Vox AC30/6 Top Boost 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 Top Boost stack: a treble path through 50 pF and a bass path through 22 nF, mixed with the normal channel through 220 kΩ. The bass pot’s wiper sits on a fixed 10 kΩ where a Fender has its middle pot. The Cut control is in the power amp, not this stack.
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 only 50 pF. It passes the top end and very little else, so it covers a narrow band of frequencies but a wide range of level.
- Bass
- A 1 MΩ pot from the bottom of the treble pot to ground, with its wiper on the 22 nF mid cap and the fixed 10 kΩ. There’s no mid control. Up to about 9 the 10 kΩ sets the scoop. At full the pot shorts it, and the mids near 1 kHz drop about 10 dB.
Parts
- CT
- 50 pF, Treble cap. 50 pF in early amps, 47 pF later.
- R15
- 100 kΩ, Slope resistor.
- C7
- 22 nF, Bass cap.
- C8
- 22 nF, Mid cap, to the bass pot’s wiper and the fixed 10 kΩ.
- Treble
- 1 MΩ, Treble pot. Its wiper is the channel’s output.
- Bass
- 1 MΩ, Bass pot. 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. The rest of the track sits across the 10 kΩ. That barely matters up to about 9, but at full it shorts the 10 kΩ and the mids near 1 kHz drop about 10 dB.
- R16
- 10 kΩ, Fixed resistor where a Bassman has its middle pot, from the bass pot’s wiper to ground. 10 kΩ is about a Fender middle control at 4. The bass pot’s lower track sits across it, which barely matters until the bass is near full.
- R20
- 220 kΩ, Mixing resistor from the brilliant channel.
- R19
- 220 kΩ, Mixing resistor from the normal channel.
Source and load
Driven by: cathode follower, 56 kΩ cathode resistor. Driving: phase inverter, 2 MΩ. Load: R20 and R19 make a 220 kΩ to 220 kΩ divider, with the phase inverter on its middle, 2 MΩ and 60 pF as modeled on the bench. R19, the normal channel’s mixing resistor, goes to ground here, as if that channel’s volume were at zero. The Load menu changes what hangs on the divider, not the divider.
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 top-boost 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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