OK - Here goes.
RANT WARNING RANT WARNING
Fuses are one of my pet peeves as they are so poorly understood as to be dangerous at least 80% of the time in my direct experience. So, a bit of a lecture follows as well.
There are three types of fuses in very broad brush:
a) A conventional fuse - consisting of a single element designed to 'blow' at a specific load with very little overshoot.
b) A "slow-blow" fuse - one that is designed to take an overload for some period of time being a function of excess load and overall fuse rating. Typically, these fuses are an inexpensive (to the fuse maker) way to protect real-estate whilst not blowing too often. They are, again typically, used for loads that come on strong, then taper down during normal operation. Tube equipment meets this definition. Overloads may occur at any time during operation, as long as there is some level of cooling time between overloads. And, small overloads may last a _VERY_ long time before the fuse actually blows. The cost of these fuses is only fractionally greater than type a).
c) Dual-Element fuses - these are fuses with an element that handles a potentially very large momentary overload, such as a motor starting or contact surge, and then steps aside for a normal fuse element. The overload function happens ONCE per cycle. Any subsequent overload and the fuse will respond as with type a). These fuses are quite costly relative to normal fuses, often 3 to 10 times the cost.
There are variations all over the place, including quick-blow fuses (very fast response), low-resistance fuses (silver or silver bearing elements) and many more.
HOW TO SIZE A FUSE:
The process is as follows:
a) Obtain a metered variac, fine-pitch ammeter, resistance box + VOM, or other device by which you are able to measure actual current drawn by a device at full load and operating voltage.
b) Ideally, you will devise a dummy-load for amplifiers that allows you to run them at/near clipping to measure the true maximum load.
Measure the load that will run across the fuse using this (these) devices.
Round it up/down to the nearest 1/2 amp, but shade low. So, 4.1A = 4.0A. 4.3A =4.0, 4.4 - to 4.8A = 4.5A and so forth. Remember, you are measuring at/close to clipping, which should not be a normal state.
Obtain DUAL-ELEMENT fuses at that rating. Install them.
1. Dual-element fuses do not like being short-cycled. The delay element *will* fail if short-cycled, so for testing (and testing only) a slow-blow is acceptable as long as you are right there with your hand on the throttle (switch).
2. Dual-element fuses are wearing parts, but less so than conventional fuses. So, if you get the fuse very close to the maximum operating value, its service-life will be measured in hundreds of hours, not thousands. But, a cheap investment as compared to a 5AR4.
3. The load-to-failure curve is not linear, but logarithmic - the higher the overload the quicker the failure.
4. An amp with a tube rectifier has two surges, very brief large with a very big overshoot (3 x quiescent), one very slow, but no overshoot. The first when the cold filaments heat up, the second when B+ comes on line. You may watch these if you meter the start.
The "why" of it: Imagine a slow-blow fuse in a cool location with a 10% overload. The fuse will likely tolerate such an overload for many minutes (using my metered variac, I once tested a 3A slow-blow at 3.3A for over 30 full minutes). 3A @ 120V = 360 watts. 3.3A @ 120V = 396 watts. That is 36 watts over, about 5 x a standard 7 watt candelabra lamp. That is a LOT of heat that is going somewhere - somewhere it is not supposed to go. Keep that up for some time and *POOF*.
My standard ST70 draws about 150 watts at full load (rated at 190 watts). I use a 1.25A dual-element fuse. The manual calls for a 3A fuse. I have not had the 1.25A fuse blow on overload yet over a couple of years, but then I baby the amp, and we are running at about 118V generally.
Good luck with it.
p.s.: Slow-blow fuses are created in the nether regions for the bedevilment of their users, and (almost) no other legitimate purpose.