Mains Fuses on amps 4-5 amp (Slow or fast blow?)

[ericb]

Hi guys, do people tend to use SlowBlo or FastBlo Mains Fuses on Amps? i.e. Marshall JMP 4amp ?

THANKS for any info .

ERIC

[AdmiralB]

Marshalls have always used fast-blow AFAIK. I use fast-blow everywhere except amps that have filament fuses - I use slo-blo there usually.

[ericb]

Marshalls have always used fast-blow AFAIK. I use fast-blow everywhere except amps that have filament fuses - I use slo-blo there usually.

THanks , that's exactly what I was hoping, as that's what I have and use. btw, nice to see you posting again after a long hiatus.


Eric

[VaughnC]

Tube amps typically have slow blow main fuses because of the high current surge when they are first turned on. Tube filaments have a very low resistance when they are cold.

[ericb]

Tube amps typically have slow blow main fuses because of the high current surge when they are first turned on. Tube filaments have a very low resistance when they are cold.

How 'bout Marshall like a 1980 JMP Vaughn ? Bob said they used fast blows? I've used fast blows in these but just wanted to make sure.

This amp does have a surge when it gets turned on , and also I have an old THD PLexi combo that also seems to have that surge like that. Both had blown mains fuses once in awhile, but then the fuse will last for years.. Strange... .
Eric

[Geetarpicker]

I always thought Marshalls usually used slow blow in both positions. For what it's worth my new 1974x 18 watt combo specifies "T" fuses, which stand for "Time Delay" aka slow blow. Most Blackface Fenders specify slow blow for what that's worth too the Marshall folks.

[AdmiralB]

There would be no reason to use a slow-blow in an HT application...unless the filters are ginormous. I've never seen a requirement for one...HT fuses are usually at least 50% overspeced, seems like.

I've never seen a Marshall that speced a slow-blow (that I knew of), and have never used one in any non-Fender amp that I can recall...EXCEPT in the filament fuses in Korg AC30s. I can't imagine any reason that using them in mains applications would be a bad thing, though. Dunno if I'd use one in HT.

[ericb]

There would be no reason to use a slow-blow in an HT application...unless the filters are ginormous. I've never seen a requirement for one...HT fuses are usually at least 50% overspeced, seems like.

I've never seen a Marshall that speced a slow-blow (that I knew of), and have never used one in any non-Fender amp that I can recall...EXCEPT in the filament fuses in Korg AC30s. I can't imagine any reason that using them in mains applications would be a bad thing, though. Dunno if I'd use one in HT.

Hey there, I'm only asking about the MAINS FUSE , not HT.... In my 1980 Jmp it's a 3 amp fuse , and in my 71 SUperlead it's a 4 amp Fuse... In my other amps most are 3 amp. Some say SLO-BLO on the back , but the Marshalls don't say. I still have a fast -blo in there, but was just curious

Eric

[AdmiralB]

I looked at the JCM 800-era schematics and they do note 'T' fuses...or slow-blow. I've never seen (that I recall) an actual amp marked that way, though.

I've never used them in Marshalls. I guess this actually isn't helping you much!

[rooster]

I've used slo-blo in all tube amp applications due to the turn-on/turn-off surges. All tube amps I've worked on were spec'd with slo-blo fuses. That's my take on it, anyway.

rooster.