Why don't you get "tone suck" from a long speaker cable?

In a word, impedance.

The source impedance from your instrument/amplifier signal, coupled with the capacitance of the cable, creates a low pass filter.

Any signal at a frequency above the cutoff will be attenuated by 1/3 relative to the input signal voltage (or -3dBV).

Guitar frequencies tend to range from ~ 30Hz-200Hz. Therefore, we want to ensure the cable creates a cutoff frequency above this range to prevent any unwanted signal loss.

The cutoff frequency can be calculated using this formula:

$F_{cutoff} = \normalsize\frac{1}{2\pi RC} $

The average guitar/speaker cable capacitance is ~30 pF/foot
(600 pF @ 20 foot):

$C\_{cable} = 0.000000000600$

• Guitar Cable

The average guitar source signal impedance is very high:

$R\_{guitar} = ~ 10K\\Omega$

$F\_{cutoff} = \\normalsize\\frac{1}{2\\pi R\_{guitar}C\_{cable}}$

$26.5Hz = \\normalsize\\frac{1}{2\\times3.14\\times10,000\\times0.000000000600}$

All signal frequencies above 26.5Hz will be significantly reduced, which is the entire frequency range of the guitar!

• Speaker Cable

The average amplifier source signal impedance is very low.

$R\_{amp} = ~ 8\\Omega$

$F_{cutoff} = \normalsize\frac{1}{2\pi R_{amp}C_{cable}} $

$33MHz = \\normalsize\\frac{1}{2\\times3.14\\times8\\times0.000000000600}$

All signal frequencies above 33MHz will be significantly reduced, which is well outside the frequency range of any instrument (and human hearing!)

You can see then that a long speaker cable (~20ft) will have no "tone sucking" effects on the signal because the signal impedance from the amp is far too low for it to create an effective low pass filter.