Hi Colin. When used for regulation duty [rather than used as an output tube] the 350b, 6f6g or any of the other tube choices should last longer than you will! That said: the rectifier will have a far greater influence on the sound of the amplifier regardless of ones choice of regulation tubes. Considering the cost of admission for a pair of strong testing 350b's, one would be miles ahead investing in the metal based version of the GZ-34. Yes, they are rather expensive....but far and away the best sounding full wave rectifier on the planet.