Transistor tremolo with a twin-T oscillator
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
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- The graphs use a steady test tone. Play sounds a picked note that dies away.
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Modeled after the Kay T-1 tremolo pedal.
One transistor amplifies the guitar, and two more in series across the signal shunt it to ground in time with a twin-T oscillator. The treadle sets the speed. There is no depth 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. Hold a note and watch the output scope. Q1 and the twin T make a slow oscillation at its collector, about 2.5 V peak to peak. Through R5 it turns the two shunt transistors on and off together, and when they are on they pull the signal at R12’s far end toward ground. The transistors switch rather than fade, so the level pulses more sharply than a tube amp’s tremolo. Speed sets the resistance in the twin T’s shunt arm and so the frequency, from about 5.7 to 17.6 times a second. There is no depth control: R5 and R12 set the depth. Q4 makes up the level lost in R12 and R13.
Download.
Parts and what they do
Stock values. Tap a part on the schematic to change it.
- R8, 33 kΩ
- Input resistor. With the base divider it sets the input impedance near 40 kΩ and cuts the stage’s gain.
- C4, 100 nF
- Input coupling cap.
- R9, 470 kΩ
- Top of Q4’s base divider.
- R10, 43 kΩ
- Bottom of Q4’s base divider.
- Q4, 2SC1815
- The input stage, common emitter with its emitter grounded, which makes up for the level the shunt network loses.
- R11, 4.7 kΩ
- Q4 collector load.
- C5, 10 nF
- Coupling cap from Q4. With the 100 k after it, it trims the lows below about 80 Hz.
- R12, 100 kΩ
- Series resistor into the shunt node. The shunt transistors pull against it, so it sets how far the level can drop.
- R13, 100 kΩ
- Pulldown at the shunt node, which halves the level with the shunt off.
- Q2, 2SC1815
- Upper shunt transistor. When the oscillator’s collector is high it turns on and pulls the shunt node toward ground.
- Q3, 2SC1815
- Lower shunt transistor, in series with Q2 and turned on with it.
- R6, 33 kΩ
- Base resistor for Q2.
- R7, 33 kΩ
- Base resistor for Q3.
- R5, 68 kΩ
- Feeds the oscillator’s collector to both shunt bases.
- C6, 220 nF
- Output coupling cap.
- R14, 47 kΩ
- Output pulldown.
- Q1, 2SC1815
- The oscillator transistor. The twin T from its collector back to its base turns the phase over at one low frequency, so the loop oscillates there.
- R1, 1.2 kΩ
- Oscillator collector load.
- R2, 10 kΩ
- Twin-T series resistor from the collector. It also carries Q1’s base bias with R3.
- R3, 10 kΩ
- Twin-T series resistor to the base.
- C1, 10 µF electrolytic
- Twin-T shunt cap from the resistor join.
- C2, 4.7 µF electrolytic
- Twin-T series cap from the collector.
- C3, 4.7 µF electrolytic
- Twin-T series cap to the base.
- R4, 33 Ω
- In series with Speed in the twin T’s shunt arm, so it never reaches zero.
- Speed, 1 kΩ pot, linear taper
- Speed, 1 kΩ, turned by the treadle, taper assumed linear. Used as a variable resistor in the twin T’s shunt arm, which sets the oscillator’s frequency.
About this version
- T-1, 2SC1815, modeled after the Kay T-1 tremolo pedal
- From a redrawn schematic. The library’s 2SC1815 stands in for the 2SC828s.
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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