Valvewizard writes however:
Merlin is the number one authority on tube amps, but his prose is somewhat convoluted .Heater Fusing
An argument can be made for not fusing the heater winding, provided it supplies nothing but AC heaters and lamps. You know, simple stuff. There is very little to go wrong with a simple AC heater supply, and the likelihood of a fault creating a permanent short is slim. Also, the inrush to heaters at switch-on is so heavy that it may be difficult to find a fuse that will withstand it. I've never seen a 20mm fuse rated at more than 10A. Beyond this you might consider automotive fuses. Traditional amp designs don't use heater fuses, and I have never heard of it being a problem.
However, if the heater supply has a rectifier for DC heaters or ancillary circuits then fusing is suddenly and strongly recommended; rectifier diodes nearly always fail short. If this means adding inrush-limiting to achieve a soft start and stop the fuse from blowing every time you switch on, so be it.
Actually the first line is saying: if the AC line only feeds filaments and lamps, it is best not to put a fuse.
And I believe him. I had two amps with this configuration, on both the fuse tested good but failed in how way to conduct. Still I cannot believe Geloso, after building this amp for many years without the fuse, suddenly added it for no reason. Geloso enthousiasts I contacted told me: if they put it there, it should be there, but still they failed to explain why.
I removed them in my amps... but maybe somebody knows why they were there...
thanks
J