Single-ended 6V6 amp with a tone control, late 1950s

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 5F2-A Princeton.

One 12AX7 stage into a volume and tone network, then a second 12AX7 stage into one single-ended, cathode-biased 6V6GT. The 5F1 Champ with a tone control and a second reservoir cap.

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. One 12AX7 stage, then the volume and tone, then the power amp. The tone pot’s wiper sits on top of the volume. One end goes to ground through .005 µF, the other to the volume’s wiper through 500 pF, so treble skips around the volume as it comes down. The power amp is the 5F1 Champ’s: the second half of the 12AX7 into one 6V6GT, cathode biased, class A, with 22 kΩ from the speaker to the second stage’s unbypassed cathode as the only feedback. The 6V6GT idles at about 35 mA, close to its 12 W plate rating, and delivers about 4 W before the harmonics pass 15 %. The reservoir has two 16 µF caps where the Champ has one, so less ripple reaches the 6V6GT’s plate.

Download.

Parts and what they do

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

R1A, 68 kΩ
The 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.
R1B, 68 kΩ
The 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.
RG1, 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.
V1a, 12AX7 / ECC83 / 7025
First half of the 12AX7, the input stage, with its cathode bypassed for full gain. The other half is the driver, in the power amp.
100k, 100 kΩ
Plate resistor for the input stage.
1500, 1.5 kΩ
Cathode bias resistor for the input stage.
25µ, 25 µF electrolytic
Cathode bypass for the input stage, full gain down to about 10 Hz.
.02µ, 20 nF
Coupling cap from the input stage to the top of the volume pot.
Vol 1M, 1 MΩ pot, linear taper
Volume, 1 MΩ linear (the layout marks it). There’s no master, so it sets how hard the driver and the 6V6GT are pushed. Its wiper is the driver’s grid, and the pot is that grid’s only path to ground.
Tone 1M, 1 MΩ pot, linear taper
Tone, 1 MΩ linear (the layout marks it), its wiper on the top of the volume. Up brings the 500 pF closer, so more treble skips around the volume to the driver’s grid. Down brings the .005 µF to ground closer, which cuts treble. The drawing doesn’t mark which way is up, so up is assumed to be brighter.
500p, 500 pF
Bright cap, 500 pF, from the tone pot to the volume’s wiper. It carries treble around the volume, so the amp gets brighter as the volume comes down.
.005µ, 5 nF
Treble cut cap, .005 µF to ground from the other end of the tone pot.
V1b, 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. The feedback from the speaker goes to its cathode. 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.
100k, 100 kΩ
Plate resistor for the driver. Its plate drives the output tube’s grid through the coupling cap.
22k, 22 kΩ
The feedback resistor from the speaker. With the 1.5k Ω below it, it sends about 6 % of the speaker voltage back to the driver’s cathode, which lowers the gain, flattens the response and tightens the bass.
1.5k, 1.5 kΩ
The driver’s cathode resistor, 1.5k Ω to ground, not bypassed. It sets the driver’s bias, and it’s also the bottom of the feedback loop: the feedback resistor from the speaker connects at the top of it, so the fed-back speaker voltage moves the cathode.
.02µ, 20 nF
Coupling cap to tube A’s grid.
V2, 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.
220k, 220 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.
470, 470 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.
25µ, 25 µF electrolytic
Bypass cap across the cathode resistor, so the cathodes hold still at the signal’s frequencies. It charges and discharges slowly as the average current changes on a hard note.
10k, 10 kΩ
Dropping resistor from B+ to the screens. It’s large, so the screens sit about 30 V below the plates and drop further when they draw more current on a hard note.
8µ, 8 µF electrolytic
Filter cap on the screen supply.
22k, 22 kΩ
Dropping resistor from the screen supply node to the node for the driver and preamp, as on the Fender 5F2-A Princeton 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.
8µ, 8 µF electrolytic
Filter cap at the node for the driver and preamp. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
OT, 8 kΩ to 8 Ω, 22.8 H primary, 86.8 mH leakage
The output transformer, 8 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, 297 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 297.3 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, 297 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, 141 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, 141 Ω
Resistance of the other half. Typical of the part, not measured.
V3, 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.
V3, 5Y3GT
The rectifier’s other plate, on the other half cycle.
16µ, 16 µF electrolytic
The reservoir caps’ first. 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.
16µ, 16 µF electrolytic
Another reservoir cap, in parallel with the first.
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. The feedback holds the output steadier against that than it would be without feedback.
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. The feedback holds the output steadier against that than it would be without feedback.
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 tube models are fitted to their data sheets: Brimar 12AX7, Tung-Sol 6V6GT and 5Y3GT. The drawing shows no figures for the transformers, so the power amp uses the 5F1 Champ’s, typical of the part, 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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