Quote:
Originally Posted by Thatvalveguy
Tube testing itself is not an accurate buisness, due to cathode interactions. most tubes that have been unused for some time will change their readings in the time following the first test, this is due to the composition of the barium surface layer changing.
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I guess that it is quite common for some new (old stock) valves to "burn" in appreciably over the first few hours of use. I once burned in an ECC81 over ~24 hours on the 163 as I wanted to use it as a standardised valve. I was a bit surprised to see that one of the two triodes changed appreciably more than the other during that process, highlighting the fact that that way they were made gave rise to significant variations, even with two identical valves in the same envelope. I recently tested an Acorn valve (out of its WWII box) and the Ia on that cycled up and down (by a small but visible amount) for several hours before settling down.
When you start looking at the papers which were published (not the mention the papers that were never published), the enormous effort put into developing and refining valves in the last century is something we no longer appreciate. You could probably fill a modest library with that stuff.
B