Today we are having a brief class on fuses courtesy of Sweetwater Sound technical guru Mark (MacGyver) Phillips.
A fuse’s job is to stop the equipment from catching on fire AFTER it has failed, NOT to save the equipment from failing. If a fuse “opens” or “blows,” replace it ONLY with the same type and size fuse. Remember that the fuse prevents fires so it must be replaced with one of the same type and size. On rare occasions an external “event” will cause a fuse to open, but most of the time the fuse opens because the equipment has failed.
You will find the type, voltage and current rating printed next to the fuse holder.
There are two types and two sizes popular in electronic gear:
SIZE:
- 3AG (fast-acting) and MDL (Slo-Blo) 1/4″dia x 1 1/4″ long. This is the older style but it’s still popular in amps.
- GMA (fast-acting) and GMC (Time Lag) 5mm diameter x 20mm long.
TYPE:
- Slo-Blo (Time Lag). These fuses are most often found in gear that has tubes and in large power amps. The “in-rush” current on initial power up of this kind of equipment is very high, so the fuse must have a mechanical shock absorber to keep it from blowing when the unit is turned on. You can often see a little spring wound inside the fuses glass case, that’s the shock absorber.
- Fast Acting. This type is most often found in solid state gear.
Fuse voltages are usually 125 or 250 volts. DO NOT USE CAR FUSES they are rated at 32 volts DC.
Current will range from 1/16 Amp to 20 Amp.
NEVER, NEVER, NEVER replace an open, fast-acting fuse with a Slo-Blo fuse, even if it has the same current and voltage ratings, you will damage something else and you may start a fire.
Notes:
- Fuses often have the type, current and voltage stamped on the end caps.
- 5mm x 20mm fuses often use the letter “T” in the end cap stamping to indicate “Time-Lag”. The “T” will precede the current rating, such as T5A, which means Slo-Blo, 5 Amp.
- Before replacing a fuse, take a look at it. Look to see if the halves of the fuse wire inside the glass are just broken, with a small gap. If so I would say a little prayer and replace this fuse because as it may have blown from some event besides equipment failure. However, if the glass is black and silver don’t even bother to waist another fuse. That means the current was so large that the fuse wire has evaporated and plated the inside of the glass. That piece of gear is most assuredly “dead” and must be serviced.