WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
here's what I intend to do. Critique please before I kill myself... breadboard the pentode strapped as a triode with a suitable set of resistances at the anode load, output, grid and cathode. Have a variable HT supply, and variable grid supply. Twiddle the supplies and plot cathode current against grid voltage and HT voltage/anode voltage (staying within the manufacturers pentode specs of course...). I figure two cheap DVMs a variable low voltage supply (for hte grid) and a variac for the HT. Anything else?
No cathode or plate resistors are required. Use a grid stopper right at the control grid pin on the socket.
Use a fixed grid voltage and vary the plate voltage in small increments, and measure the resulting plate current at as many points as you can handle until the curve has constant slope or close to it. This will generate the familiar family of curves you see in most data books.
You can also use fixed plate voltage for each curve, vary the grid voltage in small increments, and measure the resulting plate current. The change in plate current compared to the change in grid voltage is the transconductance of the tube between those points. This is usually expressed in mA/V which is also notated as a Siemen or also equivalent to 1000uMhos per mA/V. uMhos are what you will see on most older tube testers as well as in US data books. Euro and Russian databooks more often use mA/V or Siemens. (Note that this relationship is not fixed, but very linear types will have near constant slope in their linear regions and transconductance will not vary much between equidistant points.) Do for full range of plate voltages expected. (i.e close to the plate voltage you expect at cut off, and saturated.) Space the plate voltages 10 or 25V apart depending on how the first few curves look.
Incidentally you can use excel or the spreadsheet in open office to create the various characteristic curves.
Hopefully I have explained it clearly and not screwed something up..