Tube spring reverb box with a 6C4 driver

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 Danelectro 9100 Reverb Box.

A box between the guitar and the amp: a 12AX7 half and a 6C4 drive a high-impedance spring unit through a resistor, a 12AX7 cathode follower recovers it, and one pot pans between reverb and dry with its wiper grounded.

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 is gained up by V1, and its plate feeds both the 6C4 and, through 1 MΩ, the dry end of Reverb. On Normal the swing at the 6C4’s grid is large enough to pull the grid positive on peaks, so the dry signal is clipped slightly there too. The 6C4 drives the reverb unit through .01 µF and 27 k. V3 only follows the reverb unit’s output, with no gain, so the reverb comes back quiet and the 1 MΩ dry path drops the dry signal to suit. Reverb pans between the two by grounding its wiper: toward 10 it shorts the dry end and leaves only the springs. The reverb unit here is a typical high-impedance one, not Danelectro’s own, so the reverb level is approximate.

Download.

Parts and what they do

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

68k, 68 kΩ
The Normal input’s resistor to V1’s grid, 68 k.
560k, 560 kΩ
The Strong input’s resistor, 560 k. With the 220 k grid resistor it divides a hot signal down before V1.
220k, 220 kΩ
V1’s grid resistor, 220 k. With the input resistors it sets the input impedance near 290 k on Normal.
V1, 12AX7 / ECC83 / 7025
Half a 12AX7, the input stage. Its plate drives both the reverb send and the dry path.
220k, 220 kΩ
V1’s plate resistor, +120 V on the drawing.
2.2k, 2.2 kΩ
V1’s cathode resistor, +1.25 V on the drawing.
10µ, 10 µF electrolytic
Bypass for V1’s cathode.
.01µ, 10 nF
Coupling cap to the 6C4’s grid.
1M, 1 MΩ
The 6C4’s grid resistor.
V2 6C4, 12AU7 / ECC82
The 6C4, a single triode (half a 12AU7), the reverb driver. It drives the reverb unit’s input coil through .01 µF and 27 k, with no transformer.
100k, 100 kΩ
The 6C4’s plate resistor, +120 V on the drawing.
4.7k, 4.7 kΩ
The 6C4’s cathode resistor, +6 V on the drawing.
10µ, 10 µF electrolytic
Bypass for the 6C4’s cathode.
.01µ, 10 nF
From the 6C4’s plate toward the reverb unit.
27k, 27 kΩ
In series with the reverb unit’s input, 27 k. It makes the 6C4 drive the coil with a current more nearly even across frequency, since the coil’s impedance rises with frequency.
Unit in R, 190 Ω
Resistance of the reverb unit’s input coil. The current through it drives the springs, which play from a measured tank’s impulse response. Typical of a 1,475 Ω input coil, not on the drawing.
Unit in L, 235 mH
Inductance of the reverb unit’s input coil, 0.235 H, about 1,475 Ω at 1 kHz. Typical, not on the drawing.
Unit out L, 350 mH
The reverb unit’s output coil, from V3’s grid to ground. The springs come back in series with it. A measured long tank’s figures, typical, not on the drawing.
Unit out R, 181 Ω
Resistance of the unit’s output coil. Typical, not on the drawing.
1M, 1 MΩ
V3’s grid resistor, beside the output coil.
V3, 12AX7 / ECC83 / 7025
The other half of the 12AX7, a cathode follower for the recovered reverb. It has no gain, so the reverb comes back quiet.
10k, 10 kΩ
V3’s cathode resistor, +2.8 V on the drawing.
.47µ, 470 nF
From V3’s cathode to the foot switch and the reverb end of the pot.
15k, 15 kΩ
From the reverb return to the reverb end of the pot. With the pot it sets how far the reverb is shunted.
Reverb, 100 kΩ pot, linear taper
Reverb, 100 kΩ, taper assumed linear, its wiper grounded. Turned up, the wiper moves toward the dry end, shunting the dry signal and leaving the reverb end free. Turned down, the reverb is shunted.
1M, 1 MΩ
The dry path, 1 M from the 6C4’s grid to the dry end of the pot. It drops the dry signal a lot, to match the quiet reverb.
100k, 100 kΩ
From the reverb end of the pot to the output jack.
100k, 100 kΩ
From the dry end of the pot to the output jack. The two 100 k mix the ends.
TR1 a, 263 V ac, 60 Hz
One half of the power transformer’s center-tapped high-voltage winding, as a sine at the mains frequency. Typical, not on the drawing.
TR1 b, 263 V ac, 60 Hz
The other half of the winding, half a cycle behind.
PT R, 100 Ω
Resistance of one half of the winding. A typical value.
PT R, 100 Ω
Resistance of the other half. A typical value.
V4a, 6X4
One plate of the 6X4 full-wave rectifier.
V4b, 6X4
The 6X4’s other plate, which conducts on the other half cycle.
10µ, 10 µF electrolytic
First filter cap, +360 V on the drawing.
68k, 68 kΩ
Dropping resistor, 68 k, from the rectifier to the 235 V line. The small tubes draw under 2 mA in all, so it drops about 125 V.
10µ, 10 µF electrolytic
Second filter cap, on the 235 V line.

About this version

As drawn
Danelectro’s 9100 drawing. The 6C4 is half a 12AU7 inside. The reverb unit and the power transformer 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. Caps are ideal, and the battery or HT is a voltage behind a resistance.

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