On Wed, 23 Nov 2011 22:47:33 +0100, "Tony I0JX"
wrote:

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.

Your Lafayette is a simple emission tester, which apparently assumes
that most tubes fall in a fairly narrow range of cathode emission
values. The I-177 is a mutual conductance tester whose chart has
settings that cover the wider range of gm values found in tubes

Dale H. Cook, GR / HP Collector, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcity/radios/radtop.html