Compactron amp with shunt tremolo, 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 Ampeg Jet J-12-D.

One 6BK11 section into Volume and Tone, the Volume’s wiper straight on a 6BK11 floating paraphase with feedback, two cathode-biased 7591As and a silicon bridge. In the tremolo, two triode sections shunt the first stage’s feedback path.

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 gain stage with its cathode partly bypassed, then Volume, with Tone’s wiper on the Volume’s top. A path of 470 kΩ and 47 kΩ with two caps runs from the stage’s plate back to its grid, and two triode sections hang from the middle of it to ground. The oscillator swings their grids: when they conduct they pull the middle of the path down, and the stage’s gain and the grid’s level both change. Their plates have no DC on them, and the triode model passes no current with its plate below its cathode, so the caps charge the shunt point negative and the tremolo barely moves the level here, about 0.2 dB. A real tube passes a little current there. 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.

47k, 47 kΩ
Input 1’s resistor to the grid, 47 kΩ.
47k, 47 kΩ
Input 2’s resistor to the grid, 47 kΩ. With nothing plugged in, this jack’s switch puts 100 kΩ under it.
100k, 100 kΩ
On the second jack’s switch: 100 kΩ from its tip to ground with nothing plugged in. A plug opens the switch.
5.6M, 5.6 MΩ
Grid leak, 5.6 MΩ.
V2, 12AX7 / ECC83 / 7025
A section of a 6BK11 compactron, the only gain stage ahead of the inverter.
120k, 120 kΩ
Plate resistor for V2, from node C.
1k, 1 kΩ
The top of the cathode resistor, 1 kΩ, not bypassed. It trims the stage’s gain.
2.2k, 2.2 kΩ
The bottom of the cathode resistor, 2.2 kΩ, with the bypass cap across it.
25µ, 25 µF electrolytic
Bypass across the 2.2 kΩ only, 25 µF.
.02µ, 20 nF
Coupling cap to the Volume pot’s top and the Tone pot’s wiper.
Volume 1M, 1 MΩ pot, audio taper
Volume, 1 MΩ. Its wiper is the inverter’s grid, with no other grid resistor, so it’s that grid’s path to ground. The drawing marks no taper, so audio is assumed.
Tone 500k, 500 kΩ pot, audio taper
Tone, 500 kΩ. The signal comes in on the wiper. One end goes through .002 µF to the Volume’s wiper, around the Volume. The other goes through .01 µF to ground. Turned up, the wiper moves toward the .002 µF and the top end comes up. The taper isn’t marked, so audio is assumed.
.002µ, 2 nF
From the Tone pot’s bright end to the Volume’s wiper. It lets treble around the Volume, more with the Volume down.
.01µ, 10 nF
From the Tone pot’s other end to ground. With Tone down it pulls treble off the Volume’s top.
47k, 47 kΩ
From the grid toward the shunt point, 47 kΩ.
.01µ, 10 nF
In series with the 47 kΩ from the grid to the shunt point, .01 µF. It keeps DC off the grid.
.1µ, 100 nF
From V2’s plate toward the shunt point, .1 µF. It keeps the plate’s DC off the shunt sections.
470k, 470 kΩ
From the plate’s cap to the shunt point, 470 kΩ. With the 47 kΩ it feeds part of the plate’s signal back to the grid, which cuts the stage’s gain.
V1a, 12AX7 / ECC83 / 7025
Two sections of a 6BK11 in parallel, drawn as one, the shunt. Their plates have no DC on them, so they conduct only on the positive half of the shunt point’s swing, and only while the oscillator holds their grids up.
.05µ, 50 nF
Couples the oscillator’s swing from its plate to the shunt grids.
470k, 470 kΩ
From the cap to the shunt grids, 470 kΩ. With the 10 kΩ under the grids it passes about a fiftieth of the swing, and limits grid current.
10k, 10 kΩ
From the shunt grids to ground, 10 kΩ.
V1b, 12AX7 / ECC83 / 7025
The third section of that 6BK11, the tremolo oscillator.
270k, 270 kΩ
Plate resistor for the oscillator, from node B, the screens’ node.
4.7k, 4.7 kΩ
Cathode resistor for the oscillator. Its value isn’t printed, so the J-12-A’s 4.7 kΩ is assumed. Its far end goes to ground through the tremolo switch.
25µ, 25 µF electrolytic
Bypass for the oscillator’s cathode.
.05µ, 50 nF
First cap of the oscillator’s phase-shift ladder.
120k, 120 kΩ
First ladder leg to ground.
.05µ, 50 nF
Second ladder cap.
120k, 120 kΩ
In series with the Speed pot, the ladder’s second leg. It sets the top speed.
Speed 500k, 500 kΩ pot, audio taper
Speed, 500 kΩ, as a variable resistor in the ladder’s second leg to ground. Turned up, its resistance drops and the oscillator runs faster. The taper isn’t marked, so audio is assumed.
.05µ, 50 nF
Third ladder cap, straight into the grid.
270k, 270 kΩ
