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 thier 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 will
show the average, others; the minimum acceptable reading. These
readings are explained and specified in the testing data for the particular
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).
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).