I found this article from Wired earlier this year: <a href="http:&#x2F;&#x2F;www.wired.com&#x2F;2015&#x2F;02&#x2F;air-traffic-control&#x2F;" rel="nofollow">http:&#x2F;&#x2F;www.wired.com&#x2F;2015&#x2F;02&#x2F;air-traffic-control&#x2F;</a><p>It doesn&#x27;t specificaly mention vacuum tubes, but it does explain why it&#x27;s entirely plausible that they&#x27;re still around:<p>&gt;This technology is complicated and novel, but that isn&#x27;t the problem. The problem is that NextGen is a project of the FAA. The agency is primarily a regulatory body, responsible for keeping the national airspace safe, and yet it is also in charge of operating air traffic control, an inherent conflict that causes big issues when it comes to upgrades. Modernization, a struggle for any federal agency, is practically antithetical to the FAA&#x27;s operational culture, which is risk-averse, methodical, and bureaucratic. Paired with this is the lack of anything approximating market pressure. The FAA is the sole consumer of the product; it&#x27;s a closed loop.