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In Reply to: RE: More on (or is that moron?) fuses posted by 1973shovel on May 04, 2017 at 06:03:00
I will pipe in since I wrote the warning. I still own a pair of Quicksilver Silver Mono tube amps. I was using them at the time I started doing fuses. I replaced the stock glass fuse with the same value Littelfuse ceramic. It blew it on startup. I repeated it a couple more times. No inrush current protection on these amps and a number of others. I communicated with the owner of Response Audio back then about it. He had run into the same problem. Thus the warning.
I currently use a ARC VT100 mark III. It has startup surge protection. No problems with ceramics.
I ended up going one amp bigger on the fuses in the Quickies and there were no more problems with blowing them.
I'm still trying to understand the "why" of your results, which was the reason for my post. Not that the cause really matters, but it was something I simply found curious.
Also, thanks for posting the precautionary test on your site. I've never tried "boutique" fuses of any sort, and pre-testing with a standard ceramic could potentially save someone a lot of money if they're putting their toe into that water for the first time.
To be honest, I didn't do any extensive tests that could backup the basic idea of Ceramics VS glass fuses. I added that info early on to protect my customers. For years I also asked if they had big amps with not current inrush protection, but have let that practice slip since sales have dropped off. A better statement might be that not all fuses react to inrush current the same way. They might have less threshold before blowing. In my experience, fuses usually blow due to much higher amperage draws than their rated value. I don't understand why designers don't implement inrush protection. It can be done easily with relays, thermistors, and inrush current limiters. DIY people can see easy implementations in Nelson Pass DIY class A designs.
And that was the intent of my original post, to try and get a better understanding of "why".
I once posted an observation on the Asylum about the element of a 0.25A fast-blo glass fuse glowing bright orange when I first apply 24 VDC to one of my phono stages, which has a quiescent current draw of about 30mA. It does it every time I've switched the power on. The glow lasts for about a second and then looks normal, and yet the fuse has never opened (i.e. "blown").
According to Neolith in his reply to my post, ceramic fuses contain silica. Pure speculation on my part, but I wonder if that silica doesn't allow the resulting heat of a "glowing orange" fuse element to dissipate as quickly as an element in the open air of a glass fuse would, causing the fuse to blow. Still, you'd think that if that were the case, manufacturers like Buss, etc. would take that into account when assigning their fuses over-current protection ratings.
My Nelson Pass First Watt F1J (not a DIY amp) has thermistors in the power supply, and I've never had its fuse open, so your point regarding inrush current is well taken. The schematic calls for a "4A slow", and I got curious while typing this to see if the fuse was glass or ceramic. It turns out it's a 2.5A slow blow glass fuse, 1.5A under spec, and yet has never blown. I guess those thermistors do their job!
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