Single-ended 6V6 amp with reverb and tremolo, mid 1960s

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 Gretsch 6152 Compact Tremolo Reverb.

One 12AX7 stage and a Volume feed a dry path with a treble-cut Tone and a reverb send through its own stages, mixed at a driver for one cathode-biased 6V6. The tremolo’s cathode follower moves the first stage’s cathode.

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 through one 12AX7 stage and the Volume. From the Volume’s wiper the dry signal runs through 470 kΩ with .0005 µF across it, past a Tone control that pulls treble to ground through .005 µF, and through 47 kΩ to the mixing point. The reverb send runs from the same wiper through V1b to a 12AX7 driving a transformer into the spring unit. The springs are a measured tank, played from its impulse response. The mixing point drives V3b, the 6V6’s driver. The tremolo’s cathode follower sits on the first stage’s cathode, so the LFO moves that stage’s bias. There’s no depth control. The tube models are typical of the types, not measured.

Download.

Parts and what they do

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

100k, 100 kΩ
Jack 1’s resistor into the input node.
.001µ, 1 nF
Jack 2’s cap into the input node. Into about 1 MΩ it passes everything above about 160 Hz, so jack 2 is thinner and slightly brighter than the others.
100k, 100 kΩ
Jack 3’s resistor, the same as jack 1’s.
1M, 1 MΩ
The input node’s resistor to ground. The drawing shows it with no value, so a typical 1 MΩ is assumed.
2.2k, 2.2 kΩ
Grid stopper for V1a.
V1a, 12AX7 / ECC83 / 7025
Half of a 12AX7, the first gain stage. The tremolo’s cathode follower shares its cathode.
1000, 1 kΩ
Cathode resistor for V1a. The tremolo’s cathode follower pushes its current through it too, so the follower sets this cathode’s voltage and the stage’s bias (+2 V on the drawing).
35µ, 35 µF electrolytic
Bypass for V1a’s cathode. At the tremolo’s few hertz it only partly holds the cathode still, so the LFO gets through.
270k, 270 kΩ
Plate resistor for V1a, from node B3 (+180 V at the plate on the drawing).
.015µ, 15 nF
First coupling cap after V1a.
470k, 470 kΩ
To ground between the two coupling caps.
.005µ, 5 nF
Second coupling cap, to the Volume. With the 470 kΩ before it, the two caps cut some bass.
Vol 500k, 500 kΩ pot, audio taper
Volume, 500 kΩ. Its wiper feeds both the dry path and the reverb’s send. The drawing marks no taper, so audio is assumed.
470k, 470 kΩ
The dry path’s series resistor, 470 kΩ, with .0005 µF across it. Most of the bass goes through the resistor, the treble through the cap.
.0005µ, 500 pF
Across the dry path’s 470 kΩ. It lets the treble past.
.005µ, 5 nF
To the Tone control. With the control turned down it pulls treble to ground.
Tone 500k, 500 kΩ pot, audio taper
Tone, 500 kΩ as a variable resistor under the .005 µF. Turned down, its resistance drops and the cap pulls the treble off. Turned up, the cap sees more resistance and the treble stays. The taper and direction aren’t marked, so audio and brighter clockwise are assumed.
47k, 47 kΩ
From the dry path to the mixing point, V3b’s grid.
470k, 470 kΩ
From the Volume’s wiper to V1b, the reverb send.
100k, 100 kΩ
V1b’s grid resistor. With the 470 kΩ above it, it divides the send to about a sixth.
V1b, 12AX7 / ECC83 / 7025
The other half of V1, the reverb’s send stage, its cathode not bypassed.
2.2k, 2.2 kΩ
Cathode resistor for V1b, no bypass (+1.2 V).
270k, 270 kΩ
Plate resistor for V1b (+120 V).
.0005µ, 500 pF
Coupling cap to the reverb driver, small, so mostly the treble drives the springs.
220k, 220 kΩ
Grid resistor for the reverb driver.
V2, 12AX7 / ECC83 / 7025
The reverb driver, a 12AX7. The drawing shows one triode on a 330 Ω cathode resistor, so both halves in parallel are assumed, drawn as one.
330, 330 Ω
Cathode resistor for the driver, 330 Ω 1 W (+2.4 V).
35µ, 35 µF electrolytic
Bypass for the driver’s cathode.
T-2640, 25 kΩ to 8 Ω, 20 H primary, 100 mH leakage
The reverb transformer, from the driver’s plate (on node B2, +360 V there) to the spring unit. It loads the driver and the supply. Typical of a small 12AX7-driven reverb transformer, not on the drawing.
Unit in R, 800 mΩ
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, not measured.
Unit in L, 1.1 mH
Inductance of the reverb unit’s input coil. A measured 4AB3C1B tank’s figure stands in.
Unit out L, 350 mH
The reverb unit’s output coil, from the recovery grid to ground. The springs come back in series with it. A measured 4AB3C1B tank’s figure stands in.
Unit out R, 181 Ω
Resistance of the unit’s output coil, a measured tank’s 181 Ω.
V3a, 12AX7 / ECC83 / 7025
Half of a 12AX7, the reverb recovery stage. It brings the springs back up.
2.2k, 2.2 kΩ
Cathode resistor for the recovery stage (+1.2 V).
35µ, 35 µF electrolytic
Bypass for the recovery stage’s cathode.
270k, 270 kΩ
