I just don't want to throw more money down a rathole. I could order a brand new set of 572Bs from RF Parts, but I might as well go with the GI-7Bs for that much money. I started out with what I THOUGHT were 4 good spares I had received with a FL-2100B. Evidently the guy just put the dead tubes back in the box instead of pitching them. And I am NOT just some novice Service tech. I have been working on these kind of things for 40+ years now, and a Paid RF Technician with thousands of hours of bench time for the past 30 years!