Marshall 18 Watt 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.
A single tone control between the first stage and the volume: 5 nF to the volume, 10 nF to one end of a pot with its wiper grounded, and 100 kΩ from the pot’s other end to the volume.
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
- Tone
- A 500 kΩ pot with its wiper grounded. Turned down, it grounds the 10 nF from the plate and cuts treble. Turned up, it lifts that cap and grounds the 100 kΩ instead. That loads the top of the volume pot and lowers the level and bass a bit.
Parts
- C5
- 5 nF, Coupling cap to the volume. 5 nF into the volume and tone network is a high-pass in the low hundreds of hertz.
- C6
- 10 nF, Tone cap from the plate to one end of the tone pot.
- Tone
- 500 kΩ, Tone pot with its wiper grounded. Counterclockwise, the wiper grounds C6 and cuts treble at the plate. Clockwise, it grounds R1 instead, which loads the top of the volume.
- R1
- 100 kΩ, From the tone pot to the top of the volume.
Source and load
Driven by: triode plate, 100 kΩ plate resistor. Driving: volume pot, 500 kΩ. Source: both triodes of V1 in parallel, on one 100 kΩ plate resistor and a shared 820 Ω cathode resistor. Load: the 500 kΩ volume at full, then the phase inverter, whose tail bootstraps its 470 kΩ grid resistor to about 1 MΩ, with about 60 pF across it. Turn the volume down under Source and load to see the 5 nF and 100 kΩ work against it.
Values and wiring are the same as in the paralleled-triode tube amp circuit 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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