Amp fuses - Slow blow or quick blow?

[Will]

I gather slow is the glass ones usually, and quick is the ceramic + sand.
I remember my JCM900 ones that were sent from Marshall were glass, but the ones in my 2204 are ceramic (and one of the values is wrong, which shall be ammended ASAP)

Which do I want?

[Twinfan]

I use glass ones for both.  I think you want slo-blow for both mains and HT fuses?  That will avoid blowing it if there's a slight surge in power?

[Will]

Cheers, makes sense. Will hopefully get some comfirmation then order some tomorrow
JCM900 seemed to last ok with the quick ones though :p

For some reason the older ones want long fuses :?

[Twinfan]

Yeah, they're an odd size.  I get them from Maplins.

[martinw]

Slow-blow (or time lag) for the mains (usually 2 or 3A or so). This is to cope with the inrush current of the mains transformer.

Quick blow (e.g. F500mA for a typical 2 o/p valve amp) for the HT.

Both can come in glass or ceramic, and both can come in various sizes, usually 20mm or 32mm.

[Will]

Got it, exactly what I waanted to know.

Thanks guys

[Will]

Ordered, was previously searching ebay as it was hard to find specifically what I wanted through Google.
Used Maplins as suggested

[Twinfan]

Nice work  :)

Sorry about the incorrect slow-blo/HT info.  I knew going from memory was a bad idea!

[Will]

Its fine, your logic was spot on, just the fact that it only applied to the mains :P
Arrived today!!!! and one of them blew pretty quick. This is going in for a health check I think.
Will give Martin an email to see how his prices + postage will compare to Marshall factory.

On a brighter note, Cleartone cables also today :D phwoar.

[jpfamps]

Some amps also use SLO-BLO fuses on the HT if they have to cope with the charging pulses of the main filter caps. An example of this would be the old Hiwatt amps where the HT fuse is connected between ground and the bottom of the bridge rectifier (not a great place for it really).

Putting the fuse here would in theory protect the mains transformer from a filter cap short, which a fuse after the first filter cap wouldn't. Most amps have a fuse after the first filter cap to protect the amp in the event of a valve short/ bias failure which is a common mode of failure.