Hey All, re earlier (and sometimes NOS tubes), if you dig back into the earlier tube posts you'll find one I put up that talks about where tubes have been and now got to...
There are 2 main differences IMHO between the older tubes (which are now either 2nd hand or NOS (new old stock)). Back then all the tube makers had their own metallurgy departments (so they made all sorts of metal alloy combinations to get the best electrical response (anodes, cathodes, grids etc), but the main difference these days is how long the tubes are pumped (how good is the vacuum). Modern tubes sound fine (the metallurgy is being replicated (largely)), they just don't last anywhere near as long coz they are pumped for 30 mins instead of 2 to 3 hours as they used to be. And of course the manufacturers want them to wear out so we have to buy new ones. There is also a difference in the metallurgy and that also has an affect. MJMP is quite correct when he says the old tubes were better, they were just better made (good metal(s), good pumping, thicker glass), the modern designs are predominantly the same just won't last as long (or in some cases sound as good).