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.
the first thing which any DIY tube test is to see whether the heater is open (ohmeter), then check to see whether there are any shorts between plate, screen, cathode etc.
to measure transconductance, which is just <em>dI(plate)/dv(grid)</em> -- let's assume that you have a HV power supply like a Heath IP17 -- you can use the following setup from the RCA receiving tube manual in which a 1.5 volt battery is used to "shift" the grid voltage (dV=1.5).