tube burnout

I have owned a tube pre (Grant 100) for a few years and have been satisfied with the sound generally. My concern is that it eats up tubes. It takes six 6922's
with two of them in the mm phono and the others in the linestage. I have gone through about two dozen tubes in the last few years, and it does not matter what brand tube it is either.
This almost always happens to the phono tubes where they start out fresh, only to discover a month or two later these tubes start to get noisy. When replaced,
I get a month or two again before the noise starts up again and they must be replaced. While this is going on, the other four tubes in the linestage are not affected and play fine on any source other than vinyl.
I have tried 6922 from most manufacturers including new and NOS.
I had the pre checked and they said it was fine, just bad tubes to be replaced.
Which I did, and did, and did. All to no avail.
Still the problem persists. I can't believe that all the tubes I keep installing
are 'lemons'.

Does anyone have any suggestions?