Why is it, if a **28800pF** (28.8nF) ideal capacitor and 4Ω resistor are in series with a voltage source, that source rises linearly to exactly t=20µs?

[![28800pF, can see edges but timing way off](https://i.stack.imgur.com/IVUuV.png)](https://i.stack.imgur.com/IVUuV.png)

For C=**2880pF**, I expect \$5\tau=2880\text{pF}\cdot4\Omega\cdot 5 = 57.7\text{ns}\$ but the chart barely changes:

[![2880pF, can no longer see edges - timing extremely off](https://i.stack.imgur.com/RvYqC.png)](https://i.stack.imgur.com/RvYqC.png)

I tried changing most of the simulator parameters with zero effect - it's like the voltage source has an intrinsic linear ramp-up from 0-20µs. Is there a better way to simulate these very fast transients? Is waiting 20µs and closing a voltage-controlled switch the best answer?