Trainwreck Rocket 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 AC30 Top Boost network on its own channel, driven by a cathode follower. The treble wiper goes through 220 kΩ into the phase inverter, with no normal channel mixed in.
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 1 MΩ pot fed through 50 pF, as on the AC30.
- Bass
- A 1 MΩ pot from the bottom of the treble pot to ground, its wiper on the 22 nF mid cap and the fixed 10 kΩ, as on the AC30.
Parts
- 50 pF
- Treble cap, 50 pF, as on the AC30.
- 100 kΩ
- Slope resistor.
- 22 nF
- Bass cap, to the treble pot’s bottom and the bass pot’s top.
- 22 nF
- Mid cap, .022 µF, to the bass pot’s wiper and the 10 kΩ.
- Treble
- 1 MΩ, Treble pot, 1 MΩ audio. Its wiper is the stack’s output.
- Bass
- 1 MΩ, Bass pot, 1 MΩ audio. Top on the treble pot’s bottom, wiper on the mid cap and the 10 kΩ, far end on ground. Turning it up puts more track between the treble pot and the wiper, for more bass.
- 10 kΩ
- From the bass pot’s wiper to ground, 10 kΩ, as on the AC30.
- 220 kΩ
- In series from the treble pot’s wiper to the inverter. With the inverter’s 1 MΩ grid resistor it passes about four fifths of the signal, and it keeps the stack from being loaded by the inverter.
Source and load
Driven by: cathode follower, 56 kΩ cathode resistor. Driving: phase inverter, 2 MΩ. Source: the cathode follower, 56 kΩ to ground, its plate on B+4. Load: 220 kΩ in series, then the inverter’s grid, 2 MΩ and 60 pF as modeled on the bench. The Cut control is in the power amp, not this stack.
Sources
- Mark Abbott, original drawing of the Rocket circuit, from notes on cloned amps
- Matt Taylor, redrawn Rocket schematic, Version A0 (2004, updated 2008)
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 single-channel top-boost 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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