Furthermore, if we try to average the two lines together to find the line that would represent an intermediate line (in this case -40v), we find that the derived line does not match the actual curve-traced line for that grid voltage, particularly at the cutoff region. At best, our averaged lines will match the curve-traced lines across a narrow current band. How to resolve this?  One recent approach has been simply to sum two differently-valued Child's Three-Halves Power Formulas. The formulas have differing K (perveance), mu, and n (exponent) values. This makes for a set of curves that at least looks like a triode. Unfortunately, when viewed more closely it doesn't look like any particular triode. It, too, will accurately represent some portions of some triode's actual plate curves (particularly at low and high voltages), but this still falls short of truly fingerprinting a tube. 

Enter True Curves™. 
GlassWare is proud to introduce a new model we call True Curves™.  Based on a formula derived by John Broskie, it allows the modeling of triode plate curves to an unprecedented level of accuracy.  An example is provided below of how close True Curves comes to matching an actual tube tracer plot of a triode-connected 6550 (a tube not nearly as linear as the famed 300B). 

Having an accurate mathematical model of a type of triode is like having that triode hooked up to a virtual curve tracer.  With such a model of a triode's functioning, many options become available.  For example, grid line step increments of -1, -2, -3, -5,-10, -20, or -87 volts are just a mouse click away, as are changes in the maximum displayed current or voltage on the graph.  In other words, you are not limited by the curves the paper tube manual held; you can define the graph you want see. Furthermore, all the calculations of bias voltages or positive grid voltage points or distortion analysis are much more accurate and dependable because of the true rendering of the triode's curves.

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