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PostPosted: 21 Aug 2016, 11:28 
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Joined: 13 May 2016, 02:02
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Location: Javea, Spain
Intermittent fuse problem. I've been running the amp for about a week now and I'm on my 4th fuse. Sometimes it blows on switching on the amp, sometimes the amp can be on for a few hours before it goes. I'm using 500 mA slow blow fuses as suggested. Any ideas?
I need to clarify that a little... for instance, I can listen to the amp for 4 or 5 hours and then turn it off. Turn it on again 2 or 3 hours later and it's dead. Change the fuse and it's OK again. Can a fuse blow when you turn the amp off?


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PostPosted: 21 Aug 2016, 11:56 
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john55 wrote:
I'm using 500 mA slow blow fuses as suggested. Any ideas?
I just reran the numbers and it still looks like the total draw is like 40W (give or take). Assuming 80% transformer efficiency, at 240v: 40W/240V/0.8= 208mA and at 120V: 40W/120V/0.8=417mA. So I would think that the 1/2A fuse should be ok. My guess is that there is something a little nonstandard in the I^2*T value that renders the fuses marginal in this application. Increase it to 1A and you should be ok.

The thing to keep in mind is that fuses are to protect against fault currents. And fault currents which will give wiring problems (because fuses exist to protect wiring, not equipment) are measured in amps, not milliamps. We like to keep the fuse ratings low so that we limit fault induced equipment damage, but in reality using a value of an amp or two is perfectly acceptable.

On a side note... I just pulled the main fuse out of the project amp to see what was the rating. Evidently I've been running the amp with a 6A fuse in it. :blush: Never had any problem... go figure. I think I'll change that out.

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Last edited by Suncalc on 21 Aug 2016, 12:11, edited 2 times in total.

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PostPosted: 21 Aug 2016, 12:05 
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Location: Javea, Spain
Thanks Matt, just one more thing I've noticed is that one of my ammeters has crept up to 50 mA whereas the other one has remained at round about 40 mA. Both were registering about 40 mA yesterday.


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PostPosted: 21 Aug 2016, 12:10 
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john55 wrote:
I've noticed is that one of my ammeters has crept up to 50 mA whereas the other one has remained at round about 40 mA. Both were registering about 40 mA yesterday.
Recheck your power stage B+ voltages and your power stage cathode bias voltages on both channels.

It could be the tube, a resistor going, or a bad meter. Since you've only been running the amp about a week, this sounds like a component infant mortality issue.

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PostPosted: 21 Aug 2016, 13:00 
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OK, Both channels read 232V and 242V (should be 250V and 260V but the mains voltage here in Spain is significantly lower than 230V)
The bias voltages are 33V and 30.5V . The meters are reading pretty much the same now that I've turned the amp upside down and back again.


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PostPosted: 21 Aug 2016, 15:06 
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I use a 3 amp slow blow in all my builds, this is with 120V USA voltages.This equates to 1 1/2 amp with 240V. If there is a real problem such as dead short it will blow plenty fast enough. Designers always tend to rate on the conservative side for obvious reasons.


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PostPosted: 21 Aug 2016, 15:57 
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wdecho wrote:
I use a 3 amp slow blow in all my builds, this is with 120V USA voltages.This equates to 1 1/2 amp with 240V.
This is actually a common misconception. Fuses are not rated like resistors. Actually a 3A fuse is a 3A fuse.

For fault currents, fuses are actually rated by their I^2*T value. That's current squared times time. This is the rating of how long it takes the fuse to blow at a given current. (However, the IEC 127 standard for marking uses a current rating and a coded letter for response time). The continuous current rating is the current limit below which there is not normally a chance of failure because heat is radiated away and doesn't build in the fuse element. The voltage rating on a fuse is the maximum voltage (at the failure current) that the fuse can interrupt safely. If fuse is used at voltages above this rating, there is a possibility that the fuse element will not quench the fault current and arcing or plasma will result with physical damage to the fuse holder or equipment.

Little Fuse actually has a very helpful document on their particular fuses here:

http://www.littelfuse.com/~/media/electronics/product_catalogs/littelfuse_fuseology_selection_guide.pdf.pdf

There is a lot of good general information on fuses in the front sections of this document.

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PostPosted: 05 Nov 2016, 13:55 
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Location: Javea, Spain
Finally finished the full project. I used Fostex full range drivers and built the cabinets to their specs. As I had read on various posts they lack a little on bass, but clear as a bell for mids and highs. In the end a Dayton audio 8" subwoofer and plate amp, again using their cabinet specs were added to the system. The sound is absolutely perfect, just dial in a tiny amount of bottom end and everything is right. I covered everything in cowhide to match the amp. I aged the leather today (fading black dye into the brown on corners and edges). I'm going to call this project finished now, how long can you keep fiddling? Thanks to everyone on the forum, I've spent countless hours reading and rereading all the details on the 6EM7 amp DIY speaker projects.


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PostPosted: 06 Nov 2016, 03:16 
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John55, OGM, sublime! Be proud.

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PostPosted: 07 Nov 2016, 19:16 
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Hi John55. Congratulations. That's a great looking build.

Which Fostex drivers did you use?

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