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If this is stupid question and waste of time I apologize but I am kind of new to the world of vacuum tubes in the last year. My question is if there is a 12ax7 tube that is used and tests low with say approximately 75% of tube life left would that tube then be comparable to a 5751 tube that is testing new? If answer is no could someone explain to me why? Another way to ask this is when people use the 5751 tube to tame some gain in place of the 12ax7 in say the v1 position of a guitar amp what would the effect be if the low testing imaginary 12ax7 tube was used instead of a new 5751 or new fresh 12ax7?
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Tube tester readings don't always correlate with amount of usage or remaining life - a tube reading 80% COULD last longer than one reading 100%. And tube testers don't read amplification factor - they read cathode emission or transconductance, which is CURRENT amplification, not VOLTAGE. A 12AU7 (mu=20) will peg the meter if tested as a 12AX7 (mu=100) - it has twice the transconductance and higher cathode emission. Actual performance will depend on circuit - many weak 12AX7s will operated as well as new ones if the circuit is very low current - as most voltage amplifier stages are. The same weak 12AX7 may perform very poorly in a driver stage with more current.
If you are thinking that a tube that measures 75% (and 100% is new) has only 75% of the gain of a new tube that is not exactly true. It seems you think that because you reference the 5751 which is about 3/4 of the amplification factor as the 12ax7. The ability of the tube to supply the specified output lies within a range of operating conditions. Therefore when the tube measures in the "good" range, usually around 70% and up it is fully expected to satisfy those conditions. Cheers, Dak
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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