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
-PB
Blowing fuses
- Thread starter octavian
- Start date
d_composer
New member
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?
d_composer
New member
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!
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.
I don't believe you'll have any issues with a 2-3A fast acting fuse.
d_composer
New member
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!
Paul Joppa
Moderator
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.
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.
d_composer
New member
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!
d_composer
New member
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!
ZenAudio
New member
I had a similar problem with my Stereomour II. It sat idle for quite a few months after moving to a new house. Finally got around to hooking it up to my Blumenstein Orcas and subs (2013 era) and it started to intermittently blow the fuse on start up. Searched the forums and tried a 1.6A slow-blow fuse. Problem solved. And damn the Bottlehead/Blumenstein combo sounds glorious just running off my iPhone. Wowza.
Similar threads
