Which radar still used vacuum tubes in the transistor era?

Most of the MiG-25's onboard avionics relied on vacuum-tube technology — nuvistors, specifically — rather than solid-state electronics. The MiG-25P interceptor carried the TL-25 Smerch-A radar, a development of the system fitted to the earlier Tu-128. Western intelligence had assumed it was a modern solid-state pulse-Doppler unit; in reality it was built around vacuum tubes.

The Soviet electronics industry of the 1960s could not mass-produce reliable solid-state components on the scale a front-line air force needed. Tubes, by contrast, were produced in volume, could be repaired by conscript technicians at remote bases, and were replenished from stores. They also withstood the electromagnetic pulse of a nuclear detonation better, something that weighed in Soviet doctrine in a way that had no equivalent in American planning. And they delivered enormous raw power, enough to burn through the interference on which Western electronic countermeasures relied.

The original Smerch-A of the MiG-25P reached about 600 kilowatts. It lacked, however, any look-down/shoot-down capability: it could barely track targets below the aircraft against ground clutter, which limited its effectiveness against low-flying targets.