primer on buying vintage tubes?

In the days before solid state, electronics devices needed tubes to perform their various functions. There were many factories around the world manufacturing tubes, the quality was high, and the sonic signature of each brand was different. Germany, Holland, England, USA were very high quality tubes due to the fact that the military was a main purchaser for their electronics.

Now to modern times, there are very few manufacturers of tubes left, mostly Russia and former Soviet bloc countries (who have always made tubes for their military), and the newcomer for worldwide distribution; China.

That is why audiophiles in general prefer NOS for certain applications. NOS = New Old Stock, but don't start thinking these are brand spanking new tubes sitting on a shelf since 1940 or 50. It just means they had minimal use and were put back in the box for resale. You should only buy from reputable venders, and that usually comes from "word of mouth," who measure the specs of each tube in their inventory. These tubes usually have high quality readings and a trusted vender will have a warranty (usually 30 days to try the tube).

Margot, the most common way a tube goes bad is to die a natural death. It happens over time, and you'll notice that the highs in your system aren't as bright or the music isn't as dynamic as it used to be.
Very often, the tell-tale signs of a tube gone bad are; the channel is dead or the sound is cutting in and out. All of a sudden there might be hissing or crackling coming thru one of the channels.

If a tube has become microphonic, it means the tube is unstable and vibrating; you may hear unusual overtones in the music.