Avo Valve Tester

[Sparks]

I am just following the instructions from the book. It tells where to put the anode & screen switches for the correct voltages for the valve to be tested, it tells where to put the mA/V control. There is nothing else to do. Why do they put mA/V across the top of the scale? I still don't know. I really need someone who is familiar with this type of tester to explain it simply!
Bob