Sorry to be naive, but how do you tell when a tube has come to the end of its life? Does it just stop working, does the sound quality slowly deteriorate, quickly deteriorate?
I'm thinking of buying some used tubes of various lifespans (from Tubeworld) to try out different sounds, but I don't want to pop them in and have the sound slowly degrade w/o knowing what I'm hearing.
Although at times tubes stop working all of a sudden the most likely failure is slowly over time. With a tube tester you can find out how well or poor they are.
It all depends how hard a tube is driven. If an amplifier design is conservative, tubes can last for years before it starts to sound bad. On the other hand, if the amplifier is pushes a tube to it's limit, or beyond, the tube can fail in a short amount of time...
Originally Posted by Uncle Erik /img/forum/go_quote.gif My favorite is when tubes develop that purple haze shortly before failure. It is beautiful, though the consequences aren't.
This is a very important thing to note. Many tubes will exhibit that blue-ish/purple-ish glow when they are in the process of failing - it's indicative of them losing their vacuum.
Sometimes of course they just light up light a light bulb and then croak. I have only had that happen to me once, though.
As for the question of sound, with the exception of obvious distortion, it's very difficult to tell just by sonics. Some people believe tubes sound their best when new, and slowly sound worse over their usable life. Others, however, believe that tubes sound progressively BETTER right up until the point that they truly begin to fail. So while if you hear distortion you should probably change tubes, don;t go changing tubes every time you think you hear a subtle change in the sound.
And the best thing, as Miguel said, is to have a tester. The one thing a tester really is good at doing is identifying if a tube is on its last legs.
Thanks for all the help. It sounds like more art than science (with the exception of using a tester).
Since there isn't a clear diagnostic, and I don't see a tester in my near future, it seems to make more sense spending the little extra money and buying tubes that have little or no use as opposed to those that Tubeworld says "test" at say 65% (at least until I develop a better ear for the subtler changes in sound).
Originally Posted by mrarroyo /img/forum/go_quote.gif 65% is low IMO.
Even better to know!
I assumed usable life to be closer to 100% (before really pronounced sound degradation), with 65% greater than half its life left (assuming ~2-5K hrs total lifespan).
Originally Posted by mrarroyo /img/forum/go_quote.gif Although at times tubes stop working all of a sudden the most likely failure is slowly over time. With a tube tester you can find out how well or poor they are.
is there a specific tube tester you would recommend?
Buying a tube tester, unfortunately, is also a bit of an adventure, since they, too, are vintage electronics. I've bought a total of 5. One worked poorly. One worked OK. Three have worked perfectly, although one of these I bought calibrated, so while it was the most expensive, I knew it was both working and accurate. That one cost me $300. The others I all got for under $100 (on EBay), but of those four, only two were accurate (I compared them to the calibrated one).
If you just want to ensure no problems, try to get a simple emissions tester from someone who can either know how to calibrate them (I do not), or from someone who had it calibrated recently. That's good enough for someone who just wants to use tubes (as opposed to have a business selling them).
Originally Posted by Toad /img/forum/go_quote.gif is there a specific tube tester you would recommend?
They're complicated beasts and different models measure different things. There have been a few threads on them if you look. Another consideration is that there aren't any new ones - old ones often new new guts and a recalibration. And be forewarned - the really good ones are sought after and go for a pretty fair price.
Originally Posted by Skylab /img/forum/go_quote.gif This is a very important thing to note. Many tubes will exhibit that blue-ish/purple-ish glow when they are in the process of failing - it's indicative of them losing their vacuum.
Sometimes of course they just light up light a light bulb and then croak. I have only had that happen to me once, though.
As for the question of sound, with the exception of obvious distortion, it's very difficult to tell just by sonics. Some people believe tubes sound their best when new, and slowly sound worse over their usable life. Others, however, believe that tubes sound progressively BETTER right up until the point that they truly begin to fail. So while if you hear distortion you should probably change tubes, don;t go changing tubes every time you think you hear a subtle change in the sound.
And the best thing, as Miguel said, is to have a tester. The one thing a tester really is good at doing is identifying if a tube is on its last legs.
Sadly, the bluish/purplish tint does not always indicate that a tube is going. I've had some el84's glow blue and test AOK! Ultimately, the tube tester is the best single way to tell if a tube is borderline. Tubes tend to degrade slowly, and your own hearing may acclimate to the worsening sound--if any--complicating the process. Sometimes you just don't know until you experience a terminal "event."
EDIT: BTW, in my experience, the impact of a tube going "out" is generally much less exciting than it's SS counterpart. And should something else go with it (e.g. the bias resistor), it's a simple(r) solder moment.
Originally Posted by Rdr. Seraphim /img/forum/go_quote.gif Sadly, the bluish/purplish tint does not always indicate that a tube is going. I've had some el84's glow blue and test AOK! Ultimately, the tube tester is the best single way to tell if a tube is borderline. Tubes tend to degrade slowly, and your own hearing may acclimate to the worsening sound--if any--complicating the process. Sometimes you just don't know until you experience a terminal "event."
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Very true - I was trying not to confuse the issue
You're right though - the blue glow is not always indicative that the tube is losing vacuum. I also have some EL84's that glow blue and are fine. But if one does not own a tester, it's best to take a tube glowing blue out of service, IMO. If you have a tester then you can check for leaks.
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