Plate Curves & Load-Line Analyzer ⭐

What it's for

Design a complete valve stage on one screen — set an operating point, draw the load line, and read back gain, clean power and where the stage clips first.

You enter

• Tube — pick from the built-in database.
• Circuit — single-ended (SE), push-pull (PP) or PSE.
• Operation — triode, pentode or ultralinear (with UL tap %).
• Load — OPT or plate resistor, with load impedance Zp (or Zpp).
• Bias — self (cathode) or fixed.
• Operating point — B+ supply, then Set Grid Bias (Vg) or Set Current (Ia).
• Signal — Vin peak (optional; drives the waveform and clipping display).

You get

• Anode characteristic curves with the load line, the Pa-max limit and the Q-point (independent Ia / Vp zoom).
• Operating point: plate current & voltage, grid voltage, screen voltage, anode dissipation.
• Bias: self-bias Rk and its dissipation.
• AC performance: gain (× and dB), Vout peak, max clean Vin, max clean power, efficiency, estimated 2nd harmonic, and which side clips first.
• Input/output waveform with a clipping warning.
• Advanced: coupling cap, cathode bypass Ck, minimum OPT primary inductance, OPT DC resistance, B+ sag, next-stage grid resistor.

Good to know

Cutoff & clipping: the operating point (Q) slides along the load line as signal is applied. One end is cutoff — the tube switches fully off, Ia = 0 and plate voltage peaks. The other end is the grid limit, where the grid nears 0 V and starts drawing current.

Because Q is rarely centred between those two ends, one is reached before the other as you turn the input up — the side with less headroom clips first. That shorter gap sets the max clean Vin and clean power.

If it clips first on the grid side, moving Q toward the centre (rebias, or change B+/load) balances the swing and buys more clean output. Cutoff-side and grid-side clipping also sound different (even vs odd harmonics), so where it clips first is partly a tonal choice.