Gibson GA-18T Explorer 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 Bass, Volume, and Treble controls of the Gibson GA-18T Explorer, 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
- Bass
- Bass, 2 MΩ linear, a variable resistor between the tone network’s top and the volume, with 500 pF across it. The 500 pF carries the treble past it either way. Turned up, its resistance drops and the bass comes through too.
- Volume
- Volume, 500 kΩ audio. Its wiper is V1b’s grid, tied to the treble pot’s wiper. There’s no master.
- Treble
- Treble, 1 MΩ audio. One end gets the signal through 500 pF, the other goes through .01 µF to ground, and the wiper is V1b’s grid. Turned up, the wiper moves toward the 500 pF, so treble skips the bass pot and volume. Turned down, it moves toward the .01 µF, which bleeds treble off the grid. Up is assumed to be brighter.
Parts
- 10 nF
- Coupling cap from V1a to the tone network.
- 1 MΩ
- From the tone network’s top back to V1a’s grid. The plate swings opposite the grid, so this is negative feedback: with the 47 kΩ in front, the stage’s gain from the jack is held near 15 and its distortion down.
- Bass
- 2 MΩ, Bass, 2 MΩ linear, a variable resistor between the tone network’s top and the volume, with 500 pF across it. The 500 pF carries the treble past it either way. Turned up, its resistance drops and the bass comes through too.
- 500 pF
- Across the bass pot. It passes the treble around the pot’s resistance.
- Vol 500k
- 500 kΩ, Volume, 500 kΩ audio. Its wiper is V1b’s grid, tied to the treble pot’s wiper. There’s no master.
- 500 pF
- The treble pot’s bright cap, from the tone network’s top.
- Treble
- 1 MΩ, Treble, 1 MΩ audio. One end gets the signal through 500 pF, the other goes through .01 µF to ground, and the wiper is V1b’s grid. Turned up, the wiper moves toward the 500 pF, so treble skips the bass pot and volume. Turned down, it moves toward the .01 µF, which bleeds treble off the grid. Up is assumed to be brighter.
- 10 nF
- The treble pot’s cut cap, to ground.
- V1a grid
- 59 kΩ, A resistor from this network feeds back to V1a’s grid. Here that grid is a resistor to ground, about 59 kΩ. The feedback loop isn’t on this page.
Source and load
Source: 44 kΩ, the output resistance of the stage before it on the bench at 1 kHz. Load: 31 MΩ with 41 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 Gibson GA-18T Explorer 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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