Amp Fuses

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 lead 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 either within, or on the front or rear control panel of the amp. This will appear as a round screw in cartridge or built into the IEC inlet connector.

Some but not all amps have a rectangular mains input connector called an IEC inlet. Commonly, but incorrectly, also known as a kettle lead connector. Many of these IEC inputs have a small fuse drawer that slides in and out and carries a little mains fuse, and has room for a spare fuse too.

Mains fuses don’t blow too often so the only reason I would advise blindly replacing is perhaps after using a spikey power source like a generator.

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.

The HT fuse is typically a glass fuse and when it blows you can get a clue from how it has blown. If the glass is clear but the fuse wire has broken, then this indicates that it wasn’t a particularly violent blow, so it is possible that a brief surge occurred or that the fuse rating is simply too close to the normal operating range. Fuses also drift off in value over time with older fuses being a little more sensitive.

If the fuse blows and when you look at it you see a glass cartridge blackened or perhaps brown then the fuse had blown violently. Typically this happens in a valve amp when power valves are at end of life. What many people do is replace the fuse and the amp works fine for a while before blowing again two weeks later.

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 twins, hot rods since approx 2012.

Fuse Sizes and Values

Fuses come in two main physical sizes, 1 and 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:

  • 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. 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.