Amp blows 5A plug fuses, why?

[discreet]

I have a 500W/240V solid state amp which for some reason intermittently blows the fuse in the mains plug, which is rated at 5A.

It happens like this: the amp is working perfectly OK, and I switch the amp off. When I come to use the amp again, I find (occasionally) that there is no power, and the 5A plug fuse has blown.

It doesn't come on then blow the fuse, so I'm guessing it must blow the fuse at power off... I replace the fuse, and off we go again.

Would I be right in thinking that the plug fuse is generally to protect the mains cable, as the amp has its own internal 6.5A fuse (which has not blown)? Or is this just bullplop?

And if this is the case, would replacing the 5A plug fuse with a 13A fuse solve the problem? Or is it a big dangerous no-no? Could there be something wrong with the amp?

Am I being a complete spanner, or what? Thanks!

[EdwardHimself]

I doubt the power cable can't handle more than 5A. Maybe you should try replacing the cable with a different one?

[DHA]

[quote name='discreet' post='840754' date='May 18 2010, 12:37 PM']I have a 500W/240V solid state amp which for some reason intermittently blows the fuse in the mains plug, which is rated at 5A.

It happens like this: the amp is working perfectly OK, and I switch the amp off. When I come to use the amp again, I find (occasionally) that there is no power, and the 5A plug fuse has blown.

It doesn't come on then blow the fuse, so I'm guessing it must blow the fuse at power off... I replace the fuse, and off we go again.

Would I be right in thinking that the plug fuse is generally to protect the mains cable, as the amp has its own internal 6.5A fuse (which has not blown)? Or is this just bullplop?

And if this is the case, would replacing the 5A plug fuse with a 13A fuse solve the problem? Or is it a big dangerous no-no? Could there be something wrong with the amp?

Am I being a complete spanner, or what? Thanks![/quote]

you are right the mains fuse in the plug is to protect the mains cable. Try a bigger mains cable and fuse, don't just put a 13A fuse in a 5A cable.

Edited May 18, 2010 by DHA

[Dood]

Look also at the possibility that the 6.5A fuse in the amp is a 'delay' or 'slow blo' type that is designed to take an initial surge of high current. The 5A fuse in the mains plug is a fast blow type and is probably unable to handle that initial spike.

Without a lesson in egg sucking, have you checked the manual for you amplifier and does it suggest the size fuse required? Definitely heed Dave's advice - those 'computer' style IEC leads (yes - some people call them kettle leads..) are rated for 13A. Try one of those with a 13A fuse.. and if you are unlucky and the 6.5A fuse blows - don't just replace it - it's time to get your amp checked over!

[discreet]

Thanks EH, looks like you're right about this.

Thanks Dave, I'll be sure to use the correct cable.

Thanks Dood. Yes, the amp fuse is actually a 250V T6.3AL slow-blow, as you predicted. I'll try a proper 13A lead and see if that works. The amp manual doesn't mention anything about plug fuse requirements, unfortunately. Let's hope the T6.3AL remains intact!

Good advice, people! What a great resource BassChat is!

[discreet]

Was in communication with a seller of 13A IEC mains cables today, and he reckons if the amp has its own internal 6.3A slow-blow fuse, then I should be using a 7A plug fuse with a 13A-rated IEC cable. Which is what I'm going to try, unless anyone thinks it inadvisable?

Frankly I didn't know 7A plug fuses were generally available, so there you go. Every day's a school day.

Edited May 20, 2010 by discreet

[Bottle]

[quote name='discreet' post='843437' date='May 20 2010, 07:15 PM']Was in communication with a seller of 13A IEC mains cables today, and he reckons if the amp has its own internal 6.3A slow-blow fuse, then I should be using a 7A plug fuse with a 13A-rated IEC cable. Which is what I'm going to try, unless anyone thinks it inadvisable?

Frankly I didn't know 7A plug fuses were generally available, so there you go. Every day's a school day. [/quote]
It's a good suggestion - the fuse in the plug is there to protect the cable only - check that the cable is suitably rated (minimum of 1.00mm[sup]2[/sup])

3A and 13A are the most popular - 5A used to be widely available, but generally replaced by 13A now (and not always in a sensible fasion). 7A and 10A are made but finding somewhere that stocks them other than a specialist component supplier is like finding rocking-horse s***. Try RS or Farnell/CPC.

