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Fuses


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3/16/1998 7:33 AM
Jason Borders
Fuses
I have a Vox AC-50 that uses a 3 amp main supply, and a .75 amp B+ supply fuse. Would anyone know if I should use a "slo-blo" type or would a regular type work fine? Thanks.
 
3/16/1998 8:02 AM
liam

Normally you have to use slo-blo otherwise they blow as the capacitors charge up. That's what slo blo fuses are designed for I believe. However a lot of Vox amps have pretty big rated fuses considering the current that normally flows.
 
3/16/1998 11:43 AM
Doc

My experience is, if the circuit takes a slo-blow fuse it will be printed next to the fuse holder. Otherwise, if it says 3AG or just the amp rating, it is understood to mean standard fast blow.  
 
Always use a fast blow fuse in the B+. They're supposed to save your output transformer.  
 
Slow blow fuses are good where short duration overcurrent transients, or surges, would cause unnecessary "nuisance trips". Surges at turn-on can temporarily draw extra current, and are considered normal. These temporary overcurrents could look like a short circuit to a fast blow fuse if it was sized too close to the normal steady-state operating current draw.  
 
All fuses have a current vs. time curve. Slo-blo fuses will withstand an overcurrent for a longer period of time without opening than a fast blow type.  
 
Doc
 
3/17/1998 10:18 AM
Derrick Re: Fuses/ In a MARSHALL PLEXI
I've had the problem with blowing fuses in an old '67 100w Marshall plexi head sold in Europe. It says "2A", however on the american versions made for export, they say "3 0r 4A." I upped the raring to what all my other American "EXPORT" models require and it worked fine. Anyone have any input.  
Derrick...............
 
3/17/1998 2:21 PM
moocow
Power = Voltage * Current, so halving the available voltage will double the current requirements (for the same power).  
 
When your European amp runs from 220 V, it draws half the current it does than when it runs from 110 V, and the 2A fuse was adequate. But with 110 V, it will draw twice the current and blow the 2A fuse. That's why they stuck 4A fuses in the export amps.
 
3/18/1998 9:25 AM
NickM

I have had a JTM-45 reissue that was blowning fuses when it switched to standby or back. The problem was the speaker lead. I don't know if the plexi is the same but a new speaker lead, a good one fixed my problem.
 
3/17/1998 6:06 PM
Bill bolton
Re: Fuses
My mid 70s AC50 is set up with a normal fast blow fuse on the mains power and a slo-blo on the HT (B+).  
 
As far as I'm aware that is the "factory" configuration.  
 
Cheers,  
 
Bill
 

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