Blackface two-channel bass amp head with a shared third stage, mid 1960s style

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 AB165 Bassman.

Two channels on 7025s, mixed through 220 kΩ into a shared third stage with 470 kΩ of feedback around it. The bass channel has the treble, bass and Deep tone network, the normal channel a Bright switch. A 12AT7 long-tail pair with the speaker’s feedback on its grid drives two 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. Both channels have the blackface tone network hung straight off the first plate, with a fixed resistor where a middle pot would be. The bass channel’s first plate resistor has .01 µF across it, which rolls off the highs. The third stage has 470 kΩ from its plate back to the mixing node, so its gain stays near 1.5 from each channel and it takes a lot of drive to clip. The speaker’s feedback goes into the inverter’s grid, and 220 kΩ from each output plate back to the inverter adds a second loop around the output tubes.

Download.

Parts and what they do

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

R1B, 68 kΩ
The bass channel’s 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.
R3B, 68 kΩ
The bass channel’s 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.
R2B, 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.
R1A, 68 kΩ
The normal channel’s 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.
R3A, 68 kΩ
The normal channel’s 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.
R2A, 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.
27k, 27 kΩ
Dropping resistor, 27 kΩ 1 W, from the +390 V node to the +320 V node that feeds V1 and V2a.
8µ, 8 µF electrolytic
Filter cap on the +320 V node, 8 µF 450 V.
V1a, 12AX7 / ECC83 / 7025
Bass channel first stage, half a 7025. Its cathode is bypassed for full gain.
1500, 1.5 kΩ
V1a cathode resistor, 1500 Ω.
25µ, 25 µF electrolytic
V1a bypass cap, 25 µF 25 V.
100k, 100 kΩ
V1a plate resistor, 100 kΩ.
.01µ, 10 nF
Across V1a’s plate resistor, .01 µF. Above about 160 Hz it shunts the plate resistor to the supply node, so the stage’s gain falls with rising pitch: a treble cut ahead of the tone controls.
330p, 330 pF
Treble cap, 330 pF (hard to read on the drawing, so the usual value is used). The network hangs straight off the plate. Every path from the plate to ground goes through a cap.
100k, 100 kΩ
Slope resistor, 100 kΩ.
.1µ, 100 nF
Bass cap, .1 µF, five times the guitar amps’ .02 µF, so the bass pot works lower down.
.1µ, 100 nF
Middle cap, .1 µF.
Treble, 250 kΩ pot, audio taper
Bass channel treble, 250 kΩ audio. Its wiper feeds the volume. Brighter clockwise.
Bass, 250 kΩ pot, audio taper
Bass channel bass, 250 kΩ audio, a variable resistor. More bass clockwise.
8.2k, 8.2 kΩ
In place of a middle pot, 8.2 kΩ to ground (hard to read on the drawing, so the usual value is used).
Volume, 1 MΩ pot, audio taper
Bass channel volume, 1 MΩ audio.
V1b, 12AX7 / ECC83 / 7025
Bass channel second stage, the other half of V1.
1500, 1.5 kΩ
V1b cathode resistor, 1500 Ω.
25µ, 25 µF electrolytic
V1b bypass cap, 25 µF 25 V.
100k, 100 kΩ
V1b plate resistor, 100 kΩ from the +320 V node.
220k, 220 kΩ
Bass channel mixing resistor, 220 kΩ from V1b’s plate to the mixing node. There’s no cap ahead of it, so it carries a little DC between the plates and V3a’s.
.01µ, 10 nF
Coupling cap from the mixing node into V3a, .01 µF. With the 470 kΩ grid leak it rolls off below about 34 Hz.
470k, 470 kΩ
V3a grid leak, 470 kΩ.
470k, 470 kΩ
Feedback, 470 kΩ from V3a’s plate back to the mixing node. With the two 220 kΩ mixing resistors it holds V3a’s gain to about 1.5 from each channel, and it lowers V3a’s output impedance, which matters here: the speaker’s feedback goes to the inverter’s grid that V3a drives.
V3a, 12AX7 / ECC83 / 7025
The third stage, half of the middle 7025 (its other half is unused). Both channels go through it.