HTH, Ian

[Huge Hands]

[quote name='Bottle' post='843466' date='May 20 2010, 07:49 PM']3A and 13A are the most popular - 5A used to be widely available, but generally replaced by 13A now (and not always in a sensible fasion). 7A and 10A are made but finding somewhere that stocks them other than a specialist component supplier is like finding rocking-horse s***. Try RS or Farnell/CPC.[/quote]

My understanding was it is an EU directive to get rid of all fuses except 3A and 13A.

Although arguably safer in your case, you may struggle to find them at all from "buy new" shops, including those mentioned above.

I'm sure there must be a spark on here who can confirm.

[Ian Savage]

[quote name='Huge Hands' post='843476' date='May 20 2010, 07:55 PM']My understanding was it is an EU directive to get rid of all fuses except 3A and 13A.

Although arguably safer in your case, you may struggle to find them at all from "buy new" shops, including those mentioned above.

I'm sure there must be a spark on here who can confirm.[/quote]

7A is hardly ever used, 3A and 13A are indeed the 'preferred option' these days, 5A and 10A are available but you might have to hunt a bit. Although *hang on*...yep, I've got a spare 10A kicking around if you want it discreet?

[EdwardHimself]

[quote name='Huge Hands' post='843476' date='May 20 2010, 07:55 PM']My understanding was it is an EU directive to get rid of all fuses except 3A and 13A.[/quote]

That sounds like a horrendously stupid and pointless idea to me. Which i suppose makes it entirely within the realms of the EU directive.

Edited May 20, 2010 by EdwardHimself

[TimR]

My Warwick ProFet 5.1 did this. When I checked, I found it had a 5amp fuse which I put down to having loads of IEC leads in my box and had picked up an old PC or lighting one. Fuses will run for hours at up to 1.6x their rated current, but will blow instantly at 2x.

The nice people at Warwick have put a schematic on their website. The problem withy my amp is that it just has a switch between the mains and the transformer. There is no soft start and transformers take huge amounts of current at start up. 13amp has solved it. It's got a 5A time delay internal fuse anyway which leads me to belive that a normal 5A fuse on the mains plug is a bad idea!

Edited May 20, 2010 by TimR

[Ian Savage]

[quote name='EdwardHimself' post='843535' date='May 20 2010, 08:46 PM']That sounds like a horrendously stupid and pointless idea to me. Which i suppose makes it entirely within the realms of the EU directive.[/quote]


It wouldn't surprise me if it were an EU regulation, but having had a hunt around the net I can't be sure that it IS a directive; what I can say for sure is that the stuff I'm coming across for repair and in my line of work (physics teacher/audio technician) is becoming increasingly either 3A or 13A.

As I say, I haven't seen a 7A cable in a long while, still come across 5A and 10A sometimes (which for professional audio gear are the two you're most likely to want - 5A for things like amps and low-powered active speakers, 10A for LOUD stuff); everything just seems to be 13A. The cynical side of me says that it's to get people who don't realise their amp has internal fuses (or fuses in the IEC connector) to return it to the manufacturer for repair should one of these fuses go; it's just an extension of making most plugs 'moulded' to the lead so you can't do any wiring yourself.

Any technician worth their salt will be able to do the maths and work out what you need cable-wise; the problem is that if you accidentally hook up your gear with the wrong mains cable and there IS a problem (which with the antiquated wiring in some venues is more than possible) you've got to strip your amp down to replace fuses rather than just replace the IEC lead.

For anyone confused, have a look at the back of your amp - there should be a figure for power usage (NOT power output) or suchlike in Watts. Since we have a supply that's around 230-240V in the UK, divide that figure by 240 - that'll give you the power handling of the cable you need, and just go ABOVE that (so, for arguments sake, if your amp uses 1000W, the current draw approximately is 1000/240 = 4.2-ish, so you want a 5A cable and fuse).

And always carry an appropriate spare mains cable, and/or a spare fuse or two, ALWAYS. It's worth looking at what kind of glass fuses are inside the IEC connector on the amp also, and just sellotape four spare fuses (two of each, plug and internal) somewhere inside the amp/flightcase/head/whatever.

Sorry to be so Nazi about this, but I spent a good while doing live sound and trying to sort out players who blew fuses and had no idea why, and trying to simultaniously fix the problem and explain to them how to stop it occuring again...

[discreet]

Thanks very much, guys - I picked up a supply of 5A fuses from feeBay, but obviously these are of little use to me - I'll run out! Anyone want?

I've also ordered a shedful of 7A fuses, again off peeBay, and a 2m IEC cable rated at 13A, but have not had the opportunity to try them yet.

Yeah, EU... that's a whole other topic right there.

