Fuses... Slow or Fast?

[pjb]

What kind should we use for HT?
What kind should we use for Mains?
Slow Blow or Fast Acting?
-Paul

[markh]

Traditionally, slow for mains and fast for HT.

--mark

[pjb]

Well, there's a little more to it than tradition. The slow blow fuse protects the circuit while allowing a momentary inrush of current ("overload") to charge the power supply filter caps. If you use normal fast acting fuses they'd have to be a bit oversize or you'd go nuts replacing them due to that perfectly normal momentary inrush. Of course, oversizing fuses is bad for both the circuit and the musician. On the B+ side of the P/S the current inrush is not significant so you want protection without delay, ergo normal fast acting fuses are used. Good info is available here and elsewhere: http://www.circuitprotection.ca/fuseology.html

KennyO

Mmm... I know I'm not alone in not being advanced enough to understand that page... at least not until I get the week off work... In layman's terms what does the HT fuse protect and against what? Why does it need to die fast, and why are some amps minus the HT fuse altogether? (like every amp I've built so far)?

-Paul

[Phil_S]

Some amps fuse the HT center tap. The typical Marshall circuit has a 0.5A fast blow fuse between the CT and ground. If there is a short, it needs to blow right away to protect the tranny and I suppose the rest of the amp. This one helps to keep the smoke in.

It's a good idea to have it. The probable reason for omitting it is cost in manufacturing. There is every reason to put in a homebrew. It probably costs you a buck or 2.

Remember, when the HT fuse blows, you amp is still hot from the line voltage, so do shut it off and unplug it.

By contrast, the line fuse needs to be slow blow. As Kenny says, there is an inrush of current when you flick the switch and a fast blow won't stand up to it, as that's not its design. To compensate, you'd need a much higher rated fast blow and that wouldn't protect your amp in the event of an overload. Putting -- let's say a 5A -- fast blow in place of the proper 2A slow blow is just about the equivalent of stuffing a gum wrapper in the fuse holder.

[pjb]

Some amps fuse the HT center tap. The typical Marshall circuit has a 0.5A fast blow fuse between the CT and ground. If there is a short, it needs to blow right away to protect the tranny and I suppose the rest of the amp. This one helps to keep the smoke in.

It's a good idea to have it. The probable reason for omitting it is cost in manufacturing. There is every reason to put in a homebrew. It probably costs you a buck or 2.

Remember, when the HT fuse blows, you amp is still hot from the line voltage, so do shut it off and unplug it.

By contrast, the line fuse needs to be slow blow. As Kenny says, there is an inrush of current when you flick the switch and a fast blow won't stand up to it, as that's not its design. To compensate, you'd need a much higher rated fast blow and that wouldn't protect your amp in the event of an overload. Putting -- let's say a 5A -- fast blow in place of the proper 2A slow blow is just about the equivalent of stuffing a gum wrapper in the fuse holder.

Thank you... (and thank you Kenny... I didn't mean to appear rude in my reply... I will read that page when I get time, I promise). That's helps at least one dummy understand. What "mA" should the fast fuse be in the classic 18 watt trem circuit? 300 mA?
-Paul

[markh]

I said 'traditionally' because Paul seemed to be looking for an answer, not an explanation.

I'm pretty sure the reason for using the slo-blo on the mains is the big current surge due to cold heaters. The HT fuse gets 100% of the cap-charging current too, and it doe not need to be slow.

300/315 mA would be fine for an 18watt HT fuse. 500mA will work as well. There is some lower limit on the HT rating due to the surge when charging the first 32uF.

--mark

[Phil_S]

I used a 250mA fuse in mine. I am an amateur and can't tell you for certain it is the correct value, but as low as it is, I think it's OK.

[morcey2]

What kind should we use for HT?
What kind should we use for Mains?
Slow Blow or Fast Acting?
-Paul

Paul,

Is a blown fuse wrapped in tinfoil considered slow or fast?

Just kidding!!!

Here's a page from Merlin's site dealing w/ fuses:

http://www.freewebs.com/valvewizard/fuses.html

Doesn't cover much in terms of which type to use where. I usually use a 1.5A or 2A slow on the mains for an 18-watt type amp. The only amp I have a HT fuse on is the JTM45 and I think it's a 750mA fast, but I'm not positive on that.

Matt.

[pjb]

Cool, thanks guys... I bought some inline holders for the HT fuse but I'm not sure I like them now. I had BFVV that had one and the contacts inside were domed/radiused and they burned up because the radius limits the contact area... has anyone else had this problem? I want to find some of those internal clip-in fuse holders... I think is safer...?

-Paul

Edit... Also does anyone have a preference where this HT fuse should go? I know it has to go on the PT secondary ctr tap to ground but in terms of lead dress is there a place that is ideal? Or is it less critical?

[Phil_S]

Just use the same kind as for the line fuse and drill a hole in the chassis.