Two-channel bass amp with 6FQ7 drivers, 1963

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 Silvertone 1483.

Two 12AX7 channels, each with Treble, Bass and Volume, mixed through 330 kΩ into a mixer stage, a cathodyne, a 6FQ7 driving each side, and two cathode-biased 6L6GCs.

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. Each channel is one 12AX7 stage into its own tone network and Volume. Two 220 kΩ in series carry the signal, with .0022 µF from each end of the first to the Treble pot. Turned down, the pot pulls those caps toward ground and the treble goes. Then .0015 µF across part of the Bass pot: turned up, the pot’s wiper shorts across it and the lows come through. The network loses a lot on the way, so the Volumes and the 330 kΩ mixing resistors feed a mixer stage with plenty of gain left. 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.

68k, 68 kΩ
Input resistor for channel 1’s first jack.
68k, 68 kΩ
Input resistor for channel 1’s second jack, the same value.
1M, 1 MΩ
Grid resistor for V1a.
V1a, 12AX7 / ECC83 / 7025
Half of a 12AX7, channel 1’s gain stage.
2.2k, 2.2 kΩ
Cathode resistor for V1a (+1 V on the drawing).
25µ, 25 µF electrolytic
Bypass for V1a’s cathode, full gain down to a few hertz.
100k, 100 kΩ
Plate resistor for V1a (+115 V on the drawing).
.02µ, 20 nF
Coupling cap into channel 1’s tone network.
220k, 220 kΩ
First of two 220 kΩ in series along the signal path. With the .0022 µF caps and the Treble pot below them, a two-step treble cut.
.0022µ, 2.2 nF
From the start of the 220 kΩ pair to the Treble pot. With the pot turned down it shunts treble to ground.
220k, 220 kΩ
Second 220 kΩ along the signal path.
.0022µ, 2.2 nF
From the middle of the 220 kΩ pair to the Treble pot.
Treble 1 1M, 1 MΩ pot, audio taper
Treble, 1 MΩ as a variable resistor from the two .0022 µF caps to ground. Turned down, its resistance drops and the caps pull the treble off. Turned up, the caps see more resistance and the treble stays. Taper and direction aren’t marked, so audio and brighter clockwise are assumed.
.0015µ, 1.5 nF
Across the Bass pot’s first half, from the 220 kΩ to the Volume. It passes the treble around the pot.
Bass 1 1M, 1 MΩ pot, audio taper
Bass, 1 MΩ. One end is the 220 kΩ’s end, the wiper is the Volume’s top, and the other end goes to 100 kΩ to ground with .006 µF back to the wiper. Turned up, the wiper moves toward the 220 kΩ end and the lows come through the pot as well as the .0015 µF. Turned down, more of the pot sits in series and the lows drop. Taper and direction aren’t marked, so audio and more bass clockwise are assumed.
.006µ, 6 nF
From the Bass pot’s wiper to its far end.
100k, 100 kΩ
From the Bass pot’s far end to ground. With the series parts above it, a divider that sets how much comes through.
Vol 1 1M, 1 MΩ pot, audio taper
Channel 1’s Volume, 1 MΩ. Audio taper assumed.
330k, 330 kΩ
Channel 1’s mixing resistor to the mixer’s grid.
68k, 68 kΩ
Input resistor for channel 2’s first jack.
68k, 68 kΩ
Input resistor for channel 2’s second jack, the same value.
1M, 1 MΩ
Grid resistor for V1b.
V1b, 12AX7 / ECC83 / 7025
Half of a 12AX7, channel 2’s gain stage.
2.2k, 2.2 kΩ
Cathode resistor for V1b (+1 V on the drawing).
25µ, 25 µF electrolytic
Bypass for V1b’s cathode, full gain down to a few hertz.
100k, 100 kΩ
Plate resistor for V1b (+115 V on the drawing).
.02µ, 20 nF
Coupling cap into channel 2’s tone network.
220k, 220 kΩ
First of two 220 kΩ in series along the signal path. With the .0022 µF caps and the Treble pot below them, a two-step treble cut.
.0022µ, 2.2 nF
From the start of the 220 kΩ pair to the Treble pot. With the pot turned down it shunts treble to ground.
220k, 220 kΩ
Second 220 kΩ along the signal path.
.0022µ, 2.2 nF
From the middle of the 220 kΩ pair to the Treble pot.
Treble 2 1M, 1 MΩ pot, audio taper
Treble, 1 MΩ as a variable resistor from the two .0022 µF caps to ground. Turned down, its resistance drops and the caps pull the treble off. Turned up, the caps see more resistance and the treble stays. Taper and direction aren’t marked, so audio and brighter clockwise are assumed.
.0015µ, 1.5 nF
Across the Bass pot’s first half, from the 220 kΩ to the Volume. It passes the treble around the pot.
