Gibson GA-400 Super 400 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 (II), Bass (II), and Volume (II) controls of the Gibson GA-400 Super 400, 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 (II)
- Channel II’s treble, 1 MΩ audio as a variable resistor in series with .005µF to ground. Turned up, its resistance rises, the cap cuts less treble and the channel gets brighter.
- Bass (II)
- Channel II’s bass, 2 MΩ linear as a variable resistor between the tone pots’ wipers and the volume, with 500pF across it. The treble always gets across through the cap. Turned up, the resistance drops and the lows get across too.
- Volume (II)
- Channel II’s volume, 1 MΩ linear. The signal comes in on the wiper and leaves from the top, through 220 kΩ to the mixing bus. Turned down, the wiper shorts it to ground.
Parts
- 100 kΩ
- From input 1’s plate into the tone network. The two plates meet here through 100 kΩ each.
- 100 kΩ
- From input 2’s plate into the tone network.
- 20 nF
- Coupling cap from the 100 kΩ junction to the tone pots’ wipers.
- Treble 1M
- 1 MΩ, Channel II’s treble, 1 MΩ audio as a variable resistor in series with .005µF to ground. Turned up, its resistance rises, the cap cuts less treble and the channel gets brighter.
- 5 nF
- Under the treble pot, to ground.
- Bass 2M
- 2 MΩ, Channel II’s bass, 2 MΩ linear as a variable resistor between the tone pots’ wipers and the volume, with 500pF across it. The treble always gets across through the cap. Turned up, the resistance drops and the lows get across too.
- 500 pF
- Across the bass pot: the treble’s way around it.
- 220 kΩ
- Mixing resistor from channel II’s volume to the mixer’s grid.
- Vol 1M
- 1 MΩ, Channel II’s volume, 1 MΩ linear. The signal comes in on the wiper and leaves from the top, through 220 kΩ to the mixing bus. Turned down, the wiper shorts it to ground.
- V2b plate
- 53 kΩ, V2b’s plate also feeds this network. Here it’s a resistor to ground, its output resistance, about 53 kΩ.
Source and load
Source: 53 kΩ, the output resistance of the stage before it on the bench at 1 kHz. Load: 390 kΩ with 28 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-400 Super 400 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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