Blackface two-channel bass amp head, 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 AA165 Bassman.

Two channels on 7025s. The bass channel has three stages, the treble, bass and Deep tone network and a bass-boost network before its last stage. The normal channel is the blackface two-stage layout with a Bright switch. They mix through 220 kΩ into a 12AT7 long-tail pair and 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 adds a third stage behind a network that boosts the lows, and its first plate resistor has .01 µF across it, which rolls off the highs. The power amp has 820 Ω of feedback from the speaker and a 500 pF cap into the inverter, so it passes little deep bass from a guitar. The AA864 build is the later drawing. The AB165 is its own circuit, since its power amp differs.

Download.

Parts and what they do

Stock values for the AA165 version. 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 preamp node (+390 V) to the +320 V node that feeds the bass channel’s first tube.
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. The AA864 drawing leaves it out.
CT, 500 pF
Treble cap, 500 pF on the AA165 (250 pF on the AA864). 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 on the AA165, 50 kΩ audio on the AA864. Its wiper feeds the volume.
Bass, 250 kΩ pot, audio taper
Bass channel bass, 250 kΩ audio (10 kΩ on the AA864), a variable resistor.
RM, 2.7 kΩ
In place of a middle pot, 2.7 kΩ to ground on the AA165. The AA864 grounds the bass pot’s foot directly.
Volume, 1 MΩ pot, audio taper
Bass channel volume, 1 MΩ audio (250 kΩ on the AA864).
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.
470k, 470 kΩ
From V1b’s plate, 470 kΩ with .001 µF across it. With the .22 µF and 4.7 kΩ below, the highs pass through the cap and the lows through the resistor into a node that the .22 µF holds up in the bass.
.001µ, 1 nF
Across the 470 kΩ, .001 µF. It passes the treble around the resistor.
.22µ, 220 nF
.22 µF from the network’s node to the 4.7 kΩ. With the 470 kΩ above it, it sets where the mids dip: the lows see the cap as a high impedance and keep their level.
4.7k, 4.7 kΩ
4.7 kΩ under the .22 µF. V3a’s grid leak returns here, not to ground.
.047µ, 47 nF
Coupling cap into V3a, .047 µF.
1M, 1 MΩ
V3a grid leak, 1 MΩ, returned to the 4.7 kΩ’s top.
V3a, 12AX7 / ECC83 / 7025
The third stage, half of the middle 7025 (its other half is unused). It’s the bass channel’s alone.
1500, 1.5 kΩ
V3a cathode resistor, 1500 Ω.
25µ, 25 µF electrolytic
V3a bypass cap, 25 µF. The AA864 leaves it out.
100k, 100 kΩ
V3a plate resistor, 100 kΩ from the preamp node.
.003µ, 3 nF
Across V3a’s plate resistor, .003 µF (the AA864’s is unreadable, assumed to be the same). It rolls the stage’s treble off above about 1 kHz.
220k, 220 kΩ
Bass channel mixing resistor, 220 kΩ into the inverter’s input.
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 preamp node.
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.
Bass, 250 kΩ pot, audio taper
Normal bass, 250 kΩ audio, a variable resistor.
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 preamp node.
220k, 220 kΩ
Normal channel mixing resistor, 220 kΩ into the inverter’s input. The two mixing resistors carry a little DC between the two plates, as drawn: there’s no coupling cap ahead of them.
500p, 500 pF
Coupling cap into the phase inverter. With the grid resistor it sets how low the inverter goes, around 212 Hz.
V4a, 12AT7 / ECC81
12AT7, the driven half of the long-tail pair phase inverter. Its plate swings opposite the input and drives tube A.
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. 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 the feedback point for signal, so the feedback acts across the pair.
82k, 82 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.
820, 820 Ω
The feedback resistor from the speaker. With the 100 Ω below it, it sends about 11 % of the speaker voltage back to the inverter, which lowers the gain, flattens the response and tightens the bass.
100, 100 Ω
The bottom of the feedback loop, 100 Ω to ground. The fed-back speaker voltage arrives here and moves the tail.
.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.
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.
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, 47 V ac, 60 Hz
The power transformer’s bias tap, 47 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.
Bias 10k, 10 kΩ pot, linear taper
Bias adjust, 10k Ω linear. Its wiper feeds both output grids through their grid leaks. Toward the diode end the grids go more negative and the tubes idle cooler.
15k, 15 kΩ
The bottom of the bias divider, 15k Ω to ground.
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.

Versions

AA165
The AA165 drawing: the bass channel’s first plate resistor bypassed by .01 µF, a 2.7 kΩ middle resistor, and the .001 µF, 470 kΩ, .22 µF and 4.7 kΩ network ahead of its third stage. The tube models are typical of each type, not measured.
AA864, modeled after the Fender AA864 Bassman
The AA864 drawing: every preamp stage on the +390 V node, a 220 kΩ resistor ahead of a 50 kΩ treble pot, a 10 kΩ bass pot, the middle cap straight to ground, and a plain divider (220 kΩ over 330 kΩ with .001 µF) into the bass channel’s third stage, whose cathode isn’t bypassed. Its power amp is the AA165’s on a 325 V ac winding (the drawing’s figure is hard to read).

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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