Had a 12ax7 in an audio research SP-14 that was on 24/7 for 10 years. Still tested good. Do not know the brand
There has to be a range of useful hours for telefunken tubes somewhere......
I just had my hopes up that someone had seen an hour use figure stated by telefunken on an original box or maybe some manufacturers literature. I understand this varies greatly depending on how the tube is used. How many hours do you guys have on your phono stage tubes?
Alan- that is quite a surprise. I didn't think a tube could last that long. Interesting.
"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.
Agree tube life depends a LOT on the way the circuit is designed.
Some allow the tubes to totally 'loaf' along.
And just as a light bulb used in a dimmer situation will last nearly forever. so will those tubes.
Other tubes are stressed a LOT, and are short lived.
6DJ8's in a few notorious designs used to be stressed like crazy, and had short lives.
Most 12au7 and 12AX7 preamp tubes seem to be very lightly biased and last a long time.
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.
Sp-14 is a solid state line stage with hybrid phono using one 6DJ8. As I mentioned previously, Audio Research rated those tubes to last 10,000 hours and the original tubes were Chinese, however, sorted by ARC. All of those hybrid preamps were slow to warm up, but because of the long tube life you could leave it on 24/7 for the best sound and you would only have to replace the tube once a year.
10,000 hours is not very long if you have to leave your phono stage on 24 hours a day but only listen for one or two.
I have used Telefunkens and Amperex Bugle Boys from Vintage Tube Services for many years in my LAMM LL2 pre amp (which uses both 12AX7's and 6DJ8's) and find that I can get 3 to 6 years at 2 hours per night before the sound deteriorates or goes microphonic to the point I want to change - depends on your ears and mine are not the best. Phono stages may require more frequent change but mine is SS.