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 (I), Bass (I), and Volume (I) 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

Frequency response

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 (I)
Channel I’s treble, 2 MΩ linear. Its top is on the 100 kΩ junction, its wiper on the .005 µF caps from the plates, its bottom on .02 µF to ground. Turned up, the wiper moves toward the top and more of the plates’ treble reaches the network.
Bass (I)
Channel I’s bass, 2 MΩ linear as a variable resistor in the bass path, ahead of the .02 µF to ground. Turned up, its resistance drops and more of the lows get through.
Volume (I)
Channel I’s volume, 1 MΩ linear. The signal comes in on the wiper and leaves from the top, which is the mixing bus itself. 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.
5 nF
From input 1’s plate to the first tone pot’s wiper. Through these two .005 µF the plates also meet, but only for the treble.
5 nF
From input 2’s plate to the same point.
Treble 2M
2 MΩ, Channel I’s treble, 2 MΩ linear. Its top is on the 100 kΩ junction, its wiper on the .005 µF caps from the plates, its bottom on .02 µF to ground. Turned up, the wiper moves toward the top and more of the plates’ treble reaches the network.
20 nF
Under the treble pot, to ground.
500 pF
From the 100 kΩ junction straight to the output point, .0005 µF: the treble’s way around the bass path.
100 kΩ
Into the bass path.
Bass 2M
2 MΩ, Channel I’s bass, 2 MΩ linear as a variable resistor in the bass path, ahead of the .02 µF to ground. Turned up, its resistance drops and more of the lows get through.
20 nF
The bass path’s cap to ground. With the resistance ahead of it, only the lows pass.
100 kΩ
From the bass path to the output point.
50 nF
Coupling cap to the volume’s wiper.
Vol 1M
1 MΩ, Channel I’s volume, 1 MΩ linear. The signal comes in on the wiper and leaves from the top, which is the mixing bus itself. Turned down, the wiper shorts it to ground.
V1b plate
53 kΩ, V1b’s plate also feeds this network. Here it’s a resistor to ground, its output resistance, about 53 kΩ.

Source and load

Source: 60 kΩ, the output resistance of the stage before it on the bench at 1 kHz. Load: 610 kΩ with 27 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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