[emeritus]

In my youth I did once apply HT to heaters that were still warming up from cold by virtue of having strapped a silicon diode across the rectifier valve of a non-working radio. This produced crackling and miniature pyrotechnics displays in the valves, but it did actually fix the set, which had previously been silent. After a couple of futher brief cold applications of HT, and removal of the diode, it worked fine. An old hand told me that the valves had probably gone low emission and applying the full HT to cold valves had stripped the outer layer from the cathodes, revealing fresh emissive matter underneath. There was little interest in old bakelite radios then, and if I hadn't got it going, it would have been scrapped. Not to be recommended for valves that are working ok!