1500, 1.5 kΩ
V3a cathode resistor, 1500 Ω.
25µ, 25 µF electrolytic
V3a bypass cap, 25 µF.
100k, 100 kΩ
V3a plate resistor, 100 kΩ from the +390 V node.
.1µ, 100 nF
Coupling cap from V3a’s plate to the inverter’s grid, .1 µF. The speaker’s feedback joins on its far side.
V2a, 12AX7 / ECC83 / 7025
Normal channel first stage, half a 7025.
1500, 1.5 kΩ
V2a cathode resistor, 1500 Ω.
25µ, 25 µF electrolytic
V2a bypass cap, 25 µF.
100k, 100 kΩ
V2a plate resistor, 100 kΩ from the +320 V node.
500p, 500 pF
500 pF across V2a’s plate resistor. It cuts the stage’s gain above about 6 kHz.
250p, 250 pF
Treble cap, 250 pF.
100k, 100 kΩ
Slope resistor, 100 kΩ.
.1µ, 100 nF
Bass cap, .1 µF.
.047µ, 47 nF
Middle cap, .047 µF.
Treble, 250 kΩ pot, audio taper
Normal treble, 250 kΩ audio. Its wiper feeds the volume. Brighter clockwise.
Bass, 250 kΩ pot, audio taper
Normal bass, 250 kΩ audio, a variable resistor. More bass clockwise.
6.8k, 6.8 kΩ
In place of a middle pot, 6.8 kΩ to ground, as in a Deluxe Reverb.
Volume, 1 MΩ pot, audio taper
Normal volume, 1 MΩ audio.
V2b, 12AX7 / ECC83 / 7025
Normal channel second stage, the other half of V2.
1500, 1.5 kΩ
V2b cathode resistor, 1500 Ω.
25µ, 25 µF electrolytic
V2b bypass cap, 25 µF.
100k, 100 kΩ
V2b plate resistor, 100 kΩ from the +390 V node.
220k, 220 kΩ
Normal channel mixing resistor, 220 kΩ from V2b’s plate to the mixing node.
V4a, 12AT7 / ECC81
12AT7, the driven half of the long-tail pair phase inverter. Its plate swings opposite the input and drives tube A. Its grid is fed through the preamp’s coupling cap, so there’s no second one here.
V4b, 12AT7 / ECC81
The other half of the inverter. Its grid sits at the feedback point for signal, so it’s driven through the shared cathodes, and its plate swings with the input. It drives tube B.
1M, 1 MΩ
Grid resistor for the driven half, returned to the bias point above the tail, not to ground.
1M, 1 MΩ
Grid resistor for the other half.
470, 470 Ω
Bias resistor for both halves. The cathodes sit this far above the grids’ return, a volt or so.
22k, 22 kΩ
The tail resistor, to ground. It’s large, so the pair shares a nearly fixed current. When one half conducts more, the other conducts less, which splits the phase.
.1µ, 100 nF
Ties the second grid to ground for signal, so that half is driven only through the shared cathodes. The feedback comes in at the first grid.
100k, 100 kΩ
Plate resistor for the driven half. It’s smaller than the other so the two outputs come out about even.
100k, 100 kΩ
Plate resistor for the other half.
220k, 220 kΩ
Feedback, 220k Ω from tube A’s plate back to the inverter plate that drives it. The output plate swings opposite its grid, so this lowers the output stage’s gain and its output impedance. It doesn’t reach around the output transformer. It also carries about a milliamp of DC into the inverter plate.
220k, 220 kΩ
The same from tube B’s plate back to the other inverter plate.
47k, 47 kΩ
Feedback, 47k Ω from the speaker through the cap beside it into the inverter’s first grid. There it meets the preamp’s signal, turned the other way, so the two subtract. How much gets in depends on what drives that grid: the stronger the preamp’s source, the less the feedback can pull the grid. It lowers the gain, flattens the response and tightens the bass.
.1µ, 100 nF
In series with the feedback resistor, .1µF. It keeps the grid’s DC off the speaker line.
.022µ, 22 nF
Coupling cap to tube A’s grid.
.022µ, 22 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.
V5, 6L6GC
6L6GC, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply.
V6, 6L6GC
6L6GC, 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, returned to its own bias pot.
470, 470 Ω
Screen resistor for tube A. It limits the screen current when the tube is driven hard.
470, 470 Ω
Screen resistor for tube B.
4 H, 4 H
