They really were not rugged at all but they were cheap back then, a set of brand new Sylvania 6JB6 tubes was about twelve dollars. When I got the TR4CW, I had just sold a KWM2. Anyway, the 6146 tubes in the Collins rig were designed for amplifier service so the Drake took that one lesson to learn that sweeps were just not made for continuous duty. Since then, I have a set of RCA 6JB6's in my T4XB that are going on ten years and still load up to full power on 20 and 15 meters. Once I learned to be quick and let them rest for a few seconds between adjustments, they last a long time. I also have four more brand new ones just in case.