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December 12, 2019, 08:53:21 AM

Author Topic: Blowing fuses  (Read 1509 times)

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Online Doc B.

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Re: Blowing fuses
« Reply #15 on: January 14, 2018, 10:41:24 AM »
I would be inclined to try a 1.5A slo blo. The fast blo fuses may be running right on the ragged edge of their current rating at startup. A slo blo of the same 1.5A current rating should be safe to try and may handle the startup surge enough better to solve the issue.
Dan "Doc B." Schmalle
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Offline octavian

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Re: Blowing fuses
« Reply #16 on: January 15, 2018, 04:55:55 AM »
Thanks Doc. I'll try that. I can only find a 1.6amp slo blo in the right size, will that do?

Additionally, is there any reason that the startup surge suddenly started blowing fuses after 2 years? (I'm just trying to understand what I should look out for, just in case there is a problem somewhere)

Online Doc B.

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Re: Blowing fuses
« Reply #17 on: January 15, 2018, 05:52:14 AM »
Yeah, 1.6 should be OK. Hard to say what is causing the fuse to blow. I suspect sometimes it has to do with filter caps aging, or possibly tube filaments having a little more resistance (maybe clean the pins?. Or maybe your line voltage is a little high. If the slow blow holds you are probably good to go. If it does not hold, it's time to do resistance checks and otherwise dig a little deeper.
Dan "Doc B." Schmalle
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Offline Paul Birkeland

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Re: Blowing fuses
« Reply #18 on: January 15, 2018, 05:56:53 AM »
You could try a 2A fuse, just to see if a little extra headroom is helpful. 
Paul "PB" Birkeland

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Offline octavian

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Re: Blowing fuses
« Reply #19 on: January 19, 2018, 12:55:44 PM »
So far as good with the 1.6V slo-blo. I guess the general thing to do now is keep an eye on it and see how it goes?

Offline Paul Birkeland

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Re: Blowing fuses
« Reply #20 on: January 22, 2018, 06:28:14 AM »
Did you happen to change the output tubes during this time period?  It's possible that some of the more exotic 2A3s may want a little more filament current on warmup.

-PB
Paul "PB" Birkeland

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Offline octavian

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Re: Blowing fuses
« Reply #21 on: January 22, 2018, 08:17:46 AM »
No tube changes. Still using the original Sovtek 2A3s. Changed the preamp tubes, but even that was months before any of this started happening.

Offline d_composer

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Re: Blowing fuses
« Reply #22 on: January 22, 2018, 02:08:59 PM »
It’s so strange - my Stereomour II fuse just blew today too! So glad I stumbled on this thread! Ok, running to the store to find a 1.5A slow blow tomorrow... I literally know nothing about fuses - anything else I need to look for before buying it? Or just eyeball for size?

Offline d_composer

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Re: Blowing fuses
« Reply #23 on: January 23, 2018, 01:20:54 PM »
So I just tried replacing the fuse twice (both times with 1.5A 250V fuses) and each time I turned my stereomour II on it eviscerated them. Is it ok to try a 2A fuse? Not sure why this is happening as I’ve been using this amp for a while now with absolutely no problems. Thanks!

Offline Paul Birkeland

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Re: Blowing fuses
« Reply #24 on: January 24, 2018, 04:16:34 AM »
Finding the Slo Blo fuses can be a bit tricky, and I've found that my local hardware store sells fast acting fuses in the slow acting bin.

I don't believe you'll have any issues with a 2-3A fast acting fuse.
Paul "PB" Birkeland

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Offline octavian

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Re: Blowing fuses
« Reply #25 on: January 24, 2018, 10:08:03 AM »
I had to get mine from Parts Express, 1.6amp slo-blo.

Offline d_composer

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Re: Blowing fuses
« Reply #26 on: January 24, 2018, 01:22:42 PM »
Thanks! A 2A fuse did the trick! For my own education: what could be causing my fuses to be blowing all the sudden? I have a hunch that it’s the Frozen picturedisk my daughter made me buy her the other day - Stereomour was protesting the only way she knew how!

Offline Paul Joppa

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Re: Blowing fuses
« Reply #27 on: January 24, 2018, 01:49:11 PM »
It's never easy to be sure what caused something like this. Nevertheless, I'll offer a piece of science ...

With typical tube power supplies, the initial rush of current to charge the capacitors is much higher than the steady-state current. That's the most likely time to blow a fuse, if the amp is functioning properly.

The total energy in that initial burst is proportional to the current and to the time it takes. The fuse also has a time scale, related to the thermal mass of the element and the melting point of the alloy used. Slo-blo fuses have a larger chunk of metal so it takes longer to blow.

Fuses use a variety of alloys, depending on the current rating. The unfortunate thing is, that different manufacturers use different alloys, or choose the break from one alloy to another at not quite the same current. So all 1.5-amp fuses will blow eventually at the rated current, but there will be a variation between manufacturers in how long it takes at higher currents (i.e. the inrush current). If the inrush current drops before the fuse blows, then you are good to go - but an identically-rated fuse from another manufacturer might not survive the inrush.

If you look up some data sheets, you are likely to find a rating for the impulse, usually given in amps squared over seconds. For a very egregious example, a Littlefuse 313 03.2 is rated 3.2 amps and will blow with an impulse of 209 amp^2-sec, whereas their 313 004, rated 4.0 amps, will blow with an impulse of 76.1 amp^2-sec - it will blow nearly three times faster, in spite of the 25% greater rated current.
Paul Joppa

Offline d_composer

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Re: Blowing fuses
« Reply #28 on: January 25, 2018, 01:17:46 AM »
Thanks so much for that in-depth explanation, Paul! I really appreciate you taking the time to write that up. It all makes sense now! Cheers!

Offline d_composer

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Re: Blowing fuses
« Reply #29 on: January 26, 2018, 08:08:49 AM »
Wow, she just blew through my 2A fuses. I tried a 3A fuse and it worked... is this normal? I’ve honestly had zero problems with this amp for the last year or so since I built it and all the sudden this week is eating fuses like candy... I haven’t moved it, modded it (recently) or changed the pwoer situation at all... thanks!