[R-390] Tube life/replacement?

[fraserb at quasc.com]

Since I'm in the midst of my annual winter AM BCB Dxing season, I had a thought about the life of the tubes in my R-390A.

How often were tubes replaced when in service with the various govt agencies?  Were different tubes replaced on certain schedules?  Were tubes regulary tested or just pulled and replaced?

I use my R-390A for probably somewhat less than 200 hours a year.  How long are the tubes supposed to last?  How do you know when tubes are less than the minimum level required?  Are there well known symptoms that indicate a certain weak tube?

Lost of questions to start the new year!