Passive Filters in Tube Circuits
RC and LC networks for coupling, tone shaping, and frequency selection
Every tube amplifier contains filters — whether intentional or not. The coupling capacitor and grid-leak resistor form a high-pass filter that sets the low-frequency cutoff. The plate load resistor and stray capacitance form a low-pass filter that limits bandwidth. Understanding these inadvertent filters is as important as designing intentional tone controls.
For a typical preamp stage: C_coupling = 22nF with R_grid = 1MΩ gives f_c = 7.2Hz (subsonic, no audible bass loss). But C_coupling = 1nF with the same grid resistor gives f_c = 159Hz — significant bass rolloff. In guitar amps, this coupling cap value is one of the primary tone-shaping tools.
Coupling & Bandwidth Calculator
Explore how coupling caps and stray capacitance define your bandwidth
Tone Stack Frequency Response
Classic Tone Stack Circuits
FMV, Baxandall, James — the DNA of every guitar and hi-fi amp
| Tone Stack | Insertion Loss | Character | Examples |
|---|---|---|---|
| Fender (FMV) | -25 to -30dB | Scooped mids, bright | Twin Reverb, Deluxe Reverb |
| Marshall (FMV) | -18 to -22dB | More midrange, aggressive | JCM800, Plexi |
| Vox (FMV) | -15 to -20dB | Chimey, jangly | AC30, AC15 |
| Baxandall | 0dB (unity) | Flat at noon, HiFi | Quad, Leak, McIntosh |
| James (passive) | -6dB at flat | Gentle shelving | Mullard 5-10, Fisher |
| Big Muff | -10dB | Extreme mid cut | EHX Big Muff Pi |
Designing Tube Audio Filters
1. Coupling Capacitor Selection
For full-range audio: C ≥ 22nF with 1MΩ grid-leak (f_c = 7.2Hz). For guitar: C = 1-10nF intentionally rolls off bass. Capacitor type matters: film caps (Sprague Orange Drop, WIMA MKP) are neutral; silver mica adds a subtle brightness; ceramic disc can introduce distortion at high signal levels. Never use electrolytics in the signal path.
2. The FMV Tone Stack
The Fender-Marshall-Vox (FMV) tone stack is a passive RC network placed between two gain stages. It has 25-30dB of insertion loss at flat — the following gain stage must recover this. The classic Fender values: C_treble = 250pF, C_bass = 100nF, C_mid = 22nF, R_slope = 56kΩ, R_mid = 25kΩ pot. Marshall uses larger C_treble (470pF) for more high-frequency content.
3. Baxandall Active Tone Control
Unlike the FMV stack, the Baxandall circuit uses negative feedback around a gain stage to create shelving EQ with zero insertion loss at the flat position. Bass and treble controls provide boost AND cut (typically ±15dB). This is the standard in hi-fi amplifiers (Quad, Leak, McIntosh). The key advantage: flat frequency response at center position, unlike FMV which always scoops.
Testez vos connaissances
What determines the low-frequency cutoff of a coupling capacitor / grid-leak resistor combination?
References
- Paul Horowitz & Winfield Hill, The Art of Electronics, 3rd ed., Cambridge University Press, 2015. ISBN 978-0521809269Canonical reference for analog design — covers tubes in Ch. 2.4 & Ch. 3.