Knowing what type tube you are looking for would help. Tvad is dead on with the signal tubes. The older ones (generally made in the 1950s) are superior sounding. The only new production signal tube I have any experience with that sounds pretty good in my preamps is the Groove tube Mullard remake 12ax7m, these are available from several sources. When buying older signal tubes I would look for a seller who has excellent feedback and who can give you test results from a mutual conductance test and not just the standard line "tested good for shorts and emissions on my xyz emissions tester". IMHO long black plates and D or square getters are positives to look for also.
