Two-channel amp with a passive bass and treble network, 1965 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 Selmer Constellation 20.

Two channels, one ECC82 triode each, mixed into a gain stage. A passive bass and treble network follows, then a recovery stage with the speaker’s feedback on its cathode, a cathodyne, and two fixed-bias EL84s with bias tremolo.

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. Two channels, each one ECC82 triode with no cathode bypass, low gain, about 10, then its own volume. Both volumes meet at V2a’s grid through 470 kΩ each. V2a feeds a passive bass and treble network through only .001 µF. The network loses about 30 dB, and V3a makes it back. The speaker’s feedback also goes to V3a’s cathode, 5 kΩ onto 100 Ω, about 15 dB of it. V1’s supply comes through 68 kΩ, as listed in the drawing’s table, so V1’s plates idle near 40 V. Then a cathodyne and two EL84s on fixed bias, with the tremolo’s oscillator swinging their bias through the Depth pot.

Download.

Parts and what they do

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

R1, 68 kΩ
The channel 1 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.
R2, 68 kΩ
The channel 1 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.
R3, 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.
R6, 68 kΩ
The channel 2 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.
R7, 68 kΩ
The channel 2 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.
R8, 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, 12AU7 / ECC82
Channel 1’s only stage, half an ECC82. Its cathode isn’t bypassed, so its gain is low, about 8.
R4, 100 kΩ
Plate resistor for channel 1.
R5, 2.2 kΩ
Cathode resistor for channel 1, 2.2 kΩ, not bypassed.
C2, 47 nF
Coupling cap to volume 1, .047 µF.
Volume 1, 500 kΩ pot, audio taper
Channel 1’s volume, 500 kΩ.
R11, 470 kΩ
Mixing resistor from channel 1, 470 kΩ.
V1b, 12AU7 / ECC82
Channel 2’s only stage, the other half of the ECC82.
R9, 100 kΩ
Plate resistor for channel 2.
R10, 2.2 kΩ
Cathode resistor for channel 2, 2.2 kΩ, not bypassed.
C3, 47 nF
Coupling cap to volume 2, .047 µF.
Volume 2, 500 kΩ pot, audio taper
Channel 2’s volume, 500 kΩ.
C4, 420 pF
Bright cap across volume 2, 420 pF. Below full it lets treble skip around the pot, so channel 2 is the brighter one.
R12, 470 kΩ
Mixing resistor from channel 2, 470 kΩ.
V2a, 12AX7 / ECC83 / 7025
The mixing stage, half an ECC83, with a 220 kΩ plate resistor and its cathode unbypassed. Gain about 40. The other half of V2 is the tremolo’s oscillator, in the power amp.
R13, 220 kΩ
V2a’s plate resistor, 220 kΩ.
R14, 2.2 kΩ
V2a’s cathode resistor, 2.2 kΩ, not bypassed.
C5, 1 nF
Coupling cap into the tone network, only .001 µF. Into the network’s high resistances it still passes the bass.
C6, 100 pF
Treble cap, 100 pF, to the top of the treble pot.
Treble, 1 MΩ pot, audio taper
Treble, 1 MΩ (the drawing calls it Bass). It gets the signal through 100 pF, with 4700 pF to ground at its far end. Up moves the wiper to the 100 pF end, for more treble.
C7, 4.7 nF
From the treble pot’s far end to ground, 4700 pF.
R16, 680 kΩ
From the treble wiper to the output, 680 kΩ, so the two halves of the network don’t load each other much.
R17, 680 kΩ
Top of the bass section, 680 kΩ.
Bass, 1 MΩ pot, audio taper
Bass, 1 MΩ (the drawing calls it Treble). Its wiper is the network’s output. Up moves the wiper toward the 680 kΩ end, for more bass.
C8, 1 nF
Across the bass pot’s upper half, 1000 pF. Above the bass it shorts that half, so the pot only acts on low notes.
C9, 22 nF
Across the bass pot’s lower half, .022 µF.
R18, 22 kΩ
Bottom of the bass section, 22 kΩ to ground. With the 680 kΩ above, the network loses about 30 dB, which V3a makes back.
V3a, 12AX7 / ECC83 / 7025
Recovery stage, half an ECC83, after the tone network. The speaker’s feedback goes to its cathode.
R20, 220 kΩ
V3a’s plate resistor, 220 kΩ.
R19, 27 kΩ
With C10, from the supply to V3a’s plate: 27 kΩ.
C10, 250 pF
With R19 it shunts V3a’s plate above a few kilohertz, 250 pF, a gentle treble cut.
R21, 2.2 kΩ
V3a’s bias resistor, 2.2 kΩ.
C12, 25 µF electrolytic
Bypass across the bias resistor, 25 µF, so only the 100 Ω below it is in the stage’s cathode for signal.
R22, 100 Ω
The feedback point to ground, 100 Ω. With R31 from the speaker it sets the feedback.
R31, 5 kΩ
Feedback from the speaker, 5 kΩ, onto V3a’s cathode. With the Constellation’s own power amp on it runs from the speaker. On Preamp only or any other power amp its far end is grounded.
C16, 47 nF
Coupling cap to the cathodyne’s grid, .047 µF.
R15, 22 kΩ
Dropping resistor, 22 kΩ, from C18’s node to C23’s.
C23, 16 µF electrolytic
Filter cap on C23’s node, 16 µF. On the amp the oscillator’s plate resistor hangs here.
R40, 68 kΩ
Dropping resistor, 68 kΩ by the drawing’s table, to V1’s node. It drops a lot, so V1 runs on a low supply.
C1, 16 µF electrolytic
Filter cap on V1’s node, 16 µF.
V3b, 12AX7 / ECC83 / 7025
