Some tubes in power supplies handle a lot of voltage relative to their maximum voltage. As such, they will wear out relatively quickly. If I were you, I would ask Jadis what voltage that tube is exposed to in the power supply. If it is close to the maximum voltage specified for that tube type, then you are going to have to replace the tube more frequently, unless there is a more sturdy version of that tube type available (for example, the "GTA" and "GTB" versions of the 6SN7 can handle 50% more voltage than standard 6SN7's), or an alternative tube available that will work in that application and that can handle more voltage. If you have a good, trustworthy tube vendor, another alternative would be to have him screen a large quantity of 5R4's in order to find the top two or three performers - there is often great variation in performance between tubes of the same type and brand (like dogs and people), and the better performing tubes stand a better chance of withstanding the stress of the application.
Whenever a tube is run near its maximum voltage level, it will die relatively quickly because of the extra heat.
Whenever a tube is run near its maximum voltage level, it will die relatively quickly because of the extra heat.