Vox AC50 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 Normal treble and Normal bass controls of the Vox AC50 (Mk III, 1965), 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
- Normal treble
- Treble, 1 MΩ linear (the drawing marks it). Up brings the wiper toward the 47 pF, for more treble. Its wiper is the channel’s output.
- Normal bass
- Bass, 1 MΩ log. Top on the treble pot’s bottom, wiper on the mid cap and the 10 kΩ, far end on ground, as on the AC30. Up puts more track between the treble pot and the wiper, for more bass. At full it shorts the 10 kΩ and the mids drop.
Parts
- C8
- 47 pF, Treble cap, 47 pF, to the top of the treble pot.
- R10
- 100 kΩ, Slope resistor, 100 kΩ, from the follower to the bass and mid caps.
- C7
- 22 nF, Bass cap, .022 µF, to the bottom of the treble pot and the top of the bass pot.
- C6
- 22 nF, Mid cap, .022 µF, to the bass pot’s wiper and the 10 kΩ.
- Normal treble
- 1 MΩ, Treble, 1 MΩ linear (the drawing marks it). Up brings the wiper toward the 47 pF, for more treble. Its wiper is the channel’s output.
- Normal bass
- 1 MΩ, Bass, 1 MΩ log. Top on the treble pot’s bottom, wiper on the mid cap and the 10 kΩ, far end on ground, as on the AC30. Up puts more track between the treble pot and the wiper, for more bass. At full it shorts the 10 kΩ and the mids drop.
- R11
- 10 kΩ, Fixed 10 kΩ where a Fender stack has its middle pot, so the mids are always scooped slightly.
- R12
- 470 kΩ, Mixing resistor, 470 kΩ, from the normal channel’s treble wiper to the inverter’s grid. The other channel’s 470 kΩ and its tone stack load the grid too.
Source and load
Source: 2.6 kΩ, the output resistance of the stage before it on the bench at 1 kHz. Load: 690 kΩ with 63 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 Vox AC50 (Mk III, 1965) 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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