A choke from B+ to the screen supply, as in the amp this style is based on. It passes DC with little drop, so the screens sit close to B+. With the cap after it, it would filter ripple, but ripple isn’t modeled here. Typical of the part, not measured.
100, 100 Ω
The choke’s winding resistance, typical, not measured. It’s all the DC drop the screens see from B+.
20µ, 20 µF electrolytic
Filter cap on the screen supply.
1k, 1 kΩ
Dropping resistor from the screen supply node to the node for the inverter, as on the Fender AA165 Bassman 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.
20µ, 20 µF electrolytic
Filter cap at the node for the inverter. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
4.7k, 4.7 kΩ
Dropping resistor from the node for the inverter to the node for the preamp, as on the Fender AA165 Bassman 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.
20µ, 20 µF electrolytic
Filter cap at the node for 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 4 Ω, 40 H primary, 10 mH leakage
The output transformer, 4 kΩ plate to plate into 4 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, 320 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 320 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, 320 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, 30 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, 30 Ω
Resistance of the other half. Typical of the part, not measured.
D1, 1N4001
One of the silicon rectifier diodes. It drops under a volt, so B+ holds up better under load than with a tube rectifier.
D2, 1N4001
The other rectifier diode, on the other half cycle.
35µ, 35 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, 45 V ac, 60 Hz
The power transformer’s bias tap, 45 V ac, as a sine source at the mains frequency. Typical of the part, not measured.
470, 470 Ω
Series resistor into the bias rectifier, 470 Ω. It limits the charging current.
Bias diode, 1N4001
The bias rectifier, a silicon diode turned so the supply comes out negative.
50µ, 50 µF electrolytic
Filter cap on the bias supply, its plus side to ground.
Balance, 10 kΩ pot, linear taper
Hum balance, 10k Ω linear across the bias supply. Its wiper feeds tube A’s grid through its grid leak. Tube B’s comes from the middle of the two 10k Ω across the same pot, so it holds still. Turning it moves tube A’s bias against tube B’s, a few volts each way. It’s set for the least hum, which is where the two tubes pull about the same current.
15k, 15 kΩ
The bottom of the bias divider, 15k Ω to ground. With the pot and the series resistor above it, it sets the bias. There’s no bias adjust, so change this resistor to change it. Bigger runs the tubes colder.
10k, 10 kΩ
From the bias supply’s top to tube B’s bias line, 10k Ω. With the one below, it puts tube B’s grid halfway between the ends of the balance pot.
10k, 10 kΩ
From the balance pot’s bottom end to tube B’s bias line, 10k Ω, the other half of that divider.
Coil R, 4.34 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 4 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, 128 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, 232 µH
With Loss R across it, the treble rises and then levels off, like the losses in a real coil.
Loss R, 7.15 Ω
Limits what Loss L adds to the treble rise.
Loss L2, 1.06 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, 1.09 Ω
Limits what Loss L2 adds.
Tank R, 41.9 Ω
Height of the bass resonance, about 46 Ω in all at its peak. The feedback holds the output steadier against that than it would be without feedback.
Tank L, 10.6 mH
With Tank C, sets the resonance: 74 Hz.
Tank C, 440 µF
With Tank L, sets the resonance. With Tank R, it sets how narrow the peak is.

About this version

AB165
The AB165 drawing. The tube models are typical of each type, 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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