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In Reply to: RE: This might be a dumb question but that's my specialty posted by Permanent Waves on January 19, 2017 at 14:14:59
Tube life is not measured in terms of signal output. I've never noticed reduced output over time with the many tube preamps and power amps I've used over the past forty odd years.
Just for grins, I ran 12AU7s 24/7 in a Manley DAC for well over a year and quite frankly, didn't notice much difference!
As you observed, the 5751 is a low gain version of the 12AX7 - all of the time!
tubes will last a lot longer than you think. I'm running the same set of tubes in my amp and pre-amp for about 5 years now and they sound as good as ever.
Alexa, turn on the tubes... and they glow...
Not exactly. Tube life is like 1/2 a bell curve. At 75% the tube may fall off good readings fast without warning. A 75% tube also can sound a bit soft losing the great sonics of a newer tube. So you would be better with a true 5751 vs a weak 12AX7.
BTW- I do not buy used tubes. I do not know what I am getting. The tube can read good today & fall off to weak in 500 hours use or less.
Thanks for answers guys. I suspected that gain output level was separate from tube life I just didn't know for sure. I became curious because of all the listings I see for low testing tubes on eBay that always had people bidding on them. I couldn't figure out what they want them for and I guess it's still a bit of mystery to me. Just so I'm clear...if a used tube is reported to have 75% life left it should technically still have close to the original designed gain level output still?
I'll throw in some anecdotal evidence of a low testing tube/s still functioning . I bought some Genelex or MOV 12AU7s that came with the warning of marginal testing. I tested them and found they did indeed test "?" . I popped them into an integrated EL 34 PP amp as the input tubes anyway, and the sound was.....magnificent.
"if a used tube is reported to have 75% life left it should technically still have close to the original designed gain level output still?"
A P225/50R16 tire will always be a P225/50R16 tire, even when it is flat. A 12AX7 will always have a mu of 100, even when it is weak.
Tube longevity is a function of many factors and top of the list is heater voltage and heat. So, if tube life is a concern, check that the heater voltage and current consumption is within specifications and that the tube is not being over heated in its cabinet.
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