KT88 Lifetime estimation

[Salas]

Can you roughly estimate the healthy lifespan of an EH KT88 run on 15W plate, 6V (0.3V less due to drop) heater?

[Tweeker]

Your likely to be limited by the oxide cathode to a few thousand hours at best.

[Salas]

Thanks, circa 4-5 khours with such a low taxation?

[Salas]

''At current time Svet produces excellent 6550C's and no need to look further......althought I am informed the coke bottle Tungsol version can take a higher B+; i.e closer to the golden oldie A version. I don't know if this is true. Other DIY members could input....reliability etc
I'm currently using Svet 88's with a duration clock sofar accumulated 500hrs at 50mA @ 500V and all are emission stable. (a well made KT88 used under such AB conditions should last 10,000hrs min). ''

This comes from best 6550's thread. Can this be for running at 25W? Imagine me wanting 15W... It will outlive kindergarden.

[lousymusician]

The tubelab.com site has an interesting bit on tube life somewhere. He contends that running the tubes at a higher voltage but a lower bias current will give a longer life for the same power due to reduced stress on the cathode. IIRC, that means using a higher impedance primary in the output xformer (like 5000 ohms instead of 3500).