One of the stations that we do contract work for installed a Broadcast Electronics FM20T transmitter on June 6, 2001. It is still running on the original tube, a 4CX15,000A. By my calculations, that is 11 years, 7 months and 9 days, or 101,712 hours.
The hour meter shows 101,168 hours, which accounts for some maintenance, and other anomalies. Overall, the transmitter has a 99.5% up time. I do not think the transmitter suffered any failures, rather, things like the generator and the STL failed instead.
Almost twelve years on one tube is pretty impressive. I know that other Broadcast Electronics T model FM transmitters have similar tube life expectancies. I wonder what the Broadcast Electronics secret is.