Tone Stacks and EQ

An amp's tone stack is a passive filter network sitting between the preamp and power amp, and the three classic topologies — Fender's mid-scooped bass/treble, Marshall's more present bass/mid/treble, and Vox's single 'cut' control — differ enough in insertion loss and character that swapping one slope resistor is the single most effective tone-stack mod there is. This chapter covers all three, with worked cutoff-frequency examples.

A tone stack is the network of resistors, capacitors, and pots sitting between an amp’s preamp and power amp sections, and unlike the simple single-knob tone control covered in Pots, Caps, and Tone Controls, most amp tone stacks are multi-band, interactive filter networks — turning one knob changes not just that band but how the stack behaves as a whole. Three classic topologies define most of what a guitarist has ever plugged into.

Fender: mid-scooped by design, not by accident

The classic Fender tone stack pairs a bass pot with a low-pass filter capacitor and a treble pot with a high-pass filter capacitor, plus a fixed midrange resistor with no dedicated control at all. The result is a network that boosts bass and treble relative to a comparatively flat, uncontrolled midrange — the “Fender clean” scooped character isn’t a side effect, it’s the direct consequence of giving the player control over the two outer bands and leaving the middle fixed. Modding this stack follows directly from the RC-filter logic in Pots, Caps, and Tone Controls: a larger bass capacitor shifts more bass through at the same pot position, a smaller treble capacitor reduces treble rolloff at the same setting, and the mid resistor’s value sets the stack’s overall insertion loss — how much signal the network eats before it even reaches the power amp.

Marshall: less insertion loss, real midrange control

A Marshall-style stack adds a genuine midrange control alongside bass and treble, and critically uses a smaller slope resistor (commonly around 33kΩ against a Fender-style stack’s 100kΩ), which passes more signal through the network at a given gain setting — the stack itself eats less of the signal. This is why swapping a Fender stack’s slope resistor down toward Marshall spec is one of the most effective, lowest-effort tone-stack mods available: at 1kHz, that single resistor change can drop the stack’s insertion loss from roughly 18dB down toward 12dB, making the amp sound noticeably brighter and more present at the same gain and volume settings, without touching a gain stage at all.

Vox: one knob, one filter, no scoop to fight

The Vox “cut” control strips the concept down to its simplest form — a single pot and a single capacitor to ground, forming one low-pass filter with no separate bass or treble control at all. Turning the cut pot up increases how much high frequency gets shunted to ground, so the control only ever darkens the amp, never brightens or scoops it — which is exactly why classic Vox amps have a simpler, more consistent tonal character across their control range than a three-band Fender or Marshall stack does. Working the math: at maximum cut with a 1MΩ pot and a 500pF capacitor, the low-pass cutoff lands around 318Hz — well inside the guitar’s range, meaning the cut control at full rolls off nearly everything that matters. Swap to a 1000pF cap and that cutoff drops to around 159Hz, an even more aggressive darkening at the same knob position.

Why one resistor swap can be the single most effective mod on the list

Because a tone stack’s insertion loss compounds with everything happening in the gain stages around it, the mod that changes a stack’s fundamental character most efficiently isn’t always a capacitor value tweak — it’s the slope resistor, because that one component sets how much of the signal the entire network discards before the player even touches a tone knob. Converting a Fender-style stack toward Marshall spec by dropping the slope resistor from 100kΩ to 33kΩ (and optionally adding a midrange pot in place of the fixed resistor) changes the amp’s fundamental brightness and presence more directly than adjusting any single bass or treble capacitor value would.

Common mistake: treating tone stack pots as independent controls

Because a passive tone stack’s three bands share resistive and capacitive paths, the bass, mid, and treble controls interact — turning up the bass control on a Marshall-style stack doesn’t just add bass, it also changes how the treble control behaves at other settings, because the two paths aren’t electrically isolated from each other. Dialing in a tone stack by adjusting one control at a time to a target sound and assuming the others will hold steady is a losing strategy; the stack has to be tuned as an interacting system, the same caution that applies to any multi-band passive filter network sharing components. This interaction is a genuine circuit behavior, not a coincidence of a specific amp — it shows up in essentially every multi-band passive tone stack topology, and it compounds with the gain-stage interactions covered in Preamp Gain Structure rather than existing independently of them.