Gibson GA-77 Vanguard 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 Gibson GA-77 Vanguard, 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 audio. Its wiper feeds the driver’s grid. Up moves it toward the 50 pF, so more treble.
- Bass
- Bass, 1 MΩ audio, from the bottom of the treble pot to ground, its wiper on the 10 kΩ. Up grounds the bottom of the stack. Down ties it to the treble pot, and the two .02 µF with the 10 kΩ cut the bass.
Parts
- 50 pF
- Treble cap, 50 pF (.00005 µF on the drawing), into the top of the treble pot.
- 100 kΩ
- From the follower to the junction of the two .02 µF. With them it carries the mids and lows to the bottom of the treble pot.
- 20 nF
- From the bottom of the treble pot to the 100 kΩ.
- 20 nF
- From the 100 kΩ down to the bass pot’s wiper and the 10 kΩ.
- 10 kΩ
- The bottom of the stack, 10 kΩ to ground.
- Treble 1M
- 1 MΩ, Treble, 1 MΩ audio. Its wiper feeds the driver’s grid. Up moves it toward the 50 pF, so more treble.
- Bass 1M
- 1 MΩ, Bass, 1 MΩ audio, from the bottom of the treble pot to ground, its wiper on the 10 kΩ. Up grounds the bottom of the stack. Down ties it to the treble pot, and the two .02 µF with the 10 kΩ cut the bass.
Source and load
Source: 2.1 kΩ, the output resistance of the stage before it on the bench at 1 kHz. Load: 2 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-77 Vanguard 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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