Gibson GA-85 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 (I), Treble (I), and Volume (I) controls of the Gibson GA-85, 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 (I)
- Channel I bass, 1.8 MΩ as a variable resistor in the bass path. Turned up, its resistance drops and more of the lows get through. The drawing marks no taper, so reverse audio is assumed: it spreads the change across the knob.
- Treble (I)
- Channel I treble, 2 MΩ linear. The treble path comes in at its top, its bottom goes to ground through .005 µF, and the wiper is the network’s output. Turned up, the wiper moves toward the top and more treble reaches the volume.
- Volume (I)
- Channel I volume, 2 MΩ linear. The signal comes in on the wiper and leaves from the top end through 100 kΩ. Turned down, the wiper moves toward ground and shorts more of it away.
Parts
- 50 nF
- Coupling cap into the tone network.
- 150 kΩ
- Into the bass path. With the .01 µF after it, it lets through only the lows, below about 100 Hz.
- 10 nF
- The bass path’s cap to ground. The drawing hangs it on the bass pot’s wiper, which is looped to this end.
- Bass 1.8M
- 1.8 MΩ, Channel I bass, 1.8 MΩ as a variable resistor in the bass path. Turned up, its resistance drops and more of the lows get through. The drawing marks no taper, so reverse audio is assumed: it spreads the change across the knob.
- 200 kΩ
- The end of the bass path, so the bass pot at full can’t short the treble side.
- 330 pF
- Into the treble path, .00033 µF: only the highs get through.
- Treble 2M
- 2 MΩ, Channel I treble, 2 MΩ linear. The treble path comes in at its top, its bottom goes to ground through .005 µF, and the wiper is the network’s output. Turned up, the wiper moves toward the top and more treble reaches the volume.
- 5 nF
- Under the treble pot. Turned down, the wiper hangs this cap on the output and cuts treble.
- 500 pF
- Across the volume pot’s upper half, .0005 µF. With the volume down it lets the treble past the pot, so the channel gets brighter as it gets quieter.
- Vol 2M
- 2 MΩ, Channel I volume, 2 MΩ linear. The signal comes in on the wiper and leaves from the top end through 100 kΩ. Turned down, the wiper moves toward ground and shorts more of it away.
- 100 kΩ
- Mixing resistor from channel I to the driver’s grid.
Source and load
Source: 81 kΩ, the output resistance of the stage before it on the bench at 1 kHz. Load: 1 MΩ with 60 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-85 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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