What causes external fuses to blow?

[walknbluez]

I have two different amps, a SS SWR and an all tube Peavey VB-2. I've experienced both blowing the external fuse (slow blow fuse), each at different locations. The last time was just last night on the Peavey. Volume levels were not that high.....gain on 5 and master on 3-4. What is the cause of external fuses blowing?

[walknbluez]

bump for answers from our tb amp gurus

[Rick Auricchio]

What cabinets?

A lower-than-usual cab impedance can cause a high current flow, which would blow the fuse.

[anderbass]

If both amps blew fuses while using the same cab or speaker cable, I'd check that cabs jack/wiring and the speaker cable very closely for any situation that could cause an intermittent short. (like where the insulation separating the + and - wires failed and allows them to touch each other at times)

Did you already double check that your speaker cable isn't actually a guitar cord?

[walknbluez]

Originally Posted by Rick Auricchio

What cabinets?

A lower-than-usual cab impedance can cause a high current flow, which would blow the fuse.

I was playing the Peavey VB-2 on the 4 ohm setting through a 4 ohm Avatar 212 neo.

[walknbluez]

Originally Posted by anderbass

If both amps blew fuses while using the same cab or speaker cable, I'd check that cabs jack/wiring and the speaker cable very closely for any situation that could cause an intermittent short. (like where the insulation separating the + and - wires failed and allows them to touch each other at times)

Did you already double check that your speaker cable isn't actually a guitar cord?

Yes, it is a speaker cable.

I would have to take off the metal plate on the back of the cab where the jack is. I just checked it and there are two jacks on the back of the Avatar cab. There is no indication that one is in and one is out and I was told it didn't matter which one I plugged into. So I've always plugged into the left one. However, just now I noticed there is a little "wiggle room" whereas the jack on the right is tight. Could that be it?

[anderbass]

Take a look, If its ever shorted enough to blow a fuse you'll probably find black soot residue. If your using a 1/4" speaker cable, unscrew each end and make sure you dont have a potential problem there too.

Yeah Parallel speaker jacks are just wired together inside so it doesn't matter which one is in or out.

[dmusic148]

Remember fuses blow due to high current flow. So, if the venue voltage goes down, current goes up. I'm willing to bet it's due to undervoltage at the venue- very common..

[walknbluez]

Originally Posted by dmusic148

Remember fuses blow due to high current flow. So, if the venue voltage goes down, current goes up. I'm willing to bet it's due to undervoltage at the venue- very common..

It happened at a warehouse that is setup with individual rehearsal studios. Our space is used by two bands (our drummer is in both).

Perhaps a power conditioner would be a good idea?

[Rick Auricchio]

If the voltage dropped enough to cause the fuse to blow due to higher current flow, a conditioner won't help.

Virtually all power conditioners simply kill high-voltage surges, like any surge strip.

[dmusic148]

Originally Posted by Rick Auricchio

If the voltage dropped enough to cause the fuse to blow due to higher current flow, a conditioner won't help.

Virtually all power conditioners simply kill high-voltage surges, like any surge strip.

Right. There are VERY expensive ones that'll handle brownouts, but they're, well, very expensive. Like a grand or more.

[billfitzmaurice]

Originally Posted by walknbluez

However, just now I noticed there is a little "wiggle room" whereas the jack on the right is tight. Could that be it?

It could. A loose connection means higher current draw. I'd get rid of the 1/4" for Speakon, this is one of the deficiencies that Speakons fix.

[walknbluez]

Originally Posted by billfitzmaurice

It could. A loose connection means higher current draw. I'd get rid of the 1/4" for Speakon, this is one of the deficiencies that Speakons fix.

Or I could just plug into the side that's not loose?

[Jerrold Tiers]

Fuses are actually reasonably complex items......................

obviously, overcurrent causes them to blow.

Heat causes them to blow

Vibration causes them to blow.

Turn-on surges can eventually damage the fuse and cause it to later blow "early" due to heat and cumulative damage from surges.....

A fuse, even of the right nominal ampere rating can be very different from another amp rating, or the same amp rating from another manufacturer.... One may be quite safe at a given turn-on surge, the other may have a very short life with turn-on surges.

Anything from 10 to a few hundred surges and an improperly chosen fuse can be "primed" to blow. Then any stress, the next turn-on, or heat and reasonable ampere draw, etc, can cause it to blow. After some time playing, the amp and the fuse are hot, accelerating failure, so heat is a definite factor.

[walknbluez]

One of the reasons I'm asking this is because I had the faulty internal fuse of my Peavey VB-2 replaced and then got it back and then the transformer went out (had a short). The transformer was replaced, the amp was biased (conservatively) and tested. Then after using it two times, the external fuse blew. I'm just wondering if there is something wrong with the amp again.

[Jerrold Tiers]

1) if a replacement fuse blows again immediately.... yes you have a problem

2) if a replacement fuse blows, but takes a while, a number of uses / turn-on cycles, "maybe" you have a problem, or "maybe" you just have the wrong type fuse.