Fuses

[minions]

I have two questions about fuses:

Firstly, what is the difference between slow blow and fast blow fuses, and what are they each used for?

Secondly, how do you choose the value of a fuse for a given amp?

Thanks.

[ashcatlt]

Well, I'm no expert, but as I understand it, the slow-blow fuses will withstand an over-current situation a bit longer than the fast blow. That is, the fast-blow blow faster .

The fast-blow are preferable in situations where sensitive/expenive components and/or humans might be damaged by a fault. In cases where downstream components might be able to handle brief spikes, it might be desirable to use a slow-blow in order to avoid replacing a fuse over a relatively minor, short lived fault.

In a guitar amp (especially the older tube models) it is possible that the fault will mean a very high voltage flowing through the operator, I think a fast-blow is desireable. I'm not promising, though, that even these will go fast enough to save your life. At the power levels we're talking about, it doesn't take long!

[sumgai]

Sadly, tubes are the only place in guitar-dom that it's not only appropriate but necessary to use slo-blo fuses. Why?

Because as ash correctly notes, they withstand a large dose of in-rushing current, whereas a fast-blo fuse will simply give up the ghost, as in, like, right now.

This inrush is a result of the filaments being powered up - when they're cold, they act like very low-value resistors. When warm, they act much more reasonable, but for that short time, the incoming current is high indeed. (Note - it's not the voltage that's high during this time, it's the current.... which is what fuses are sensitive to.)

Keep in mind that while the foregoing is true for most tube amps, and most tube types, there are some exceptions. But I doubt that you'd find any of them in our neck of the world.

More to come, about designing and such, but for now, I gotta get some sleep! Manana.



sumgai