1. OD3 150V gas regulator: The voltage changes 4V as the current changes from 5 to 40 mA.

4V change / 35mA change = 114.3 Ohms of dynamic resistance (not negative resistance).

The 114.3 Ohms is relatively constant over the complete audio range (DC to > 20kHz).

The only negative resistance of a gas voltage regulator is before it first ignites, ignites, (at a voltage much higher than the regulation voltage, and then it ignites and draws current at the regulated voltage).

High voltage, low current; lower voltage, high current = negative resistance.

That negative resistance that occurs between non ignited and ignited states is why a capacitor connected across the gas regulator tube will oscillate (relaxation oscillator).

2. Capacitive Reactance, Xc = 1/(2 x pi x f x C). For 100uF:

At 20 Hz, Xc = 1/(2 x 3.14 x 20 x 100e-6) = 79.6 Ohms.

At 10 Hz, Xc = 159.2 Ohms.

At 5 Hz, Xc = 318 Ohms.

3. Show us your schematic, please.

A gas voltage regulator is not a capacitor.

Gas voltage regulators regulate voltage.

Capacitors Store Charge (which can supply current to the load).

4. Want to regulate a screen voltage, OK, a gas regulator is fine.

Want to power a triode output tube plate current, a gas regulator is Not OK.