Tube testing question

[Clearspider]

Besides a tube-tester, is there any possible way to tell if a tube has a lot or a little life left in it?

[Old Tele man]

..."life" in vacuum tubes is typically equated to transconductance value, which gradually diminishes with use (especially heavy overloaded use).

...short of periodically measuring the output power, your EARS are the only way of telling things are declining.

[Clearspider]

Is there a gradual decline in sound quality over the entire life of a tube, or is it a fast ramp-down at a certain point?

[Blue Strat]

Is there a gradual decline in sound quality over the entire life of a tube, or is it a fast ramp-down at a certain point?

It's usually very gradual but can be instantaneous (shorted tube)....and I'm sure there are a few cases where the ramp down is fast.

Nice answer, huh? I don't make the news, I just report it ;)

[Old Tele man]

...well, the "weezel-wording" answer is: "...it's rather gradual until failure, then it's rather sudden."

...but, yes, it's usually a gradual decline, but not 'technically' significant until the transconductance has fallen to "...70%..." of it's original value (per RDH3, Chapter 30, page 243) for power tubes.

...FWIW, I favor Blue Strat's approach: buy a matched QUAD set of power tubes and periodically swap the "working" pair with the "spare" pair...if the volume or tone noticibly changes, the "working" pair has started to decline...if, not, swap them back in as they're still OK...a simple comparison test!


Blue Strat -- Ha,ha! you beat me to it while I was searching for my RDH references!