amp tech's I need your help

[recto-robbie]

Hey, we plugged in a Peavey Valveking head earlier today and after 15 minutes or so, actually while looking at the head I saw a small flash of light and it went completely dead. No weird smells and no noise when it went off. Sounded pretty good for the 15 minutes.
We found that the fuse had blown, searched around for a replacement and found one same size. Amps original fuse was a quick blow 250v,5 amp and the replacement was a slow blow 250v, 2.5 amp.
Anyways we wound up trying it to see if the heater filaments would simply light,, they did,, and needless to say we went a step further to turning it on. It worked just fine,, we only played it for about 10 minutes with the incorrect fuse but it sounded just as good as before. We played it hard for the last few minutes to boot,, no issues.
What do you guys think could have been or is the problem?
I personally havent ever had a fuse blow in an amp.
Needless to say a new fuse will be bought for it before its turned on again.
Is this a common problem with some amps? Thanks for any help you can offer.

[Blue Strat]

Fuses cost about 1/10th of a cent to make. Consequently, they're not precision components.

Sometimes they just fail for no particular reason. Get the correct tube and try again. Also, be sure that the CORRECT fuse is not a slow blow. That can make a lot of difference.

[SatelliteAmps]

Fuses rarely fail for no particular reason. Only in extremely rare cases. Try replacing with the correct fuse, and playing it for 15+ minutes to see if the problem repeats. If not, then it is one of those rare cases. If it does repeat itself, then it is time to figure out why the fuse blew.

[Ronsonic]

In the trade it's called a "nuisance fuse blow." There isn't a tech alive who can accept such a thing without questioning it. We want to know the amp is okay. The use of a smaller fuse for testing is safe, a bigger one is not. Part of the checklist on my bench is to check fuses for correct value on everything that comes in. You'd be amazed at how many times I've found some 25A car fuse in there.

My official recommendation is to carry one (1) spare fuse of the correct value. If a fuse blows look at it, if the metal wire inside has plated itself onto the glass, it's likely you've got a real problem. If not, put in the spare. If it blows. Pack the amp up and go to a back up. Don't feed it fuses the fault won't heal. You'll just burn more parts.

[recto-robbie]

Thanks to all of you guys who helped,,, I wound up buying some correct replacement fuses and it seems to work just fine now.
Put some ear plugs in, cranked it up LOUD and played both clean and gain channels with pedals and without, boost pedals, you name it. This went on for an hour straight without a hiccup. Im hoping its safe to assume that it was a faulty fuse. Many thanks again, Rob

[donnyjaguar]

I've heard of a combination of dust in the amp and humid environment causing an arc. If you were playing balls-out you may not have heard the snap of the arc before the fuse blew. Gotta confess I've only seen this happen on old and shagged tv sets though.

[recto-robbie]

Thanks for the reply,, im not sure if thats what happened or not. At the time it blew I wasnt playing too loud, I didnt hear anything but I was actually looking at the amp while it happened. I simply saw a bright white flash exactly where the fuse is located. I believe that light I saw was from in between certain parts of the amp that didnt have a complete seal.
Anyways it had one of those white ceramic fuses in it originally and you cant tell if those blow. I swapped it with a new same spec glass fuse and played it hard yesterday with no issues. I searched the web for others who may have had the same issue with their VK and I did run into a few others that stated their fuse blew soon after buying it and no problems again after installing the replacement fuse.
Anyways the amp was bought brand new 2 weeks ago for back up and never opened so im not to sure its dust. Thanks again

[donnyjaguar]

At this point I'm leaning towards defective fuse and the problem is solved with the replacement. Let's just blame the Chinese sweat shop that made the original fuse. :)