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Lifespan of NOS telefunken 12AX7?

Hello,
I just purchased a quad of NOS telefunken smoothplate 12AX7's from brent jesse. I have seen some 12AX7 tubes rated for up to 10,000 hours. These have been some medical grade tubes such as the mullard 10M series. I have some mullard longplate 12AX7's and 12AU7's that I have been using on a regular basis for the past two years. I would say they are getting close to 1200 hours on them. They are still going strong and sound good. I was just wondering what the general lifespan of a standard telefunken 12AX7 tube was. I am using them in a rogue ares phono stage. How many hours would you guess they should last. I understand there is no set number of hours but, I am interested in knowing a tubes useful lifespan. I tried to find some info on google but, I couldn't really find anything that stated a lifespan of standard telefunken tubes. Thanks in advance.

Andy
andyprice44
rrog
1,958 posts
 
"Alan- that is quite a surprise. I didn't think a tube could last that long. Interesting."

Audio Research rated the tubes in their hybrid preamps to last 10,000 hours. However, I have never seen a tube manufacturer give their tubes a lifespan. It really depends on the application. That same tube that lasts 10,000 hours in the Audio Research preamp may only last 500 in another preamp.
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andyprice44 OP
133 posts
 
thanks for the info Rrog and elizabeth.

I might call rogue audio on monday just to ask them some questions about the circuit design on the ares phono I am using. Maybe they can tell me about how much stress the tubes will be under.

Andy
lewm
13,764 posts
 
As Rrog and Elizabeth inferred, it would depend upon the plate voltage (voltage difference between anode and cathode) and DC current being run through the tube. The product of those two parameters is "plate dissipation" in Watts. Look up a tube data sheet for a 12AX7. It will give you the max numbers for each of those 3 parameters. If the actual plate voltage, plate current, and plate dissipation (information which you would have to obtain from the manufacturer, if you don't know how to measure) are in the midrange of the recommended max values, then the life of the tube will more depend upon sample to sample variations within the tube itself.

I am surprised at the story about the SP14, because Audio Research was notorious for running their tubes very "hard". I guess that applied to the output tubes in their amplifier designs. The old Dual 50 and its variants used to blow output tubes all the time.
sbrown
148 posts
 
Hmm. The SP14 uses 6DJ8's not 12AX7's - could explain longer than normal life.
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