Dumble Overdrive Special #124 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.

Stack and the source and load

The Middle, Treble, and Bass controls of the Dumble Overdrive Special #124 (1984), 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

Frequency response

Every tone setting Part tolerance

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

Note Chord

Guitar

Match loudness Speaker

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

Middle: Middle, 100 kΩ as a variable resistor to ground (its top lug isn’t connected). Turned up, its resistance rises and less of the mids go to ground. Audio taper is typical, not on the drawing.
Treble: Treble, 250 kΩ. Its wiper is the stack’s output, the top of Volume. Audio taper is typical, not on the drawing.
Bass: Bass, 250 kΩ. On Rock with the PAB relay at rest its wiper is tied to its top, so it’s a variable resistor. Audio taper is typical, and 500 kΩ audio after 1988.

Parts

330 pF: Treble cap, from V1b’s plate to the top of Treble. 330 pF in parentheses on the drawing, .002 µF after 1988.
100 kΩ: Slope resistor, from V1b’s plate to the bass and middle caps. 100 kΩ in parentheses on the drawing, 150 kΩ after 1988.
100 nF: Bass cap, to the top of Bass.
50 nF: Middle cap, to Middle’s wiper. .05 µF in parentheses on the drawing, .01 µF after 1988.
Middle: 100 kΩ, Middle, 100 kΩ as a variable resistor to ground (its top lug isn’t connected). Turned up, its resistance rises and less of the mids go to ground. Audio taper is typical, not on the drawing.
Treble: 250 kΩ, Treble, 250 kΩ. Its wiper is the stack’s output, the top of Volume. Audio taper is typical, not on the drawing.
Bass: 250 kΩ, Bass, 250 kΩ. On Rock with the PAB relay at rest its wiper is tied to its top, so it’s a variable resistor. Audio taper is typical, and 500 kΩ audio after 1988.
1.8 kΩ: Under Bass, to ground. With Bass at 0 the stack still has 1.8 kΩ at the bottom. 10 kΩ after 1988.
22 MΩ: Across the PAB relay’s first contact, from Treble’s bottom to the Rock/Jazz switch. It keeps the open contact at the same DC level, so the relay doesn’t pop.
22 MΩ: Across the PAB relay’s second contact, from Bass’s wiper to the Rock/Jazz switch. It keeps the open contact at the same DC level, so the relay doesn’t pop.
270 kΩ: Deep network, from the Rock/Jazz pole (Bass’s top on Rock). With Deep off its far end hangs open.

Source and load

Source: 40 kΩ, the output resistance of the stage before it on the bench at 1 kHz. Load: 990 kΩ, 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 Dumble Overdrive Special #124 (1984) 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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