10A I didn't know about at all, but hey. Thanks for the offer of a 10A Ian, will gladly accept and add to the equation... PM'd.

TimR, thanks, it's a ProFet 5.1 I'm talking about, as you no doubt knew from my sig. Reassuring to know the problem may soon be history, thanks! 5A certainly does seem to be a bad idea...

Will let you know how it goes. Thanks again.

Edited May 20, 2010 by discreet

[discreet]

[quote name='Ian Savage' post='843725' date='May 21 2010, 12:22 AM']And always carry an appropriate spare mains cable, and/or a spare fuse or two, ALWAYS. It's worth looking at what kind of glass fuses are inside the IEC connector on the amp also, and just sellotape four spare fuses (two of each, plug and internal) somewhere inside the amp/flightcase/head/whatever.[/quote]

Already swiftly coming to this conclusion. Very good advice!

[Ian Savage]

[quote name='discreet' post='843731' date='May 21 2010, 12:29 AM']Already swiftly coming to this conclusion. Very good advice! [/quote]

I wish more musicians had come to that conclusion already Personally when doing live sound I have a toolbox containing EVERYTHING that might be needed to keep the gig going (fuses, screwdrivers, guitar strings, full soldering setup, spare cables and adaptors of all varieties, batteries, Allen keys, drum key, even a spare guitar strap - the works!) but I know that the majority of sound guys don't...

[TimR]

[quote name='discreet' post='843727' date='May 21 2010, 12:25 AM']Thanks very much, guys - I picked up a supply of 5A fuses from feeBay, but obviously these are of little use to me - I'll run out! Anyone want?

I've also ordered a shedful of 7A fuses, again off peeBay, and a 2m IEC cable rated at 13A, but have not had the opportunity to try them yet.

Yeah, EU... that's a whole other topic right there.

10A I didn't know about at all, but hey. Thanks for the offer of a 10A Ian, will gladly accept and add to the equation... PM'd.

TimR, thanks, it's a ProFet 5.1 I'm talking about, as you no doubt knew from my sig. Reassuring to know the problem may soon be history, thanks! 5A certainly does seem to be a bad idea...

Will let you know how it goes. Thanks again. [/quote]

I hadn't noticed your sig. I'm going to contact Warwick tomorrow. Seems a bit of a coincidence. Maybe there's a problem with the transformers. BUT why have a 5amp slow blow internally and then put a 5amp normal fuse in the lead?

What is also interesting me is that it appears that IEC connector on the amp may only be rated at 10amps, but I'm not sure that matters too much. There are also IEC leads that are only 0.75mm2 and therefore only rated at 5amps don't put a bigger fuse in them! If we are putting fuses bigger than 5amps in them we should be using 1mm2 leads.

More investigation required!

Edited May 21, 2010 by TimR

[discreet]

[quote name='TimR' post='843764' date='May 21 2010, 01:39 AM']More investigation required![/quote]

May be quite wrong here, but since peering closely at my IEC mains cables I've noticed the ones that are fitted with 5A fuses bear the legend '5A' at the plug end, but '10A' at the other end. Do I take it that this informs you firstly which fuse is fitted, and secondly, maximum ampereage handling for the cable? Or does it mean something else? Should I technically be able to use 5, 7 or 10A fuses with these cables?

TimR, I should point out that when the amp is getting power, it works perfectly well with no issues. The 5A fuse blows rarely, but inevitably it is very inconvenient when it does, and is not a great confidence-builder. I'm hoping my problem will be solved as yours seems to have been, by uprating the plug fuse.

Would still be very interested to know what Warwick say, though. Probably, 'Buy a 5.2'

[cameltoe]

My Hartke combo did this with it's 3A fuses. I put a 5A in and haven't had any problems since.

Weird

[TimR]

Haven't contacted Warwick yet. I'm not convinced that the lead I had came with the amp.

Inrush current is the current that flows in the transformer when you initially power it up. It can be up to 10x the transformer rating for 0.1s

The power rating of the transformer is 500W so 2A under full load but possibly up to 10A on start up. A 5A will blow straight away if you catch it on the wrong point of the cycle, but this is going to be random hence it being only occasional. A 10A fuse is going to be fine.

The important thing is to protect the lead as that's the most likely part to get dammaged and you want the fuse to blow before the lead catches fire. I'm sticking to a 13A cable 1mm2 and a 13amp fuse (only because 10A fuses seem to be difficult to get hold of, I'll swap it for a 10A when I next see one, or I'm at the electrical wholesalers). The amp is protected by the 6.3A slow blow fuse (marked as a 5A on the Warwick drawing!)