Grid leak for the oscillator, and the ladder’s last leg.
V3a, 12AX7 / ECC83 / 7025
12AX7, the driven half of the floating paraphase inverter. Its plate swings opposite the input and drives tube A. The feedback from the speaker goes to its cathode. Its grid connects straight to the preamp’s mixing resistors, which are its path to ground.
V3b, 12AX7 / ECC83 / 7025
The other half. Its grid is fed from the middle of the divider, so its plate swings the other way and drives tube B.
470k, 470 kΩ
Grid resistor for the other half, to ground.
500p, 500 pF
Across the other half’s grid resistor, 500pF, as on the drawing. Above a few kilohertz it shunts the signal from the divider’s middle, so the second half gets less of it and tube B’s side gets less treble than tube A’s.
1k, 1 kΩ
Cathode resistor for both halves. It sets their bias, a couple of volts.
25µ, 25 µF electrolytic
Bypass cap across the shared cathode resistor. The cathodes hold still, so the second half gets its signal only from the divider.
120k, 120 kΩ
Plate resistor for the driven half.
120k, 120 kΩ
Plate resistor for the other half.
470k, 470 kΩ
Top of the divider, from tube A’s plate. With the resistor below it, from tube B’s, the middle sits at the average of the two sides. With the two swinging equal and opposite, the middle holds still. Whatever is left goes to the second half’s grid, which corrects it, so the inverter keeps itself even as the tube ages.
510k, 510 kΩ
Bottom of the divider, to tube B’s plate.
.02µ, 20 nF
From the middle of the divider to the second half’s grid. It keeps the plates’ DC off that grid.
6.8k, 6.8 kΩ
The feedback resistor from the speaker. With the 220 Ω below it, it sends about 3 % of the speaker voltage back to the inverter, which lowers the gain, flattens the response and tightens the bass.
220, 220 Ω
The driven half’s own cathode resistor, 220 Ω, between its cathode and the shared cathode resistor, not bypassed. 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 that cathode. It lowers that half’s gain too.
.02µ, 20 nF
Coupling cap to tube A’s grid.
.02µ, 20 nF
Coupling cap to tube B’s grid.
1k, 1 kΩ
Grid stopper for tube A. It keeps the tube from oscillating and softens grid current.
1k, 1 kΩ
Grid stopper for tube B.
V4, 7591
7591A, tube A of the push-pull pair, its cathode on the shared cathode resistor, so it biases itself.
V5, 7591
7591A, tube B of the pair.
270k, 270 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.
270k, 270 kΩ
Grid leak for tube B.
140, 140 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.
25µ, 25 µF electrolytic
Bypass cap across the cathode resistor, so the cathodes hold still at the signal’s frequencies. It charges and discharges slowly as the average current changes on a hard note.
1k, 1 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.
22k, 22 kΩ
Dropping resistor from the screen supply node to the node for the inverter and the preamp, as on the Ampeg Jet J-12-D 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 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, 6.6 kΩ to 8 Ω, 30 H primary, 12 mH leakage
The output transformer, 6.6 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, 280 V ac, 60 Hz
The power transformer’s high-voltage winding, 280 V ac end to end, into a bridge of four silicon diodes. It’s drawn as two sources of 280 V ac a half cycle apart into two diodes, which charges the reservoir the same way at one diode drop (under a volt) higher. The winding’s marked value, on the drawing.
PT, 280 V ac, 60 Hz
The same winding on the other half cycle. It runs opposite the first source, so the bridge charges the reservoir on every half cycle.
PT R, 40 Ω
Resistance of the whole winding, plus the primary’s share. In a bridge every charging pulse runs through the whole winding. With the diodes’ drop it sets how far B+ sags on a hard note. Typical of the part, not measured.
PT R, 40 Ω
The same winding’s resistance, on the other half cycle. 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.
40µ, 40 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. 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
Two 6BK11 compactrons, three triodes each, run on the 12AX7 model, which matches the type’s published figures for its high-mu sections. The cathode resistor on the oscillator isn’t printed, so 4.7 kΩ is assumed.

Sources

  • Joe Piazza, redrawn J-12-D schematic, 1996

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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