half and Tone into the other end of Mixer, and Mixer’s wiper is the output to the amp. The springs are a measured tank, played from its impulse response. The unit has its own power transformer, a half-wave rectifier, a choke and three filter caps. Tube models and the transformers are typical of the types, not measured.

Download.

Parts and what they do

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

1M, 1 MΩ
From the input jack to ground, 1 MΩ. The guitar sees this beside the grids.
V1a, 12AT7 / ECC81
First half of the 12AT7, the send’s first stage, grid straight on the input jack.
100k, 100 kΩ
V1a’s plate resistor, 100 kΩ, to the send tap. The drawing reads +105 V on the plate.
10k, 10 kΩ
The other 10 kΩ of V1a’s plate load, from the tap to the 250 V node. The send comes off the tap, so it carries only the plate current times 10 kΩ, about a tenth of the plate’s swing.
1500, 1.5 kΩ
V1a’s cathode resistor, +1.8 V on the drawing.
25µ, 25 µF electrolytic
Bypass for V1a’s cathode, 25 µF 25 V.
.01µ, 10 nF
From the send tap to the top of Dwell.
Dwell, 250 kΩ pot, linear taper
Dwell, 250 kΩ linear. It sets how hard the send chain and the springs are driven. Turned up, the 6K6GT clips on hard notes.
V1b, 12AT7 / ECC81
Second half of the 12AT7, the send’s gain stage. Its cathode resistor isn’t bypassed.
100k, 100 kΩ
V1b’s plate resistor, +120 V on the drawing.
1500, 1.5 kΩ
V1b’s cathode resistor, not bypassed, +2 V on the drawing.
.002µ, 2 nF
Coupling cap to the 6K6GT, .002 µF. Into 220 kΩ it passes everything above about 360 Hz, so the springs get little bass.
220k, 220 kΩ
The 6K6GT’s grid resistor.
V2, 6K6GT
The 6K6GT, a small power pentode, the reverb driver. It drives the reverb unit’s input coil through TR2. Its screen sits on the 295 V node.
1000, 1 kΩ
The 6K6GT’s cathode resistor, 1 kΩ 2 W. The drawing reads +24 V, about 24 mA for plate and screen.
25µ, 25 µF electrolytic
Bypass for the 6K6GT’s cathode, 25 µF 25 V.
TR2, 7.5 kΩ to 8 Ω, 10 H primary, 50 mH leakage
The reverb transformer, from the 6K6GT’s plate to the reverb unit’s 8 Ω input coil. The drawing shows only its part number. 7.5 kΩ to 8 Ω is a 6K6GT’s usual load, and the primary’s 400 Ω is near the drawing’s 10 V across it. Typical, not measured.
Unit in R, 800 mΩ
Resistance of the reverb unit’s input coil, 0.8 Ω. The current through it drives the springs, which play from a measured tank’s impulse response. Its figures are a measured long tank’s, typical, not measured on this unit.
Unit in L, 1.1 mH
Inductance of the reverb unit’s input coil, 1.1 mH, a measured long tank’s figure.
Unit out L, 350 mH
The reverb unit’s output coil, from the recovery grid to ground. It’s the grid’s only path to ground. The springs come back in series with it. Its figures are a measured long tank’s.
Unit out R, 181 Ω
Resistance of the reverb unit’s output coil, 181 Ω, a measured long tank’s figure.
V3a, 12AX7 / ECC83 / 7025
First half of the 7025, the reverb recovery stage. Its grid sits on the reverb unit’s output coil with no resistor of its own.
100k, 100 kΩ
Plate resistor for the recovery stage, +160 V on the drawing.
1500, 1.5 kΩ
Cathode resistor for the recovery stage, +1.2 V on the drawing.
25µ, 25 µF electrolytic
Bypass for the recovery stage’s cathode.
.1µ, 100 nF
Coupling cap from the recovery plate to the top of Tone.
Tone, 50 kΩ pot, linear taper
Tone, 50 kΩ linear, across the reverb’s path with .01 µF at its foot. Down, the wiper moves onto the cap and the reverb gets darker. Up, it’s brighter. Up is assumed to be clockwise.
.01µ, 10 nF
From the foot of Tone to ground. It’s the treble cut Tone moves toward.
250p, 250 pF
Only 250 pF from Tone’s wiper to Mixer’s reverb end, so the reverb reaching the output is mostly mids and treble.
Mixer, 250 kΩ pot, linear taper
Mixer, 250 kΩ linear, with the reverb on one end and the dry signal on the other. Its wiper is the output. Turning it up adds reverb and lowers the dry signal.
.05µ, 50 nF
From the input jack to the dry follower’s grid.
2.2M, 2.2 MΩ
From the 250 V node to the follower’s grid. With the 2.2 MΩ below it, it holds the grid near half the supply, +120 V on the drawing.
2.2M, 2.2 MΩ
From the follower’s grid to ground, the bottom of the divider.
V3b, 12AX7 / ECC83 / 7025
Second half of the 7025, a cathode follower for the dry signal. It gains slightly under 1 and drives Mixer’s dry end from a low impedance.
100k, 100 kΩ
The follower’s cathode resistor, 100 kΩ. The drawing reads +130 V on the cathode.
.1µ, 100 nF
From the follower’s cathode to Mixer’s dry end.
TR1, 237 V ac, 60 Hz
The power transformer’s high-voltage winding, one end grounded, as a sine source at the mains frequency. The drawing shows no figure, so 237 V ac is set to put the first cap near the drawing’s +300 V. Typical, not measured.
PT R, 100 Ω
The winding’s resistance, with the primary’s share. Typical of a small transformer, not measured.
D1, 1N4001
The first of three rectifier diodes in series, half wave. In series they stand off the winding’s peak between them.
D2, 1N4001
Second rectifier diode.
D3, 1N4001
Third rectifier diode.
40µ, 40 µF electrolytic
The first filter cap, 40 µF 450 V, +300 V on the drawing. It charges once a cycle, so it carries more ripple than a full-wave supply’s would.
Choke, 4 H
The filter choke. With the second cap it removes most of the ripple. Typical, not on the drawing.
Choke R, 150 Ω
The choke’s winding resistance. Typical, not measured.
40µ, 40 µF electrolytic
The second filter cap, 40 µF 450 V, on the 295 V node that feeds the 6K6GT.
10k, 10 kΩ
Dropping resistor, 10 kΩ 1 W, to the 250 V node that feeds the small tubes.
40µ, 40 µF electrolytic
The third filter cap, 40 µF 450 V, on the 250 V node.

About this version

As drawn
Fender’s 6G15 drawing. The 6K6GT model is typical of the type, set at the tube manuals’ published points. The transformers and the reverb unit are typical, not on the drawing.

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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