Bass 2 1M, 1 MΩ pot, audio taper
Bass, 1 MΩ. One end is the 220 kΩ’s end, the wiper is the Volume’s top, and the other end goes to 100 kΩ to ground with .006 µF back to the wiper. Turned up, the wiper moves toward the 220 kΩ end and the lows come through the pot as well as the .0015 µF. Turned down, more of the pot sits in series and the lows drop. Taper and direction aren’t marked, so audio and more bass clockwise are assumed.
.006µ, 6 nF
From the Bass pot’s wiper to its far end.
100k, 100 kΩ
From the Bass pot’s far end to ground. With the series parts above it, a divider that sets how much comes through.
Vol 2 1M, 1 MΩ pot, audio taper
Channel 2’s Volume, 1 MΩ. Audio taper assumed.
330k, 330 kΩ
Channel 2’s mixing resistor to the mixer’s grid.
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. 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.
68k, 68 kΩ
Plate resistor for the driver.
2.2k, 2.2 kΩ
Cathode resistor for the driver. It sets the driver’s bias, a volt or so.
.01µ, 10 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.
68k, 68 kΩ
The cathodyne’s plate resistor, the output to tube B.
2.2k, 2.2 kΩ
The cathodyne’s bias resistor. The grid returns below it, so the cathode sits this far above the grid.
68k, 68 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.
.01µ, 10 nF
Coupling cap from the inverter to side A’s driver.
.01µ, 10 nF
Coupling cap from the inverter to side B’s driver.
2.2k, 2.2 kΩ
The drivers’ cathode resistor, 2.2k Ω to ground for both halves, with no bypass cap. The two cathodes share it, so the pair works somewhat like a long-tail pair: what one half gains in current the other loses, which evens out the two sides’ swings.
1M, 1 MΩ
Grid leak for side A’s driver, to ground.
V3a, 6CG7
6CG7, side A’s driver: a gain stage that raises the inverter’s signal about 9 times on its way to tube A’s grids. The other half drives side B, and the two share one cathode resistor.
100k, 100 kΩ
The driver’s plate resistor, 100k Ω from the node for the drivers.
.01µ, 10 nF
Coupling cap from the driver’s plate to tube A’s grids.
1M, 1 MΩ
Grid leak for side B’s driver, to ground.
V3b, 6CG7
The other half of the 6CG7, side B’s driver.
100k, 100 kΩ
The driver’s plate resistor, 100k Ω from the node for the drivers.
.01µ, 10 nF
Coupling cap from the driver’s plate to tube B’s grids.
V4, 6L6GC
6L6GC, tube A of the push-pull pair, its cathode on the shared cathode resistor, so it biases itself.
V5, 6L6GC
6L6GC, tube B of the pair.
560k, 560 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.
560k, 560 kΩ
Grid leak for tube B.
270, 270 Ω
The shared cathode resistor. Every output tube’s current runs through it and lifts the cathodes above the grids, and that’s the bias. Driven hard, the pair draws more current on average, so the cathodes climb and the tubes run colder, which softens the clipping.
1.5k, 1.5 kΩ
Dropping resistor from B+ to the screen supply. The screens sit slightly below the plates.
20µ, 20 µF electrolytic
Filter cap on the screen supply.
15k, 15 kΩ
Dropping resistor from the screen supply node to the node for the drivers, as on the Silvertone 1483 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.
10µ, 10 µF electrolytic
Filter cap at the node for the drivers. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
68k, 68 kΩ
Dropping resistor from the node for the drivers to the node for the inverter and the preamp, as on the Silvertone 1483 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.
5µ, 5 µF electrolytic
Filter cap at the node for the inverter 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, 4 kΩ to 8 Ω, 30 H primary, 10 mH leakage
The output transformer, 4 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, 360 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 360 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, 360 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.
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.
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.
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 the Sears 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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