[Hallicrafters] When do you replace tubes?

[Waldo Magnuson]

Hi,
    My question is when do you replace tubes in Hallicrafters receivers. 
  I use a Hickok 6000A tube tester which is a dynamic mutual conductance 
and reads out Gm.  As an example in a SX-100 receiver I'm working on I 
read some of the following measurements:  For a 6AU6 (book value) 2300 
(measured) 1475 or 64%, For a 6C4 (book) 2200 (meas.)1700 or 77%, And 
for a 6SC7 (book) 1000 (meas.) 350 & 200 or 35% & 20%, and so forth.
    My inclination is to replace any tube below 75% Gm reading of a new 
tube.  What are other opinions?  Thanks.
73,  Skip Magnuson  W7WGM