Soldano SLO-100 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 Marshall-type stack after the gain stages, fed by a cathode follower. Both master volumes hang off the treble wiper.
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
- Treble
- A 470 pF treble cap and a 47 kΩ slope resistor. That’s between the values in the JTM45 and the ones in the lead Marshalls.
- Bass
- A 1 MΩ variable resistor into the bass path, through 0.02 µF.
- Middle
- A 25 kΩ divider with the mid cap on its wiper, as in the Marshall stacks.
Parts
- CT
- 470 pF, Treble cap.
- R11
- 47 kΩ, Slope resistor, 47 kΩ. Otherwise it’s the same stack as the 1970s master-volume amp.
- CB
- 20 nF, Bass cap, 0.02 µF.
- CM
- 20 nF, Mid cap, 0.02 µF, on the mid pot’s wiper.
- Treble
- 250 kΩ, Treble pot, 250 kΩ. Its wiper feeds the master.
- Bass
- 1 MΩ, Bass pot, a variable resistor.
- Middle
- 25 kΩ, Mid pot, with the mid cap on the wiper and the bass pot returning to ground through the whole track, as on the SLO-100 drawing. Turned down, it leaves slightly more bass than a pot wired as a variable resistor.
Source and load
Driven by: cathode follower, 100 kΩ cathode resistor. Driving: volume pot, 1 MΩ. Load: the overdrive master (1 MΩ) at full. Behind its wiper sit the normal master (1 MΩ, at full) and the phase inverter’s 1 MΩ grid resistor, 500 kΩ together.
History of this network
People often call this the FMV stack, for Fender, Marshall and Vox, or the TMB stack, for treble, middle and bass. Nobody knows who designed it first. The earliest use we know of is Gibson’s GA-77 amp, whose schematic is dated 1953 to 1954. Its first channel has bass and treble controls.
Fender added a middle control to the Bassman’s stack in 1957. Marshall’s JTM45 of 1962 to 1963 followed the 5F6-A Bassman of 1958 closely, and that’s how the network got to Marshall. The Marshall versions here show how its values changed after that.
Vox’s Top Boost for the AC30 is credited to Dick Denney and was first drawn up formally at JMI in December 1961. It uses the same network with a fixed 10 kΩ where the middle pot would be. The bass pot’s wiper goes to that 10 kΩ and its far end to ground, so at full it shorts the 10 kΩ. Accounts differ on where it came from. One Vox history names the Gibson GA-77, another says Denney worked from a Fender Twin, and others call it the Bassman’s stack without the middle control.
Passive bass and treble networks were common in radio and hi-fi gear by the early 1950s, so the idea may be older than the GA-77. We don’t know of an earlier drawing of it.
Values and wiring are the same as in the five-stage cascaded overdrive 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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