Valve
Circuit
How it works
This calculator builds the complete small-signal model of a common-cathode stage and evaluates it as a complex transfer function at 260 frequencies. Three mechanisms shape the curve:
1 · Cathode bypass shelf
Below the bypass corner, Ck stops shorting Rk and local degeneration returns. The valve is modelled as a Thévenin source µ·Vg in series with rp, so the cathode impedance Zk appears multiplied by (µ+1):
2 · Coupling capacitor high-pass
Cc and the next stage's grid leak RL form a first-order high-pass filter, driven from the stage's output impedance (Ra ∥ rp):
3 · Miller effect roll-off
The tiny grid–plate capacitance is multiplied by the gain of the stage. Together with the source impedance (previous stage output + grid stopper) it forms a low-pass filter. High-µ valves like the 12AX7 suffer most, which is why guitar amps use 68 kΩ grid stoppers as gentle treble taming.
The −3 dB points are read directly from the computed curve, so interactions between the filters are fully accounted for.