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In Reply to: RE: I'm thinking no posted by zacster on January 19, 2017 at 16:05:39
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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