Dumb fuse question

[MikeMcK]

I have an old Traynor and when I happened to be in Rat Shack I grabbed some spare fuses.

For some reason I thought this amp said "SLO-BLO" next to the fuse receptacle and that's what I got. Should SLO-BLO fuses only be used where they're specifically called out?

[67super]

Typically you want to use SLO-BLO in a guitar amp. Fast blow fuses will blow just turning the amp on.

[VaughnC]

Generally speaking, a tube amp should have a slo-blo type fuse for the mains. Its purpose is to allow for the brief current surge as the tube heaters warm up. A standard fuse might work and it can be argued that it will offer slightly better protection in case of a circuit short...but, due to the tubes cold filament surge, a standard fuse may occasionally blow when you first turn the amp on.

[Pearly Gator]

There is a BIG difference between fast blow and slow blow fuses. Use only the type and rating suggested by the manufacturer.

A fast blow fuse will open when current exceeds the marked rating. A slow blowing type can allow up to a 100% overload before going open. For example, a 2A slo-blo may not open until it exceeds 4 amps depending on the duration of the overload.

Next, pay attention to the voltage rating of the fuse. Do not use low voltage rated (IE - automotive type) fuses in high voltage circuits. (100 volts and above.) This is because an open low voltage fuse could arc after blowing in a high voltage circuit.

Sorry for the long winded post. Last week I was given a free Ampeg VT120 amp by a customer who put a 15 amp fuse in place of the 5 amp mains fuse. The repair estimate exceeded the value of the amp. An output tube shorted, taking out the cathode bias resistor, the negative bias circuit went next and, with no where else to go, the current burned up the filament hum divider circuit. Various areas of the PCB turned to charcoal. The 15 amp fuse was the only thing that survived. $350 in amp repairs blew to spare a 50 cent fuse.

Gary

[John Phillips]

+10 to everything Gary said. Like most techs I expect, I've also many times had to do (or not do) repairs costing hundreds of dollars because someone put a way over-high fuse - or wrapped the old one in foil or something - in an amp with an initial fault that might have been as simple as cheap to fix as a blown tube or rectifier diode.

Do NOT EVER increase the fuse value to 'get you through the gig' or because you don't have the right one handy. All that will happen is that it still won't get you through the gig, and probably seriously damage the amp as well. A fuse blowing is almost always (99% or more of the time) a sign of something else wrong, not a 'bad fuse'.

The exception is using a fast-blow fuse (of the same value) in place of a slow-blow - the worst that will happen is that the fuse will just blow again. I would probably risk a slow-blow in a fast-blow application too if you have no other choice, because a continuous overload will still blow the slow-blow one, probably before any other damage occurs. But it's still a risk. (And not in any amp which specifically says 'Do Not Use Slo-Blo', specifically.)

In fact, the reason slow-blow fuses exist is actually to offer better protection, because it allows you to use a lower value than you would otherwise have to in order to keep a fast-blow fuse from tripping on the power-up surge.

Mains (power, primary) fuses in tube guitar amps are usually slow-blow. HT fuses are often fast-blow, although not always. If there's only one fuse it will be a primary.


This was not a dumb question BTW. The dumb question is "oh... was it because I put a car fuse in my amp that it's now going to need a new PT/circuit board/something else expensive?" :).

[SatelliteAmps]

Also, never, ever, ever, ever solder a wire to bypass the fuse. Had a mint blackface Fender amp that came into our shop once that had melted it's PT. A customer had recently purchased it from a local store. Couldn't figure out why the fuse didn't pop. Took about 15 minutes to find that someone knew the amp had problems, but wanted to sell it and carefully soldered a solid wire underneath the fuse holder bypassing the fuse. Frsutrating.

Fuses are rated to be exactly what is necessary in the amplifier. It's not jsut to keep things working, but also to help let technicians know what may be wrong with a broken amplifier.