Dale H. Cook wrote:
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

Right. The emission tester isn't testing the gain of the device or the
transconductance... all it is testing is how effective the cathode is
at emitting electrons.

How effective that is has to do with the surface area of the cathode, with
the temperature of the cathode, and with the composition of the cathode.
But you can be reasonably sure that most tubes of a same general technology
will have the same general emission. So if you pull some generic octal
tube or some generic miniature 9-pin tube, you can make a pretty good guess
what the emission is going to be.

Those testers are basically useless, though, since all they do is detect
one sort of tube failure, they can't detect any of the others. The ones
you used to see in supermarkets and drug stores tended to be calibrated
such that new tubes would test marginal, also, in an attempt to increase
sales...
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."