Testing Vacuum Tubes

"NOS" test?

above for display purposes only
(not relevant to the article)

Just for the sake or understanding, when testing a vacuum tube, there is no such thing as "new tube, near NOS or NOS" reading. I know some who sell tubes on eBay and other venues use such terms but they really don't know what they are talking about.

There is much to say about vacuum tube quality and testing but without going deep into theory & technical details, here's what I have to say in simple terms:

One should always check for shorts and internal element leakage before evaluating the quality of a tube. If a  short or serious leakage is present, discard the tube and DO NOT PROCEED FURTHER (or serious damage to the tube tester may result)! When it comes to evaluating a tube's quality. The way to do so is to measure the Gm (for tubes other than diodes and voltage regulators) on a tube tester that designed to measure such). An additional test (significant in some circuits) is a test for gas.

New tubes (just like used) ones will vary considerably in their Mutual conductance (measured in micromhos) or Gm (for short). Test results will vary depending on the  type and brand of tube checker. Emission type checkers will  only measure  emission and this reading is very unreliable and will vary significantly with various tube tester models. When testing rectifiers and diodes there's no such thing as a Gm test. Diodes & rectifiers make poor amplifiers.

So with this in mind, when a seller states a tube's quality in terms of; "like new", "NOS" or "near NOS" (or in terms other than "Gm") is not a knowledgeable seller. I'm not saying they are dishonest, but I'm stating that there is a lack of understanding of their product or the proper way to evaluate same.

Standards were set up long ago as to what the minimum acceptable tube test readings should be (and the readings will vary with brands and models). Some tube testers will list the average, others; the minimum acceptable or expected readings. These readings are explained and specified in the testing data for the particular model tube tester in their manual.

I use a Hickok model 539C, considered buy many to be one of the most accurate and desirable testers available (sans a laboratory model) the posted values on it's charts are; minimum acceptable.

Here's an example:

The minimum acceptable reading for a #27 tube is 630 Gm. If you have one that tests 1100 Gm, that's  well above the acceptable reading and is about as good as this particular tube will measure (NOS or otherwise). A tube testing a bit below the minimum (or expected value) is not necessarily a candidate fro the trash can, NO! it may work just fine in many circuits, so don't throw it away.

Tube Testing Info: