Models the stage saturation plugins actually imitate: a single-ended triode preamp running into its curved region. The waveshaper is y = (tanh(K·(x+B)) − tanh(K·B)) / (K·(1−tanh²(K·B))) evaluated per channel with FFmpeg aeval val(ch) expressions — a biased tanh with unity small-signal gain, so quiet material passes clean while loud peaks bend into the curve. The bias B (0–0.4) is the even/odd balance: any asymmetric curve generates even-order harmonics (2nd, 4th — octave-related and consonant) alongside the odd ones, and bias sets how far the operating point sits off center; 0 gives a symmetric tanh with purely odd harmonics, higher values tilt toward the 2nd-harmonic warmth single-ended stages are famous for (push-pull stages cancel exactly these components by design). The curve runs inside a 2× oversampled chain (resample to 96 kHz, shape, back down) so the harmonics the shaper sprouts do not fold back into the audio band as aliasing; a 20 Hz high-pass removes the DC offset the biased curve naturally produces. Drive (1–10) is pre-gain into the curve, compensated after so overall gain stays near unity; a post-shaper tone switch adds a gentle warm or bright shelf; mix blends the shaped signal against the untouched dry path for parallel saturation; output gain trims ±12 dB and a limiter guards the sum. Before/after, an FFT probe measures the harmonic growth in the 300–900 Hz and 8.5–11.5 kHz zones plus RMS/peak/crest-factor deltas — crest factor falling a few dB while RMS holds is the visible fingerprint of saturation.