10A on the IEC relates to continuous current and is not an issue, the 5A sticker on the plug top relates only to the size of fuse fitted.

I am going to open my amp up on Monday to look at the transformer and associated wiring to see what I can see.

Edited May 21, 2010 by TimR

[discreet]

[quote name='TimR' post='844166' date='May 21 2010, 02:01 PM']10A on the IEC relates to continuous current and is not an issue, the 5A sticker on the plug top relates only to the size of fuse fitted.
I am going to open my amp up on Monday to look at the transformer and associated wiring to see what I can see.[/quote]

Thanks for this. Let me know what you see in your amp...

Edited May 21, 2010 by discreet

[4-string-thing]

Not sure if this is relevant, but any electrical wholesaler such as the one I work for, will be able to sell you all sorts of fuses and give you good advice on slow blow and time delay fuses etc.

You could try glass fuses instead of the ceramic ones you normally get for 13a plugs, these are obviously see-through and allow you you to see if the fuse has melted or blown. If there is still fuse wire visible inside the fuse (just broken in the middle) then the fuse has melted, usually caused by overload, ie, the fuse is rated too low for the current drawn. If there is no visible fuse wire and the inside of the glass looks burnt or brown in colour, then it has blown, usually due some kind of short circuit.

If it's just melted, you could uprate it by a couple of amps and try again, if it's blown it would be best to get it looked at.

[discreet]

[quote name='4-string-thing' post='844619' date='May 21 2010, 09:07 PM']You could try glass fuses instead of the ceramic ones you normally get for 13a plugs, these are obviously see-through and allow you you to see if the fuse has melted or blown. If there is still fuse wire visible inside the fuse (just broken in the middle) then the fuse has melted, usually caused by overload, ie, the fuse is rated too low for the current drawn. If there is no visible fuse wire and the inside of the glass looks burnt or brown in colour, then it has blown, usually due some kind of short circuit.
If it's just melted, you could uprate it by a couple of amps and try again, if it's blown it would be best to get it looked at.[/quote]

Thanks, this could be a good way to find out more about what's happening...

EDIT: Am currently trying a 13A cable with a 7A fuse - all OK so far - time will tell.

Edited May 24, 2010 by discreet

[TimR]

The transformer has 4 taps; 2@59v rated at 5.2A and 2@18v rated at 0.63A. The taps are combined to give 118v and 36v. So it looks like its a 640VA transformer not a 500VA one!

500W is on the limit of what you would put a soft start or startup current limiter on. The 5.1/5.2 don't have one, the 10.1 Extreme does! Warwick are obviously trying to keep costs down here. They do have a delay before the speakers are switched on though, which is nice of them. The screws holding the case together are shocking quality as well but the chrome knobs are a nice touch. Still it's a really nice sounding amp for the price.

[discreet]

[quote name='TimR' post='845774' date='May 23 2010, 02:33 PM']The transformer has 4 taps; 2@59v rated at 5.2A and 2@18v rated at 0.63A. The taps are combined to give 118v and 36v. So it looks like its a 640VA transformer not a 500VA one![/quote]

Er... bit techy for me, but if I understand rightly, VA (Volt-Ampere) is a measure of power in a circuit, so a 640VA transformer is more likely to blow a fuse than a 500VA transformer at power-up, without a current limiter? Or am I just mincing about in a bikini because I'm mentally ill? Thanks!

[TimR]

[quote name='discreet' post='845784' date='May 23 2010, 02:45 PM']Er... bit techy for me, but if I understand rightly, VA (Volt-Ampere) is a measure of power in a circuit, so a 640VA transformer is more likely to blow a fuse than a 500VA transformer at power-up, without a current limiter? Or am I just mincing about in a bikini because I'm mentally ill? Thanks![/quote]

Yes VA is Watts in this case. I'm guessing that the transformer has to be rated higher than the power amp and from what I've read it is the transformer rating that governs the inrush current, not the power amp rating.

Anyway, I posed a Q on the Warwick support forum. Its Sunday in Germany so I guess all their designers are out in the sunshine on the Nurberg Ring in their Porches. Hopefully,knowing their efficiency, they will reply at 8.01 tomorrow morning.

[url="http://forum.warwick.de/20-warwick-amp-customer-support/11987-pro-fet-5-1-fuses.html#post185930"]http://forum.warwick.de/20-warwick-amp-cus...html#post185930[/url]

Edited May 23, 2010 by TimR