[Electronic M]

I see them as a valuable service aid. Aside from HV rectifiers in TVs (which no tester comes close to giving a test under designed operational conditions), some oscillator tubes (the tester can tell how well a tube will amplify but not how well it will oscillate), and some tubes in TV horizontal deflection stages (those circuits can be REALLY finicky about tubes sometimes) it has been my experience that my tester is pretty darn accurate at pointing out tubes that are weak or bad.

Swapping the ones that test really bad for spares usually brings a stage, that has been otherwise recapped and gone through, back to life. If I test tubes before a recap and power up I usually put electrical tape on tubes that test in a range on my tester that I consider the marginal range as a mark so if the stage they are in seems to be not working well enough for my tastes or at all I notice the tube did not test well, and try a strong testing spare in that spot before diving into deeper passive component trouble shooting.

One case where a tube tester can give a semi-false condemnation of a tube is in the case of AA5 radio rectifiers. There are three heater terminals on those tubes one of which is a dial light tap which is tapped nearer to one of the other heater terminals so just enough power is bleed off to the light. If the section of the rectifier's heater that is in parallel with the filament is open then the tube will not light on some testers, and the tube can be considered to be bad by some schools of thought. However if one uses a beefier dial light in the radio and the previously mentioned problem is all that is wrong with the tube then the tube can still work in a radio. I recall reading an article that said that prior to WWII tubes with that defect would be routinely replaced, but once the war started and repair men could not order any more replacement tubes the dial light trick became standard operating procedure for nearly all repair men...Though some tried to fix open heaters through welding...