Interestingly, most of the 18 watt schematics I've seen do not use a HT fuse. However, I suppose you could use a 0.5A FB between the power supply center tap and ground, similar to the JTM.
Read this from the old 18 watt forum:
https://www.18watt.com/viewtopic.php?p=192209&sid=980af41036e6008bd5744b3a94d096c1#p192209Here's some good stuff from Merlin (Valve Wizard) on fuses. He opts for 2 fuses on the HT winding prior to the rectifier rather than fusing the center tap:
http://www.valvewizard.co.uk/fuses.htmlYou could also place safety diodes on the rectifier socket (in series) if you are worried about a short causing PT damage:
https://www.tdpri.com/threads/rectifier ... e3.833572/