I have a Lafayette tube tester that works in a very simple manner: it
connects all the tube elements, but the plate, to the cathode. It then
measures the current flowing through the diode formed by the plate on the
one hand and by all the other elements on the other hand. Very simple.

The tube tester has a potentiometer that permits to adjust the current meter
sensitivity.

What is unclear to me is why ther tube tester manual instructs to set that
potentiometer at position "30" (over a 100 scale) for almost all tubes. I
would have expected that each tube requires a different setting (like in the
I-177 tube tester), but it is not so: the magic "30" setting is valid for
almost all tubes, big or small it does not seem to matter.

Can any one please help me to understand the reason for that?

Thanks and 73

Tony I0JX
Rome, Italy