What causes a fuse to blow?

lotsolowend · #1 02-27-2011, 12:17 AM

I recently purchased a Mesa D-180 in EXCELLENT condition. It has played fine at rehearsals and one gig a few weeks ago. Sounds amazing.

I was at a gig last night, and about halfway through our set (while i was noticing how good the amp sounded ironically enough) the bass began to fade out so I leaned in front of my amp to hear what the deal was. The sound continued to fade and the amp lost power. I switched to DI and finished the gig so it was mostly just an embarrassing situation.

When I got the amp home today, the first thing I checked was the fuse. It looked like this:

Luckily, I had a replacement fuse handy. Replaced the fuse and it fired up without any problems.

I understand that fuses get worn out, and since this amp is new to me I don't know how old this particular fuse was. My question is: What could cause a fuse to blow? Maybe I was pushing the amp too hard? Perhaps the amp got so hot that the fuse expanded and broke? Could it be tube problem that caused the fuse to blow?

At any rate, I won't suspect that I have a problem unless this fuse blows in the near future.

SurferJoe46 · #2 02-27-2011, 12:26 AM

Fuses do NOT get worn out - no moving parts. All they do is detect over-current conditions and they aren't like a lightbulb or fresh bread.

A fuse blows for a reason - either it's defective or it really REALLY sees something's wrong.

If it blows another - then you have two options - neither are good, but you need to find out why and if you are not an electrical-type person, then you haul your used unit to an amplifier repairman - OR you go get a new unit that hasn't got a mysterious background of possible abuse, misuse or hate by the previous owner and will also have a warranty.

It is used and someone sold it for a reason - however varied that may be - but someone didn't like it when it was new - what chance is there that it's 100% OK now?

Did they need the money?
Was it a lemon?
Did it blow 60 fuses for them and they got tired of it?
Was it the wrong color?

Who knows??? - Youse takes yer chances with used gear.

dincz · #3 02-27-2011, 02:05 AM

Quote:

Originally Posted by SurferJoe46

Fuses do NOT get worn out - no moving parts.

Repeated expansion and contraction as they heat up and cool down can result in metal fatigue.

Caca de Kick · #4 02-27-2011, 03:37 AM

BS, fuses don't wear out.

My experience with the several older Mesa tube heads:
power tube failure, or broken grid resistor.

dincz · #5 02-27-2011, 04:12 AM

Quote:

Originally Posted by Caca de Kick

BS, fuses don't wear out.

Really? So what in your informed opinion makes them immune to metal fatigue? You should do some research before you post.

Caca de Kick · #6 02-27-2011, 04:42 AM

?

A Mesa head does not develop any detrimental heat on the front face panel where the fuse is mounted to fatique the fuse. And likewise I have inspected many vintage amps that still have their original or very-very old fuses in them.
Also....I restored many 60's cars that had their original glass fuses still in place and in flawless shape, and cars have seen more extreme temperature swings in the fuse panel area than an amp does. So yeah, I've dealt with glass fuses in more than one application in my life....that there is my "informed opinion" buddy!

Seriously dude, I've been playing these Mesa heads for 21 years now. Maybe you should have years of experience with them before spouting back to me. I've had to repair quite a few of these.

Jaco who? · #7 02-27-2011, 04:51 AM

I'm an idiot, but I do any scheduled maintenance on my automobiles myself mostly because I'm pretty poor and cheap by nature. I've never seen an owners manual for a vehicle (or an amplifier) that recommends any scheduled replacement of fuses. I would think that if metal fatigue was an issue, it would recommend a mileage/ number of hours usage when they should be changed. They don't. But like I said, I'm an idiot.

dincz · #8 02-27-2011, 04:57 AM

Quote:

Originally Posted by Caca de Kick

A Mesa head does not develop any detrimental heat on the front face panel where the fuse is mounted to fatique the fuse.

Seriously dude, I've been playing these Mesa heads for 21 years now.

Fuses work because they generate heat when they pass current - just like any conductor. There's no escape from physics. They don't have to be mounted in a hot location to get hot.

Seriously dude, I've been using and repairing a wide range of audio and RF electronic gear for a very long time and it sometimes happens.

I'm not saying this is the case in the OP's amp - just that your "BS" comment is BS.

MAMMOTHvolume · #9 02-27-2011, 05:02 AM

fuses don't go because of old age they go because somethings gone wrong. Sounds like a valve may have flashed over indicating it's on its way out.

4to5to6 · #10 02-27-2011, 05:04 AM

I'm not familiar with the Mesa D-180. Is this a tube amp or solid state? You may have a rectifier tube going (shorting out). In both tube and transistor amps a capacitor going would blow the fuse as well which could be indicated by bulging or leaking of the bad cap. The plastic insulation where the transistors mount can get a hole blown in them although very rare but very intermittent and hard to find. There's lots that go wrong depending on the circuit and components.

I would get this amp to a tech to check out before it really blows. Don't risk it.

