When a tube is tested, the tester usually has you set up the tester for a standard bias (by setting all the controls as stated in the setup for the tester). This will only tell you if the tube had emission or not. Others, like most of the Hickock testers tell you what the mutual conductance should be for that particular tube. If the bias control on the tube is lowered to the range specified on the chart, and the tube does not have meet the mutual conductance as a minimum, then it's performance will be less than the purpose of its design. Most manufacturers will play with bias to set the tubes class of operation, where is operated on it's particular design curve, etc. With my tube collection, if it doesn't meet the minimum mutual conductance, I throw it out.

Donnie

:I am replacing Tubes on a 1960 RCA stereo, the unit has a total of 14 tubes for the power supply and amplifier. As I test the individual tubes with a tube tester, I am finding the variations one would expect after 45+ years.
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:I have tubes that have tested "GOOD" at 65% and others at 85%. Also, only 2 that tested "BAD" at 25%-35%
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:So, one would simply conclude that any test results showing in the tester Green Scale as "GOOD" is acceptable. But we all know any tube less than 100% is showing signs of age and possible performance.
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:I/m looking for feedback from the experts, what level should be the lower threshold of acceptability. Should tubes below 65% be replaced and those above installed ?
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:I want to have some level of confidence after replacing all the capicators and bad/questionable Tubes this unit should work correctly as expected.
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:So, what is "GOOD" enough ?
:Comments ?