What kind of cathode current are you pulling? What's the TPO of the station?
I liked this transmitter (and a lot of people hated it) but I never liked the driver of this transmitter and it had some quirky behaviour. I think I remember something like, if you pulled 400mA, the tubes would last 18 mo, but if you pulled 475mA, they were dead in 4mo. Very touchy. We always had to keep a couple of sets of spares because they could fail at almost any time.
The finals just about always gave 2 years.