About the use of PTE files
The best tube tester ever made, to my opinion is the SOFIA. When making a tube test, this tester always generates a PTE file first. This is a Sofia formatted file, with all data inside, needed to produce the tube curves. The program you need for this is freeware, but it runs under DOS only. The input data for the program is the PTE file, which you can make yourself with a SOFIA tube tester, or otherwise download it from the internet. The advantage of this software is: First, the tube curves will appear scaleable. So you can zoom in or out, by changing the horizontal and vertical axis scales. Second, there is a mouse moveable marker, which you can set at YOUR desired operating point, and read all tube data from that point. Like, Gm, Rp, gain, Plate dissipation.
The PTE file for the GM57 tube is here
This is how such a tube curve looks like. Only this is a screen shot, with the marker at 93mA / 396Volt. When you have it live, you can shift the marker where you want. That will give you great insight about which operating points are not good, as such will have values for gain and Gm, very dependant of small moves of the marker, which means: distortion. So when you move the marker with the mouse along the load line you have in mind, you can already see by the stability of the dynamic parameters, if this is a good load line, and how far you can move along this load line before it gets unpleasant.
Mmm... lets say if you never missed this feature.... be happy as you are. But if you were never sure what load line is a good one.... you should try this software.
As said before, you do need a DOS capable PC, and some experience with DOS. I run the DOSBOX "Megabuild 6" version under Windows 7, it that works fine. It is what I printe the curves with, as you see below
A detailed instruction is here: http://www.emissionlabs.com/datasheets/SOFIA/SOFIA-PTE-FILES.htm
GM75 tube curves, made with the SOFIA tube tester.