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Old 27th June 2013, 10:53 AM   #13191
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Quote:
Originally Posted by AndrewT View Post
I find it difficult to believe you have missed all the references to soft starting and close rated fusing and all the other references to normal transformer fuse rating when a soft start is not adopted.

Fuse value for a transformer or motor ~ THREE times the VA rating divided by the supply voltage.

For a 300VA the fuse required for long term reliable "no blow" start ups is 3.75A use either 3.15A or 4A.

If you adopt soft starting, then you can get away with T1.25A, if you select an appropriate current limiter value for close rated fusing.
Ahhhhhhh! Now that's useful! The First Watt General Power Supply simply states that it must be a 1.25A fuse, and no other information is given.

I will use a 3.15A fuse and hopefully be done with it, as the amp is silent and works perfectly in all other regards. Many thanks.
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Old 27th June 2013, 11:05 AM   #13192
AndrewT is offline AndrewT  Scotland
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How much current will that 3.15A fuse pass without blowing for say 30minutes while you are at the front door trying to persuade some unwelcome caller they are not wanted?

I always recommend close rated fusing. That almost always requires adoption of soft start.
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Old 27th June 2013, 01:04 PM   #13193
Zen Mod is offline Zen Mod  Serbia
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naah ........

Lucas could just put nail instead of fuse

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Old 27th June 2013, 01:25 PM   #13194
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Are you suggesting that 3.15A will not protect anything in the event of a surge, for example, by being too nail-like?
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Old 27th June 2013, 01:29 PM   #13195
Zen Mod is offline Zen Mod  Serbia
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NP is putting CL60 as soft start in his commercial products , meant to be both bullet and idiot proof as much they can ;

is that something telling to you ?

with one CL60 in line , I'm using T4A on 800VA Donut , combined with 8x33mF , in one of my amps , on 220Vac line

never had fuse failure , even in case of black-out ( worst case scenario for sole NTC based soft start)
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Old 27th June 2013, 02:27 PM   #13196
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Again, Please see my earlier post.
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Old 27th June 2013, 02:34 PM   #13197
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But all seems to be solved by simply adding the higher value fuse. Why complicate things? My amp has nowhere near your huge transformer, or capacitance, and I have a smaller fuse, and all is well.
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Old 27th June 2013, 02:41 PM   #13198
AndrewT is offline AndrewT  Scotland
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The 3.15A fuse should stop the cable feeding a faulty F5 from setting fire to the house.

I don't like that. That's why I prefer close rated fusing.
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Old 27th June 2013, 02:44 PM   #13199
bcmbob is offline bcmbob  United States
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Just Sayin'
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Old 27th June 2013, 03:19 PM   #13200
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Hi guys...a question on a different topic.
What is the lowest input impedance the F5 can have and still operate properly? Typically it is either 100k or 50k....what about 1k???
Don't be concerned about the pre-amp having to drive the 1k load....Just consider the F5 here.

Last edited by valvesound; 27th June 2013 at 03:22 PM.
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