There can be several fuses on an amplifier. They are there to protect against serious damage by breaking a part of the circuit when some fault occurs. So when a fuse blows, don’t replace it blindly but make sure you understand the reason for it blowing.
Here are the fuses to be aware of:
Mains plug fuse. In the UK we have a fuse in the mains plug. This will usually be the last thing to blow and is really there to protect against fire damage should the mains lead get damaged.
Most amps have a mains fuse on the front or rear panel. This fuse protects against a number of fault conditions in the amp and is kind of a last resort for a serious failure in the amp. Typically if the amp draws about 500mA in normal use then the mains fuse will be 3 or 4 times that value. It can take a number of forms: it can be internal, enclosed in a circular screwdriver or thumbwheel contained on the rear, or front panel or if your amp has an IEC mains inlet socket (kettle lead) then you will likely find a fuse holder drawer built into the socket. This little drawer can contain the active fuse and a spare one.
When fuses blow there are two distince modes of blowing… BLOWING usually has visi8ble evidence in the form of black or brown smoke in within the glass and then just breaking with the glass looking clean. This is a very good indicator of how you should treat a fuse blow. If it is smokey in appearance then it means that fuse has blown due to a massive current inrush and something is very wrong in the amp but if it a clear blow, then it might be just a marginal thing e.g. like a power surge or a fuse being outside its working rating. Fuses also drift off in value over time with older fuses being a little more sensitive.
Mains fuses in the amp do not blow too often so when they do you need to think about what is the reason for this. If you have been exposed to a bad power source like a generator then that will sometimes cause the fuse to go and there is no other issue. But that is on a good day, so expect that when a fuse goes you need to sit up and listen. The fuse is telling you that there is a fault in there.
HT Fuse – stands for high tension, and meaning high voltage, is a fuse that protects the amp from damage when a valve shorts internally. In a valve amp it is possible for an internal short to occur within the valve. When this happens amps with am HT fuse will blow the fuse. Amps without an HT fuse will potentially do damage to the mains transformer or other sections.
Common causes of mains fuse blowing include:
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- Internal short in the valve
- Short circuited mains filter
- Shorted power transformer
The HT fuse in an amp is only applying to the HT (High Tension) voltage in he power supply that powers the valves. What many people do when the HT fuse goes is replace it and find the amp is fine again… but then it goes again two weeks later.
Typical reasons for HT fuse blowing are:
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- power valves at end of life and occasionally shorting out internally.
- pre-amp fuse with an internal short
- HT supply problem like a bad cap
- HT supply problem like a bad rectifier
There can be other fuses too. For example many mains transformers have a built in fuse. It is not replaceable and is there as a fire prevention measure.
Heater fuses are commonly found on Marshal amps eg JCM2000 range and Fender too on later reissue twins, hot rods since approx 2012. This is a safety precaution to protect the power transformer in the event of tube failure affecting the heater filament. But because of the heavy current found in the heater circuit the fuse connections frequently struggle to carry it and cause their own issues.
Resettable fuses are used in a few amps. One notable example is Laney. Laney uses something called a thermal fuse that detects over current and turns the amp off for about 30 minutes. There have been reliability issues with this kind of circuit protection. Replacing this thermal fuse is a simple enough job if you have a Laney that will not behave.
Fuse Sizes and Values
Fuses come in two main physical sizes, 1-1/4″ (32mm) long in older amps and 20mm long in more modern amps..
They also have a current rating and speed characteristic. The current rating defines how much current the fuse can handle before blowing and the speed indicates whether it will blow quickly or slowly.
Speed is usually labelled on the fuse as either:
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- F for Fast
- T for Trage, Time Delay or Slow Blow.
Typical fuse values would be F2A meaning fast two amp or T500mA meaning half an amp.
You can buy fuses in packs of 10 for about £3-5. Worth having spares at least of your HT fuse just for an interim fix.
A really bad idea is to replace a blown fuse with a larger value and hope it was a one off fault. It might be just that, but it is an expensive gamble.