Small push-pull 6V6 amp with a 6AT6 input, 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 5F10 Harvard.

One 6AT6 stage into a volume and tone network, then a 12AX7 driver and cathodyne into two fixed-bias 6V6GTs, with a 5Y3GT rectifier. The 5F11 Vibrolux without its tremolo, on lower voltages.

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 6AT6 stage, then the volume and tone, then the power amp. The three jacks feed one grid through 68 kΩ each, and each empty jack ties its tip to the next, so input 2 is about 3.5 dB down from input 1 and input 3 about 9.5 dB down. The 6AT6 is a single triode with a mu of about 70, so the first stage gains about 43 times, where a 12AX7 half in the same spot gains about 58. 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. In the power amp, a 12AX7 driver and cathodyne feed two 6V6GTs biased at −27 V, each idling at about 17 mA, and 56 kΩ from the speaker to the driver’s cathode cuts the gain about 2.7 dB. Flat out it delivers about 9 W.

Download.

Parts and what they do

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

68k, 68 kΩ
Input 1’s resistor into the grid. With jacks 2 and 3 empty their tips tie to this one, so a guitar in input 1 sees all three 68 kΩ in parallel, 22.7 kΩ. There’s no grid resistor to ground, so the guitar is the grid’s only path to ground.
68k, 68 kΩ
Input 2’s resistor into the grid. With a guitar in input 2, jack 1’s tip is grounded, so its 68 kΩ goes to ground and the input is a divider, about 3.5 dB down.
68k, 68 kΩ
Input 3’s resistor into the grid. With a guitar in input 3 the other two 68 kΩ go to ground, so the input is about 9.5 dB down.
V1, 6AT6
The 6AT6, a single triode with about 70 % of a 12AX7’s gain, the only gain stage ahead of the volume. Its two diodes aren’t used.
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 output tubes 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, audio taper
Tone, 1 MΩ audio, 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.
V2a, 12AX7 / ECC83 / 7025
12AX7, the driver. A cathodyne has no gain of its own, so this stage raises the preamp’s signal about 30 times, enough to drive the output tubes. 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.
.02µ, 20 nF
Coupling cap from the driver to the cathodyne.
V2b, 12AX7 / ECC83 / 7025
The cathodyne, or split-load inverter. The same current runs through its plate resistor and its cathode resistors, which are about equal, so the plate swings one way and the cathode the other by about the same amount. The cathode drives tube A, the plate tube B. Its gain is slightly under 1 on each side.
56k, 56 kΩ
The cathodyne’s plate resistor, the output to tube B.
1.5k, 1.5 kΩ
The cathodyne’s bias resistor. The grid returns below it, so the cathode sits this far above the grid.
56k, 56 kΩ
The cathodyne’s cathode load, the same value as its plate resistor. The cathode above the bias resistor is the output to tube A.
1M, 1 MΩ
Grid resistor for the cathodyne, returned to the junction of its cathode resistors so the grid sits near the cathode’s high DC voltage.
56k, 56 kΩ
The feedback resistor from the speaker. With the 1.5k Ω below it, it sends about 3 % 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. 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. It isn’t bypassed.
.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.
V3, 6V6GT
6V6GT, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply.
V4, 6V6GT
6V6GT, 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
Dropping resistor from B+ to the screen supply. The screens sit slightly below the plates.
16µ, 16 µF electrolytic
Filter cap on the screen supply.
22k, 22 kΩ
Dropping resistor from the screen supply node to the node for the inverter and preamp, as on the Fender 5F10 Harvard 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.
16µ, 16 µF electrolytic
Filter cap at the node for the inverter 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, 10 H primary, 10 mH leakage
The output transformer, 8 kΩ plate to plate 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, 267 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 267 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, 267 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, 70 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, 70 Ω
Resistance of the other half. Typical of the part, not measured.
V5, 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.
V5, 5Y3GT
The rectifier’s other plate, on the other half cycle.
16µ, 16 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, 32 V ac, 60 Hz
The power transformer’s bias tap, 31.9 V ac, as a sine source at the mains frequency. Typical of the part, not measured.
6.8k, 6.8 kΩ
Series resistor into the bias rectifier, 6.8k 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.
56k, 56 kΩ
The bias supply’s load, 56k Ω to ground. With the 6.8k Ω above it, it sets the bias. There’s no bias pot, so change this resistor to change the bias. Bigger runs the tubes colder.
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
Every value is from Fender’s drawing. The 6AT6 model is fitted to the tube manuals’ published points, the 12AX7, 6V6GT and 5Y3GT to their data sheets. The transformers aren’t on the drawing, so their figures are 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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