Gibson GA-50 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 control of the Gibson GA-50, with every part as on the bench. The Gibson GA-50T has the same network.
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
- Bass, 1 MΩ as a variable resistor to ground. The cap across it shorts it for treble, so its resistance only adds to the divider for bass. Turned up, its resistance rises and more bass reaches the grid. The drawing marks no taper or direction, so audio is assumed, up more bass.
Parts
- 120 kΩ
- From the grid down to the bass pot. It’s the bottom of the divider when the bass cap shorts the pot.
- Bass 1M
- 1 MΩ, Bass, 1 MΩ as a variable resistor to ground. The cap across it shorts it for treble, so its resistance only adds to the divider for bass. Turned up, its resistance rises and more bass reaches the grid. The drawing marks no taper or direction, so audio is assumed, up more bass.
- 1 nF
- Bass cap across the bass pot. Its value isn’t marked on this drawing. The GA-50T drawing marks .001 µF in the same place, so that value is used.
Source and load
Source: 150 kΩ, the output resistance of the stage before it on the bench at 1 kHz. Load: none. The network hangs on one node, so the source alone sets what it does.
Values, wiring and drawing are taken from the Gibson GA-50 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.
© 2026 Swell Amplification LLC. All rights reserved. The simulator and tools, their circuit models, drawings, presets and text are the property of Swell Amplification LLC and may not be copied, reproduced, redistributed or used to build another product without written permission. The original circuits are historical designs documented in public sources, and no claim is made to them. Everything here is provided as is, without warranty of any kind, for study and personal use. Results are simulations of typical parts, not measurements of any real unit. Don’t rely on them for the safety of any build or repair. Tube amplifiers carry lethal voltages.
Product and company names are trademarks of their owners. They are used here only to identify the circuits these models are based on; this site is not affiliated with or endorsed by any of them.
From Swell Amplification, swellamps.com.