Gibson GA-50T 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 Volume (instrument), Volume (mic), and Treble controls of the Gibson GA-50T, 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
- Volume (instrument)
- Instrument volume, .5 MΩ. The signal comes in on the wiper and leaves from the top end, which is the bus. Turned down, the wiper shorts the signal to ground. The drawing marks no taper, so audio is assumed.
- Volume (mic)
- Mic volume, .5 MΩ, wired like the instrument volume. The instruction sheet says to turn it all the way off when the mic jack isn’t in use. The drawing marks no taper, so audio is assumed.
- Treble
- Treble, 1 MΩ. Its wiper feeds the grid through .001 µF. Turned up, the wiper moves to the cap from the bus, so treble goes around the 470 kΩ. Turned down, the wiper moves to ground and the second cap bleeds the grid’s treble away. The drawing marks no taper or direction, so audio is assumed, up brighter.
Parts
- 10 nF
- Coupling cap to the instrument volume’s wiper. At .01 µF into the volume pot it cuts some deep bass, and most of what’s left of the tremolo’s slow swing.
- Vol .5M
- 500 kΩ, Instrument volume, .5 MΩ. The signal comes in on the wiper and leaves from the top end, which is the bus. Turned down, the wiper shorts the signal to ground. The drawing marks no taper, so audio is assumed.
- 250 pF
- Bright cap, 250 pF from the wiper to the bus. Below full volume it lets the treble around the pot.
- 50 nF
- Coupling cap to the mic volume’s wiper.
- Vol .5M
- 500 kΩ, Mic volume, .5 MΩ, wired like the instrument volume. The instruction sheet says to turn it all the way off when the mic jack isn’t in use. The drawing marks no taper, so audio is assumed.
- 1 MΩ
- From the bus to the inverter’s grid. With the bass network below the grid it makes a divider, so most of the level is lost here.
- 1 nF
- Treble cap, .001 µF from the bus to the treble pot.
- Treble 1M
- 1 MΩ, Treble, 1 MΩ. Its wiper feeds the grid through .001 µF. Turned up, the wiper moves to the cap from the bus, so treble goes around the 470 kΩ. Turned down, the wiper moves to ground and the second cap bleeds the grid’s treble away. The drawing marks no taper or direction, so audio is assumed, up brighter.
- 1 nF
- Treble cap from the pot’s wiper to the grid.
- V1 plate
- 190 kΩ, V1’s plate also feeds this network. Here it’s a resistor to ground, its output resistance, about 190 kΩ.
Source and load
Source: 99 kΩ, the output resistance of the stage before it on the bench at 1 kHz. Load: 180 kΩ with 76 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-50T 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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