Not knowing the history, of the tube, it takes, a true emission tester, that the applied voltage, can be varied, and a working knowledge(experience) to determine, to any truthful degree, that a tube is NOS. Mutual conductance testing, is a great way to match tubes(except power tubes), but is very unreliable, in ascertaining, whether a tube is NOS.@fletchj You probably already know this, since this is a zombie thread that is ancient, but you have the tube tester types backwards in this quote above. Emissions testers make the least reliable test. Mutual Conductance (Transconductance) testers are more reliable.
Tube myths
Many have asked me the same question, "How does the tube sound?" Well, the sound of the tube, is affected by numerous factors, such as circuit interaction, voltage applied to the tube, system setup, use of the tube, etc. All these things, combine, to make the sonics that one's hears, in his system. A bright sounding tube, in a mellow system, will appear to sound neutral. A tube that accentuates, the presence region, in a room with a dip, in that same region, will sound neutral. A neutral sounding tube, in a etched sounding system, will pass the etched sound along, and appear to sound etched. See what I am getting at? It's very hard, to know how a tube will sound, to the end user, in his/her room, and his/her system, based on how it sounds, in yours.
Different tube manufacturers, had a "house sound", whereas the tubes, they manufactured, tended to follow. But to what degree, the end user, will hear this sound, is solely influenced, by the various factors.
Another question frequently asked, "Is the tube NOS?" Manufacturers published a "spec sheet", that listed a set of parameters, that determined, a NOS(New Old Stock) tube. In order to determine if a tube meets this specification, one must determine, the test conditions, and invariably, test for voltage gain(emission). The voltage applied to the tube, must meet the manufacturers specs, to determine, if the emissions, are what the manufacturers specified. The NOS spec, is 99% of the time, lower, than the actual emissions, of a true NOS tube. A true NOS, unused tube, tested, according to the manufacturers spec, will almost always exceed the NOS spec, by approx 10%. A tube with an emission reading of 100%, tested accordingly, usually indicates, some use. However small, it is still used. Not knowing the history, of the tube, it takes, a true emission tester, that the applied voltage, can be varied, and a working knowledge(experience) to determine, to any truthful degree, that a tube is NOS. Mutual conductance testing, is a great way to match tubes(except power tubes), but is very unreliable, in ascertaining, whether a tube is NOS. The cosmetic condition(the way a tube looks) means nothing, as far as to whether it is NOS.
I will periodically, post more info, on the various, tube myths, that are floating around the web. My best advice, is to find a tube dealer, who is willing to stand 100%, behind his products, and knowledgeable, about the product, they are offering.
Different tube manufacturers, had a "house sound", whereas the tubes, they manufactured, tended to follow. But to what degree, the end user, will hear this sound, is solely influenced, by the various factors.
Another question frequently asked, "Is the tube NOS?" Manufacturers published a "spec sheet", that listed a set of parameters, that determined, a NOS(New Old Stock) tube. In order to determine if a tube meets this specification, one must determine, the test conditions, and invariably, test for voltage gain(emission). The voltage applied to the tube, must meet the manufacturers specs, to determine, if the emissions, are what the manufacturers specified. The NOS spec, is 99% of the time, lower, than the actual emissions, of a true NOS tube. A true NOS, unused tube, tested, according to the manufacturers spec, will almost always exceed the NOS spec, by approx 10%. A tube with an emission reading of 100%, tested accordingly, usually indicates, some use. However small, it is still used. Not knowing the history, of the tube, it takes, a true emission tester, that the applied voltage, can be varied, and a working knowledge(experience) to determine, to any truthful degree, that a tube is NOS. Mutual conductance testing, is a great way to match tubes(except power tubes), but is very unreliable, in ascertaining, whether a tube is NOS. The cosmetic condition(the way a tube looks) means nothing, as far as to whether it is NOS.
I will periodically, post more info, on the various, tube myths, that are floating around the web. My best advice, is to find a tube dealer, who is willing to stand 100%, behind his products, and knowledgeable, about the product, they are offering.