Plate resistor for the recovery stage (+120 V).
.02µ, 20 nF
Coupling cap from the recovery stage toward the Reverb control, through the reverb switch.
Reverb 500k, 500 kΩ pot, audio taper
Reverb, 500 kΩ. It sets how much of the recovery stage reaches the mix. Its wiper feeds the mixing point through 470 kΩ, so it loads the mix slightly at every setting. Audio taper assumed.
470k, 470 kΩ
The reverb’s mixing resistor into V3b’s grid.
V4a, 12AX7 / ECC83 / 7025
Half of a 12AX7, the tremolo’s oscillator.
270k, 270 kΩ
Plate resistor for the oscillator (+140 V).
1000, 1 kΩ
Cathode resistor for the oscillator (+.4 V).
35µ, 35 µF electrolytic
Bypass for the oscillator’s cathode.
.03µ, 30 nF
First cap of the oscillator’s ladder.
100k, 100 kΩ
In series with Speed, the ladder’s first leg to ground. It sets the top speed.
Speed 500k, 500 kΩ pot, audio taper
Speed, 500 kΩ as a variable resistor in the first leg. Turned up, its resistance drops and the oscillator runs faster. Audio taper assumed.
.01µ, 10 nF
Second ladder cap.
1M, 1 MΩ
Second ladder leg to ground.
.01µ, 10 nF
Third ladder cap.
1.5M, 1.5 MΩ
Third ladder leg, the oscillator’s grid leak.
2.2k, 2.2 kΩ
Grid stopper for the oscillator.
.005µ, 5 nF
Couples the LFO from the oscillator’s plate.
1M, 1 MΩ
In series with the LFO, ahead of the tremolo switch.
2.2M, 2.2 MΩ
With the 220 kΩ below it, a divider: about a tenth of the LFO reaches the follower’s grid.
.01µ, 10 nF
At the foot switch jack, to ground. It rounds the LFO.
.05µ, 50 nF
On the follower’s grid side, to ground. It rounds the LFO further.
220k, 220 kΩ
The follower’s grid resistor to ground. There’s no Intensity control: this divider sets how deep the tremolo goes.
2.2k, 2.2 kΩ
Grid stopper for the cathode follower.
V4b, 12AX7 / ECC83 / 7025
The other half of V4, a cathode follower. Its cathode is V1a’s cathode (C on the drawing), so as its grid swings with the LFO it pushes more or less current through V1a’s 1000 Ω and moves that stage’s bias (+2 V).
V3b, 12AX7 / ECC83 / 7025
12AX7, the driver. With one output tube there’s no phase inverter, only a gain stage to raise the preamp’s signal enough to drive the output grid. Its grid connects straight to the preamp’s output with no coupling cap, so its path to ground is through the preamp’s volume and tone network.
270k, 270 kΩ
Plate resistor for the driver. Its plate drives the output tube’s grid through the coupling cap.
3.9k, 3.9 kΩ
Cathode resistor for the driver. It sets the driver’s bias, a volt or two.
.01µ, 10 nF
Coupling cap to tube A’s grid.
V5, 6V6GT
6V6GT, the output tube, single-ended: it carries the whole signal, and its idle current runs through the transformer all the time. Its cathode resistor biases it.
470k, 470 kΩ
Grid leak for tube A, to ground. The bias comes from the cathode resistor. Driven into grid current, the coupling cap charges through it and the bias goes colder for a moment. That’s blocking distortion.
330, 330 Ω
The cathode resistor. The tube’s current runs through it and lifts the cathode above the grid, and that’s the bias. Driven hard, the tube draws more current on average, so the cathode climbs and the bias goes colder.
1k, 1 kΩ
Dropping resistor from B+ to the screen supply. The screens sit slightly below the plates.
10µ, 10 µF electrolytic
Filter cap on the screen supply.
15k, 15 kΩ
Dropping resistor from the screen supply node to the node for the driver and the preamp, as on the Gretsch 6152 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.
10µ, 10 µF electrolytic
Filter cap at the node for the driver and 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, 5 kΩ to 8 Ω, 14 H primary, 30 mH leakage
The output transformer, 5 kΩ from the plate to B+ 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, not measured.
PT, 331 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 331 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, 331 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, 100 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, 100 Ω
Resistance of the other half. Typical of the part, not measured.
V6, 5Y3GT
One plate of the 5Y3GT 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.
V6, 5Y3GT
The rectifier’s other plate, on the other half cycle.
40µ, 40 µF electrolytic
The reservoir cap. It feeds the output transformer’s B+ end. 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.
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. With no feedback, the voltage at the speaker rises and falls with its impedance.
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. With no feedback, the voltage at the speaker rises and falls with its impedance.
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

As drawn
The 12AX7 model is fitted to its data sheet (Brimar). Values from Gretsch’s 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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