Although it is theoretically possible that a fuse could wear out through thermal expansion and vibration it is extremely unlikely. Fuses have been around for a long time with the bugs being worked out a long time ago.

Looking at the photo, this is a slow blow type fuse and I would guess this fuse had a lot of current going throughout it when it blew.

dincz · #11 02-27-2011, 05:06 AM

Quote:

Originally Posted by MAMMOTHvolume

fuses don't go because of old age they go because somethings gone wrong.

Almost always, yes.

billfitzmaurice · #12 02-27-2011, 06:33 AM

Fuses do on occasion fail for no apparent reason. Replace it with the correct value and type, and don't be overly concerned unless it blows again. If that happens it's tech time.

Rockmusician · #13 02-27-2011, 06:47 AM

Fuses do wear out. When your amp is powered on, there is current flowing through the fuse. They are subject to metal fatigue. Ever have a fuse blow in your car? When you replace it, everything works again. I have only had a fuse blow out once. I replaced it with a new fuse and everything worked perfectly.
However, fuses can blow due being over loaded or if there is a short somewhere.

Jostein · #14 02-27-2011, 06:47 AM

I've had the fuses blown on my powered PA speakers a few times. Mostly when someone pulled out the power chord without switching the unit off first. And once because my bandmate brought a power strip that had a piece of metal flying around inside, he thought "it wouldn't matter".

Just saying the fault isn't necessarily inside your amp.

**xed** · #15 02-27-2011, 06:59 AM

Question #1: was the fuse that went bad the CORRECT fuse that was supposed to be in that amp? Maybe the previous owner decided to bandaid the amp with a non spec fuse to limp through a gig or something. If non spec then replace with correct fuse and then see what happens.

**f64** · #16 02-27-2011, 07:06 AM

The other thing you’ll want to check here is the schematic or printing adjacent to the fuse holder for the correct size (resistance). A really dangerous situation is created when someone increases the size to a higher load to get more life before it blows. I've run across this when someone is dumping an amp that they know blows fuses. Buying used amps this is one of my first check points. What’s dangerous is that if there is an issue you've now moved what is going to fail to a component in the amp. The fuse is there for a reason. It becomes the insurance policy against going up in smoke. Good Luck!

Bassmec · #17 02-27-2011, 07:17 AM

There are two types of fuse used in amps fast and delayed/ slo-blow,
both types are known to fail occasionally due to repeated heat cycles.
I would check the value and type of fuse against the manufactures spec. Then run the amp on test checking for red plating because I think you may have an output tube that is failing.
Is this a fixed Fixed Bias amp like other Mesa tube amps
or has it been altered to fixed variable bias?.

PJSShearer · #18 02-27-2011, 07:24 AM

+1 - make sure the fuse that blew was the coreect one and, either way, take your amp to a pro for a check over.

Quite often if a fuse blows people will replace it with what they have to hand at the time, rather than the correct replacement - these tend to pop in the middle of a gig. Usually people have the best intentions - I WILL go and get the correct fuse when Radio Shack opens tomorrow and put it in, but ... I've seen a fuse wrapped in the foil from a cigarette pack as a replacement and the guy wondered why he was seeing smoke coming out of his amp a few gigs later.... not funny !

lotsolowend · #19 02-27-2011, 01:29 PM

Wow, thanks for all the responses - and sorry if i stirred some conflict about whether or not fuses age!

This amp, although it's used, is in excellent condition - that much is true. The original owner took fantastic care of it and it was hardly gigged. I don't know very much about tube biasing but I know the tubes have been biased by a professional (haha, sorry for my ignorance). I'm not sure what red plating is but i'll do some research. I replaced the fuse with the correct value according to the specifications on the schematic and the front chassis of the amplifier (6.25 amperes). Can't remember which voltage I grabbed but it was enough (either 240 or 120 i think - both more than enough for this amp). The original owner supplied a "spare-parts" kit consisting of a matched pair of power tubes, some covers, replacement knobs, resistors, and *fuses* (thank god) -- another testament to his care for the amp.

Thanks for all the responses, I'll do some more research and be sure to get it checked out if the fuse blows again. I'm really grateful some genius decided to put a fuse there, otherwise I'd REALLY be in trouble!

anderbass · #20 02-27-2011, 04:52 PM

lotsolowend, there's lots of possibilities but one real easy thing you should check is to try pushing all your powertubes down into they're sockets. Sometimes they can work their way loose part way out of the socket and arc causing the fuse to blow. This has happened to me twice, once with a Mesa 400+ and also to my old Fender Twin Reverb.

If you find any powertubes nearly falling out of their sockets, use a magnifying glass to inspect the tubes pin connectors. If you find any with a small melted spot surrounded with a little black soot you've just found the reason your fuse blew. You'd then just need to squeeze each half of the tube retaining clamps in tighter so they grip the tube basses, and then carefully but firmly plug your power tubes all the way back in.