Fuses: fast or slow? How to determine it?

[Latole]

How is it originally calculated whether the circuit requires a fast or time-delay fuse on power transformer primary ?

Many (all?) vintage Fender amps use time-delay fuses.
As do all the Marshalls I've seen in my research.

Are there any all-tube amps of this type that use fast fuses?

How ignorant I am!   

[pdf64]

Many of the more recent Fenders use a NTC current surge suppressor and a fast acting mains fuse https://el34world.com/charts/Schematics/files/Fender/Fender_65_super_reverb.pdf

They work fine until the power gets cut accidentally and then gets quickly reinstated, ie before the thermistor has cooled down. As can happen on busy stages at set up / sound check. Then the inrush to recharge the magnetic circuit can pop the fuse.

[Dave]

For just a moment, when the power is turned on, the reservoir caps look almost like a dead short. A fast acting fuse might burn because of that. That's why you use slow burn fuses there.


Dave

[Merlin]

How is it originally calculated whether the circuit requires a fast or time-delay fuse on power transformer primary ?

Fast fuses are never required for power supply use, they will just blow with inrush. Pretty much the only place where you might want a fast fuse is in series with the power valve cathodes or output transformer CT, and even that is debatable.

[Latole]

Thank you all for your answers,

This is probably the cause of the amp problem in this topic , The 2 A fuse without it being written that it must be slow -blow ;

https://el34world.com/Forum/index.php?topic=31676.0