Sola Sound Tone Bender Mk III 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 Tone control of the Sola Sound Tone Bender Mk III, 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
- Tone
- A 100 kΩ linear pot between the two paths. Clockwise is the full path, so it’s darker, counterclockwise the treble path.
Parts
- R6
- 18 kΩ, Q3’s collector load, to the divider node.
- R7
- 10 kΩ, Top of Q3’s supply divider. The kit has 1 kΩ and a 50 kΩ trimmer, set to about 10 kΩ. The model uses one fixed 10 kΩ.
- R8
- 10 kΩ, Bottom of Q3’s supply divider.
- C5
- 100 nF, From the divider node to the Tone pot’s bass end. The node carries the collector’s whole signal, divided down.
- C6
- 2.2 nF, From Q3’s collector to the Tone pot’s treble end.
- R9
- 10 kΩ, From the treble end to ground. With C6 it passes only the highs, above about 7 kHz.
- Tone
- 100 kΩ, A 100 kΩ linear pot between the two paths. Clockwise is the full path, so it’s darker, counterclockwise the treble path.
- R10
- 220 kΩ, Series resistor from the Tone wiper into the Level pot.
Source and load
Source: 580 kΩ, the output resistance of the stage before it on the bench at 1 kHz. Load: 98 kΩ with 40 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 Sola Sound Tone Bender Mk III on the bench, so the two always match.
How a tone stack works
Two ways through. In the FMV stack the treble cap is small, so it passes only the highs, straight to the top of the treble pot. Everything else goes through the slope resistor, then the bass and mid caps. The bass cap feeds the bottom of the treble pot, and the bass pot sits between there and the top of the mid pot. Turned down, it shorts the bass cap’s end toward the mid pot, and the lows drop away. The mid pot sits between the mid cap and ground, so turning it up lifts the whole lower path off ground. The output is the treble pot’s wiper, a mix of the highs at the top of its track and the lows and mids at the bottom.
The mid scoop. Where a stack passes the bass and the treble by different paths, there’s a stretch in between where neither path passes much. That dip in the mids is the scoop. Its depth and center move with the knobs. In the FMV stack the slope resistor and treble cap set where it sits, and the mid knob fills it in. A plain low-pass or high-pass filter has no scoop.
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. For one resistor and one cap, the corner calculator finds the corner from the parts, or the parts from the corner.
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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