12AX7, a cathodyne, or split-load inverter, with no driver ahead of it. 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, so the preamp has to bring the whole swing the output tubes need.
68k, 68 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.
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.
470k, 470 kΩ
Grid resistor for the cathodyne, returned to the junction of its cathode resistors so the grid sits near the cathode’s high DC voltage. It’s the grid’s only path to ground, and the cathode follows the grid, so the preamp sees much more than 470k Ω.
.047µ, 47 nF
Coupling cap to tube A’s grid.
.047µ, 47 nF
Coupling cap to tube B’s grid.
V4, EL84 / 6BQ5
EL84, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply.
V5, EL84 / 6BQ5
EL84, 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.
1.5k, 1.5 kΩ
One screen resistor shared by both tubes, with no cap after it. It carries both screens’ current. The two screens’ signal currents are opposite and cancel in it, so its drop follows the even harmonics and grows as the stage is driven harder.
22k, 22 kΩ
Dropping resistor from B+ to the node for the preamp and inverter, as on the Selmer Constellation 20 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.
32µ, 32 µF electrolytic
Filter cap at the node for the preamp and inverter. 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 15 Ω, 40 H primary, 10 mH leakage
The output transformer, 8 kΩ plate to plate into 15 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, 250 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 250 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, 250 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, 150 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, 150 Ω
Resistance of the other half. Typical of the part, not measured.
V6, EZ81
One plate of the EZ81 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, EZ81
The rectifier’s other plate, on the other half cycle.
32µ, 32 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, 250 V ac, 60 Hz
The power transformer’s bias tap, 250 V ac, as a sine source at the mains frequency. Typical of the part, not measured.
330k, 330 kΩ
Series resistor into the bias rectifier, 330k 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. It also holds the Intensity pot’s bias end still, so the LFO swings the wiper.
39k, 39 kΩ
The bias supply’s load, 39k Ω to ground. With the 330k 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.
V2b, 12AX7 / ECC83 / 7025
The tremolo’s oscillator, half a 12AX7. The three caps after its plate turn the signal around at one frequency, and the oscillator runs at that frequency, a few times a second. In the amp it’s the other half of V2, whose first half is the preamp’s mixing stage.
470k, 470 kΩ
Oscillator plate resistor, from the inverter’s supply node.
4.7k, 4.7 kΩ
Oscillator cathode resistor.
25µ, 25 µF electrolytic
Oscillator bypass cap. At a few hertz it isn’t a full short, and it adds some of the phase shift that keeps the loop oscillating.
.022µ, 22 nF
First ladder cap.
Speed, 2 MΩ pot, linear taper
Speed, 2M Ω linear, used as a variable resistor. Less resistance runs the oscillator faster.
100k, 100 kΩ
Sets the top speed. It’s all that’s left with Speed on 10.
.01µ, 10 nF
Second ladder cap.
1M, 1 MΩ
Middle resistor of the ladder, returned to the cathode. The footswitch grounds this point.
.01µ, 10 nF
Third ladder cap.
1M, 1 MΩ
Oscillator grid leak.
1M, 1 MΩ
Couples the LFO from the oscillator’s plate. It keeps the bias network from loading the oscillator much.
.01µ, 10 nF
Blocks the oscillator’s DC, so only its swing reaches the Intensity pot.
Depth, 250 kΩ pot, linear taper
Depth, 250k Ω linear. One end gets the LFO, the other sits on the bias supply, and the wiper is the bias line for both output tubes. At 0 the wiper is on the bias supply and the bias holds still. Turned up, more of the LFO rides on the bias, so both grids move together and the output stage’s gain rises and falls. Which way it turns isn’t marked on the drawing. Up is assumed to be more.
Coil R, 16.3 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 15 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, 480 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, 870 µH
With Loss R across it, the treble rises and then levels off, like the losses in a real coil.
Loss R, 26.8 Ω
Limits what Loss L adds to the treble rise.
Loss L2, 3.99 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, 4.11 Ω
Limits what Loss L2 adds.
Tank R, 157 Ω
Height of the bass resonance, about 173 Ω in all at its peak. With no feedback, the voltage at the speaker rises and falls with its impedance.
Tank L, 39.6 mH
With Tank C, sets the resonance: 74 Hz.
Tank C, 117 µF
With Tank L, sets the resonance. With Tank R, it sets how narrow the peak is.

About this version

As drawn
ECC82 (12AU7) for the two input stages, ECC83 (12AX7) for the rest, as the drawing’s table lists them. The tube models are typical of the 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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