Valve Operating Modes¶
Overview¶
Swell's sonic versatility comes from multiple controls that define the valve's operating point—bias condition, gain structure, and signal chain position. These interact to shape the valve's contribution to the filter, from transparent enhancement to extreme wave-shaping.
Quick Start Presets¶
Clean, transparent filtering with warmth: - Bias: COLD | Mode: T | Boost: OFF | Gain: Low
Classic tube saturation: - Bias: COLD | Mode: T | Boost: OFF| Gain: High
Aggressive guitar-amp character: - Bias: COLD | Mode: P | Boost: ON | Gain: High
Extreme asymmetric wave-folding: - Bias: HOT | Mode: P | Boost: ON | Gain: High
Switches¶
Bias¶
The bias switch sets the valve's quiescent operating point, fundamentally changing its response to input signals.
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COLD: Valve is biased to place the operating point to provide good signal swing and headroom whilst adding some compression. In Triode mode, the signal swing remains largely linear with subtle softening near the lower extremes — warm character with minimal distortion. In Pentode mode, both hemicycles are subject to asymmetric compression: the pentode kink acts on the positive swing, cutoff proximity compresses the negative. The result is soft limiting that adds density without obvious harmonic distortion.
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HOT: Valve is biased toward higher quiescent current, shifting the operating point upward on the load line. The effect differs significantly between modes:
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In Triode mode, HOT bias moves the operating point away from the cutoff non-linearity, giving both half-cycles more headroom. This produces the most linear response of all four mode combinations — open and transparent across the full signal swing.
- In Pentode mode, the elevated operating point brings the positive grid swing into the A2 region more readily. Grid current through the series grid resistor folds the positive peaks back, while the negative hemicycle compresses toward cutoff — the most aggressive, asymmetric character available. At high resonance and sufficient input level, the grid driver itself saturates — the indicator LED turns red as a reliable visual marker. This is intentional behaviour, not a fault. See Indicator LED for full LED state reference.
HOT bias reduces overall output level, most significantly in Pentode mode. Use the BOOST and GAIN switches together to recover full output (10Vpp).
Mode¶
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T (Triode): The valve is configured in triode mode, which in this low-voltage circuit provides higher gain and more linear transfer characteristics. Saturation produces predominantly even-order harmonics (2nd, 4th, 6th) for warm, musical coloration. Use for mellow tones, subtle enhancement, or when you want resonance with weight and body.
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P (Pentode): The valve operates in its native pentode configuration. The characteristic kink in the pentode transfer curve generates more odd-order harmonics (3rd, 5th, 7th), delivering the aggressive, biting character of classic guitar tube amplifiers. Use for cutting tones, harmonic aggression, or to add edge to resonant sweeps.
Bias × Mode: Combined character¶
Bias and Mode interact — the four combinations each have a distinct sonic character:
| Triode | Pentode | |
|---|---|---|
| COLD | Warm — linear with subtle softening at extremes | Compressed — soft asymmetric limiting on both hemicycles |
| HOT | Clear — most linear of all four combinations | Wild — wavefolding on positive peaks, cutoff compression on negative |
| Triode | Pentode | |
|---|---|---|
| COLD |  |
 |
| HOT |  |
 |
All traces captured at maximum drive level. At moderate levels the operating point stays within the linear region — distortion character remains but with lower amplitude.
Valve-to-valve variation in HOT + Pentode: The intensity of the grid current effect in HOT + Pentode varies between individual valves. Across production units we have observed that some valves exhibit pronounced wavefolding and grid saturation at the positive peak; others compress smoothly without a strong grid current kink. Both behaviours are normal — the valve will still saturate and compress the signal strongly in all cases. This is inherent device variance in components manufactured to Soviet military tolerances decades ago, not a defect. Bartola Instruments does not hand-select valves by this characteristic: the variation is part of what makes every Swell its own instrument.
Gain¶
The primary valve amplification gain control is the Gain switch. This sets the post-valve gain stage independently of the BOOST switch. Overall module gain is the combination of BOOST, GAIN, and the master GAIN knob.
- Low: Standard post-valve gain. Used when cleaner sounds are desired.
- High: Increased post-valve gain. Primarily used in Pentode modes — in HOT + Pentode, use together with BOOST to compensate for the significant output level reduction caused by the elevated operating point and wavefolding behaviour.
Boost¶
The Boost switch reconfigures the valve stage between two gain configurations:
- OFF: Standard gain
- ON: Higher gain. Increases saturation intensity and drives the valve harder into its non-linear region. Primarily used in Pentode modes — in HOT + Pentode, required alongside GAIN High to recover full output level. Heavy compression and/or distortion is expected when using Boost.
Due to its amplification level, expect the noise floor to become audible subject to your Eurorack Case and Power Supply noise.
Gain Knobs¶
There are 2 out of the 5 knobs in Swell which are dedicated to the filter gain settings:
Gain¶
Master level control for the module. Routes input signal gain either into the valve (Pre mode) or into the filter (Post mode), depending on the Pre/Post switch setting.
- 0 (fully CCW): Silent
- 3 o'clock: Maximum clean gain
- Beyond 3 o'clock: Extreme saturation and hard clipping. Output is clipped to Eurorack standard 10Vpp maximum levels.
Drive¶
Pre-valve gain stage that increases input signal level before it hits the valve grid. Higher drive settings push the valve deeper into saturation and—in HOT + Pentode mode—force the grid into A2 operation more readily, intensifying the asymmetric wave-folding effect. Works across all bias and mode settings to add grit, compression, and harmonic complexity.
Sonic Impact: Start at zero for clean tones. Gradually increase to add harmonic saturation. In combination with HOT + Pentode, high Drive settings create extreme asymmetric wave-shaping and grid-current limiting. Experiment with Drive + COLD Pentode for controllable compression, or Drive + HOT Pentode for aggressive folding and distortion.
Chain location (Pre/Post)¶
The Valve switch places the valve either before or after the filter, fundamentally changing what each stage processes and how they interact.
Pre mode¶
The input signal passes through the valve stage first, then into the filter. The valve's output drives the filter input directly, with no intervening gain control — the filter's input transconductance cell is driven by whatever the valve produces, making the filter overdrive character a function of valve operating point and drive level rather than a user-adjustable parameter.
The FM feedback path is unaffected by the valve in this mode: the normalization loop runs from the filter output back to the fm input and cutoff — the valve is upstream and outside this loop.
Overall system gain is lower in Pre mode, as there is no makeup gain stage between the valve output and the filter input. A saturation and soft-limiting stage at the filter output keeps module output within 10Vpp under all conditions.
The valve generates rich harmonic content; the filter then shapes it. Resonance bites into a complex spectrum, producing thick, evolving sweeps.
Post mode¶
The signal feeds the filter input first. The Gain knob sets the filter input level in this mode, directly controlling how hard the filter's input cell is driven and therefore the degree of filter overdrive.
The filter output feeds directly into the valve's grid driver without any intervening gain adjustment, so the filtered signal drives the valve into saturation. In this configuration the FM normalization loop — filter output → fm input → cutoff — now passes through the valve, meaning the valve's saturation becomes part of the self-modulation character when FM feedback is active. This produces different timbral results from the same FM settings compared to Pre mode.
Resonance peaks can push the valve into saturation dynamically, creating timbral shifts that track the filter cutoff.