Call conrad johnson. They are very helpful.
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I know this is an old thread but here is my 2c.
KTK-1 for later production
BBS-1 for earlier production
KTK is 1/8" longer than the BBS, other than that they are the same.
The fuse will blow if a tube blows. That is what its for.
Voltage spikes and such can also take one out.
Your amp may need service if it is blowing them for no reason. I, myself, have only ever replaced a fuse after loosing a tube.