Three-transistor 18 V boost with active tone controls, 1970 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.
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.
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.
3Press Play
Press Play through the circuit to hear a picked note or chord. Play the string is the guitar by itself.
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.
5Tap a wire
Tap a wire to put the probe there. The Probe graph and the spectrum show the signal there.
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
- Not sure what a control does? Hover over it, or hold a finger on it, for a short note.
- For step-by-step walkthroughs, open Start here at the top of the page.
- To put together a pedal or preamp of your own, use the Builder tab.
- Your switches stay where you left them the next time you open that circuit, in this browser only.
- A tube amp opens whole, with its power amp. Pick Preamp only in the Power amp menu to hear the preamp alone, or another power amp to try this preamp on it.
- The graphs use a steady test tone. Play sounds a picked note that dies away.
- Part values are typical of their type. Each circuit’s notes say which schematic it’s based on.
Modeled after the Colorsound Power Booster.
A two-transistor feedback pair with a Drive pot in the first emitter, then a bass and treble network wrapped around a third transistor, on 18 V. No volume control.
Try this
Common mods
Changes people often make to this circuit. Try it plays the note before and after, at the same loudness. Keep it adds the mod to Your changes, where it can go on a switch.
Input
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.
Your build
Stack blocks from the guitar down to the output. Tap + Add a block wherever you want one. The coupling caps and the supply are wired in for you.
Your blocks. These are blocks you saved with Save block, values and part changes included. Add one to any build of the same kind with + Add a block. They stay in this browser.
Your builds stay in this browser, on this device, and 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 to a file to keep a copy. Open a file, or drop the .bench file on the page, brings it back on any device.
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.
Tap where each leg goes. Tap both ends of a part to put the new one across that 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.
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.
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.
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. Q1 and Q2 act as one amp. R7 feeds Q2’s collector back to Q1’s emitter, so the pair’s gain is 1 + 12 kΩ over what sits in Q1’s emitter: about 3.6 with Drive at 0, much more near full. On 18 V the pair has twice the Overdriver’s supply to swing in, so most of the turn is a clean boost. The tone stage after it is a Baxandall network wrapped around Q3, with Bass and Treble lifting or cutting each end. There’s no volume control: at the default settings a 200 mV pick comes out at about 1.7 V. The model leaves out the batteries’ sag.
Download.
Parts and what they do
Stock values for the Later version version. Tap a part on the schematic to change it.
- C1, 220 nF
- Input coupling cap. There’s no pulldown resistor. Q1’s base is biased through R2.
- Q1, BC109C
- First half of the feedback pair. Its collector drives Q2’s base directly.
- R1, 120 kΩ
- Q1’s collector load.
- R3, 4.7 kΩ
- Q1’s emitter resistor. With R7 it sets the pair’s least gain, about 3.6.
- C2, 22 µF electrolytic
- Couples the Drive pot to Q1’s emitter, so the pot changes the gain without moving the bias.
- Drive, 10 kΩ pot, linear taper
- A 10 kΩ linear pot, a variable resistor from C2 to ground. Turning it up lowers the resistance in Q1’s emitter and raises the gain.
- C3, 220 pF
- Collector to base on Q2. It keeps the pair from oscillating and rolls off the top end at high gain.
- Q2, BC109C
- Second half of the feedback pair.
- R4, 1.8 kΩ
- Q2’s collector load.
- R5, 470 Ω
- Unbypassed part of Q2’s emitter resistance. It keeps Q2’s own gain to a few.
- R6, 1.2 kΩ
- Bypassed part of Q2’s emitter resistance.
- C5, 22 µF electrolytic
- Bypasses R6.
- R2, 150 kΩ
- DC feedback from below R5 to Q1’s base. It sets the pair’s bias.
- R7, 12 kΩ
- Feedback from Q2’s collector to Q1’s emitter. It sets the pair’s gain, 1 + 12 kΩ over what sits in Q1’s emitter, and since it comes before C4 it also carries DC.
- C4, 10 µF electrolytic
- Coupling cap from the pair into the tone network (4.7 µF in the early version).
- R8, 4.7 kΩ
- Input end of the bass leg.
- Bass, 100 kΩ pot, linear taper
- Bass. Toward the input end it boosts, toward the feedback end it cuts.
- C6, 100 nF
- Across the whole bass pot, so above a few hundred hertz the pot is shorted out.
- R9, 4.7 kΩ
- Feedback end of the bass leg.
- R10, 39 kΩ
- From the bass wiper to the center node.
- C7, 10 nF
- Input end of the treble leg.
- Treble, 100 kΩ pot, linear taper
- Treble. Toward the input end it boosts, toward the feedback end it cuts.
- C8, 10 nF
- Feedback end of the treble leg.
- R11, 5.6 kΩ
- From the treble wiper to the center node.
- C9, 100 nF
- Couples the center node to Q3’s base.
- R12, 180 kΩ
- Top of Q3’s base divider.
- R13, 33 kΩ
- Bottom of Q3’s base divider, about 2.8 V at the base.
- Q3, BC109C
- The tone stage’s amplifier. Its collector feeds back to its base through the tone network, so the network sets its gain at each frequency.
- C11, 100 pF
- Collector to base on Q3. It rolls off the very top.
- R14, 3.9 kΩ
- Q3’s collector load.
- R15, 1 kΩ
- Q3’s emitter resistor.
- C12, 10 µF electrolytic
- Bypasses R15 (4.7 µF in the early version).
- C10, 22 µF electrolytic
- Couples Q3’s collector to the feedback end of the tone network.
- C13, 220 nF
- Output coupling cap, straight to the output jack (4.7 µF in the early version).
Versions
- Later version
- Positive supply 18 V, negative ground. BC109C transistors. 10 µF from the pair and in Q3’s emitter, 220 nF at the output.
- Early version
- The first units: 4.7 µF from the pair and in Q3’s emitter, and 4.7 µF in place of the 220 nF at the output.
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. Caps are ideal, and the battery or HT is a voltage behind a resistance.
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