Transconductance/Tube testing?

[jbutler]

Someone asked me a question I couldn't answer and didn't make complete sense to me about testing NOS/Vintage tubes intended for use as replacements of other vintage tubes: whether it's better to test mutual conductance or plate current?

I've never really gotten too far into tube theory, but as I understand it, mutual conductance (transconductance.. the opposite of resistance: Mhos) of a tube is a ratio measure of how the tube controls the plate current, so if all else is equal, if the mutual conductance is the same as the old tube, the plate current with the tube replacing it should be the same (Unless of course there's something else wrong with one of the tubes.)

So my assumption has always been that if one can test the transconductance of the tube, it's the better measure of the tubes overall integrity, while testing the plate current would be how you'd go if you couldn't actually test the transconductance.

Am I missing something here? Am I totally wrong about all of this? As I understand it, plate current is a function of transconductance. This hasn't really concerned me until now since I stick with quality vintage reissue matched tubes, but the person who asked, while not a tech himself (yet) has far more discerning ears than most people ever will and insists on NOS/Vintage replacements. Bottom line: when matching tubes for replacement, which is the better measure?

So if any of you tube "Gurus" could give me a concise, "For Dummies" explanation, or links to one, I'd really appreciate it.

Thanks.