Selmer Constellation 20 tone stack
Tone stacks and tone controls from amps and pedals, from their schematics, with the stage that drives each one and what it feeds. Turn the knobs and watch the curve. Pin a setting to compare, change any part, and listen through a speaker. Pickups and guitar knobs are on the Guitars page.
The Treble and Bass controls of the Selmer Constellation 20, with every part as on the bench.
Swipe sideways for the whole drawing. Tap a part to change it. The source drives the end marked in, and the load hangs on the end marked out.
At this setting
0 dB is the source’s voltage with nothing hooked to it, so the curve shows what the stack cuts. A tube source changes the curve only through its output resistance, not its gain.
Pin this curve keeps what the graph shows, up to three pins in their own colors. Pins stay when you switch stacks, so you can lay a Marshall treble sweep over a Vox one. In a sweep, the lightest line is the knob at 0 and the darkest is 10.
Listen
You hear a picked string through a guitar on full, with 15 feet of cable. Play without it leaves the stack out.
Source and load
Driven by
Driving
The knobs
- Treble
- 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.
- Bass
- 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.
Parts
- 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, 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, 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.
Source and load
Source: 130 kΩ, the output resistance of the stage before it on the bench at 1 kHz. Load: 3.5 MΩ with 52 pF across it, the input of what follows it on the bench, from its impedance at 1 and 10 kHz.
Values, wiring and drawing are taken from the Selmer Constellation 20 on the bench, so the two always match.
Warnings. The summary above the graph turns red if the stack can’t be solved as set, usually from a part at zero or a shorted pot track.
How this tool works. We solve each stack with the circuit bench’s engine: the source stage’s operating point first, then every part at 161 frequencies from 20 Hz to 20 kHz. A stack that’s also in a bench circuit is built from that circuit file, so the two always match.
The curves are small-signal. They don’t show clipping or grid current in the next stage. For the stage before or after a stack, try the gain stage tool.
Spot an error, or want a circuit added? Email info@swellamps.com. A schematic or a clear photo of one helps. Anything you send may be used to improve these models and our desktop app.
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