I have sat on my fingers until now, giving it both some thought, and hoping for some points to be covered. Most here have seen most of this before:
There are two types of delayed-action fuses. The first is the typical
SLO-BLO fuse, which consists of a wire element wound around a bit of ceramic. They look like this:
https://assets.alliedelec.com/c_scale,w_400,f_auto,d_no_image.png/70184163.jpg The second is a
TIME DELAY fuse, AKA a "Dual element" fuse. They look like this:
https://assets.alliedelec.com/c_scale,w_400,f_auto,d_no_image.png/70149818.jpg The former will tolerate up to a 100% overload for tens of seconds, a 50% overload for tens of minutes and a 20% overload for far too long. These fuses should not be let anywhere near tube equipment unless its
ONLY purpose is to protect real-estate. These fuses will tolerate short-cycling quite well. So, if this fuse type was in place, then short-cycling is likely not the issue.
The latter will accept a substantial, but very brief overload at turn-on (when the filaments are cold) but act as a conventional fuse thereafter. These fuses do not like short-cycling, and should be allowed to cool for several minutes, at least before a re-start. Such as these have some ability to protect equipment as well as real-estate.
Next: Fuses are wearing parts. They get old, and they fail. It is a function of start/stop cycles, how close to full rating they operate, and actual operating voltage as to whether that failure is 1 wee, 1 year, or 30 years down the road, but fail they will. Those fuses sized closest to actual operating parameters will fail soonest, but will also protect the equipment best.
And: Pretty much every regular operator of tube equipment should have the wherewithal to measure within very small increments. By such means, one may tell whether or not there is an impending failure mode - a dozen watts too many, and something is up.
And: Whereas I agree that a red-plated tube may be toast, at least with post-blight devices, I have any number of pre-blight tubes that have been through all sorts of trauma in equipment that has crossed my bench over the years - and as I have a decent tube-tester, I am able to separate the wheat from the chaff. IF (and only if) you have the means to make a reliable test, do so. Otherwise, yes, toss the tube.
Also: Even the best polarized cap does not like reverse voltage. They are cheap enough not to take the risk. NP caps don't care. I will use screened NP caps when practical and optimal, mostly on solid-state circuits, however.
Also: On a dead-cold amp, try tightening the transformer bolts (holding the laminations together) and if that does not work, obtain some super-thin super-glue and drip that over the top of the windings. A little goes a long way. This should reduce mechanical hum.
Lastly: Lose the 3-way line cord. A polarized cord with the hot to the switch is OK, however. Even suggested.
And, Enjoy!