CT-100 on eBay

[old_tv_nut]

The theoretical relationship of gun current to drive voltage is a power law with a particular exponent, which has the same shape even if the three guns are driven more or less hard. The need for a series resistor indicates that the curve of light output is not exactly a power law, because the resistor will change the shape of the curve and make it more linear. This could be either an effect of the gun curve itself, or a non-linear effect of the phosphor. I think it is the gun, as phosphor effects usually consist of a loss of efficiency at high currents.
There is also some difference between grid drive (used for RGB in the CT-100) and cathode drive of the luminance (used in later sets), but apparently the later sets also found it useful to put a resistor in one or two of the cathodes even though they used cathode drive.