Bottlehead Forum
Bottlehead Kits => Stereomour II => Topic started by: octavian on January 09, 2018, 03:13:24 PM
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Hi all,
My Stereomour II has started exhibiting a really weird issue after 2 years of functioning flawlessly. It's blowing fuses. But here's the kicker, it's doing it when turning off! While on, it functions perfectly, no weird noises, usual super-low noise floor etc. I'm stumped. What could be wrong with it? Could the switch have gone bad and is shorting somehow while being flipped to off (not sure this would even make sense as the cause)?
Thanks all.
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What replacement fuses are you using?
It is indeed strange that you are blowing a fuse at power off. What voltage is your AC mains, and are your outlets grounded?
-PB
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Outlets are grounded, same fuse as original 1.5AMP Fast Blow 250V. Outlets are grounded, it's plugged into a power filter bar with surge protection (as it's been for the last year).
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Can you test to see if the amp still blows fuses if you pull the power cord to turn it off instead of using the power switch?
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Just tried to turn it off by yanking the power cord, still blew the fuse, so I guess it's not the switch.
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I'd be interested in seeing photos of your build, particularly of the wiring around the IEC power entry module, and the STP that runs up to the front power switch.
Can you share where you're located and what your AC mains voltage is?
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Thanks! I haven’t checked the voltage in a while, but I’m in Chicago, and if I remember right it tends to be around 118. I’ll have to take some photos tonight, but in the meanwhile, here’s a photo from my original build thread: https://bottlehead.com/smf/index.php?action=dlattach;topic=8117.0;attach=9346;image . Could one of the power tubes cause this issue? I guess they’re getting up there in hours.
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There isn't a Stereomour II in existence that could have been operated long enough to exhaust a pair of 2A3s (unless they were the ultra cheap Chinese ones).
You could certainly power up, then turn off the amp with no tubes installed.
Can you plug in one of those 3 light outlet testers into your outlet, just to be sure there isn't anything weird going on with the power?
-PB
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I don’t have one if thise testers, but I can do a quick voltage check with a multimeter and the power bar tests for ground. Could that be enough? The weird part is that I’ve had no issues with any of the ither devices that are plugged into it, only the amp.
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Of course, there is the other option. It could be blowing fuses when turning it on. Would the cause be more straightforward in this case? (Though it’s never done this after changing a fuse)
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If the fuse blows when you turn it on, then the amp would turn off and you'd get no sound out.
You can actually test your outlets with your DVM. Set it to AC volts (at least 200V AC if it's not autoranging). Put the black probe into the round ground hole, then measure the AC voltage at the narrow slot and wide slot. The bigger slot should show 0V and the smaller slot ~120V.
-PB
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Okay, so I looked closely underneath and there seem to be zero issues - no bug causing a short, etc. It looks just like the day I finished it. I also figured out that it is blowing fuses on turning on. I put a fresh fuse in, turned the power on, then the fuse blew pretty much instantly. I haven't been able to test the socket yet (neither of the sets of probes I have for my DVM fit in a socket! Sigh, I'll have to get an extra set). However, the other stuff I have plugged into this same power bar isn't exhibiting any problems or blowing fuses, so I'm thinking it's probably something in the amp.
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So have you been able to listen to the amp at all when you've had this problem? When you posted that the fuse blew when you turned the amp off, there was a presumption that you were listening to the amp prior to having this issue.
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It's difficult to guess what has happened without more consistent description of when the fuse is blowing, but I would be inclined to investigate the switch and the IEC socket to see if something has come loose and able to short because of it.
One also needs to always be cautious of trying fuse after fuse without determining what is causing them to blow, because that can cause expensive damage like a blown power transformer. If a rectifier has blown or a cap has shorted it must be replaced before powering up the amp.
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Hi both,
So I originally (wrongly) suspected the fuses blew when turning it off, as it had never blown when replacing the fuse until the last time. So here's where I'm at. The amp sometimes works just fine, but occasionally blows a fuse on start up. When it's working, it exhibits zero issues, no weird noises etc. I checked the underside and all of the wiring seems just fine on both the IEC and the switch. It blows very quickly when turning it on - the tubes don't light up at all etc. Could it be a defective power tube? Is the switch still a likely culprit?
Thanks all. Sorry for the perplexing puzzle.
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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.
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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)
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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.
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You could try a 2A fuse, just to see if a little extra headroom is helpful.
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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?
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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
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No tube changes. Still using the original Sovtek 2A3s. Changed the preamp tubes, but even that was months before any of this started happening.
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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?
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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!
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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.
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I had to get mine from Parts Express, 1.6amp slo-blo.
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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!
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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.
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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!
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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!
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Hmmm, with a stock build and no new tubes? Really try to find a Slo Blow 2A fuse to take out a potential fuse quality question and maybe do the voltage checks just for peace of mind.
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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.