How to Checking and Changing the Fuse in Your Tube Amplifier – Reviews and Tips

A blown line fuse could be the source of your tube (or solid state) amplifier’s failure to “switch on.” How to Checking and Changing the Fuse in Your Tube Amplifier. The line fuse, also known as a mains fuse, is the fuse that connects your power transformer to the main AC supply. When there is a problem between the wall outlet and your power transformer, the main fuse can and will blow in an otherwise operating amp (PT). When there is a short somewhere in the amp, a fuse can also blow. In either instance, when the fuse blows, it is doing its job and safeguarding the amplifier by preventing it from running when a fault exists that could destroy the circuit.

Tips for checking and changing the Fuse in Your Tube Amplifier:

If you’re using a known-good wall outlet (and if you’re using a surge protector, make sure it’s working), and your amp has a working power cable, but it still won’t come on, you should check your fuse. Before attempting to remove the fuse cap, make sure your amp is turned off and disconnected, as with all other amp inspection checks we discuss. Removing the fuse is a shock danger otherwise.

The fuse holder (seen above in a Fender Super Six) is usually found near where the power cord enters the amplifier. Almost always, the fuse cap will be the type that requires inward pressure before unscrewing (counterclockwise). If you don’t apply inward pressure, the fuse holder will spin, twisting the main wires connecting to it, potentially causing a worse problem than the one you started with. Consider the safety cap found on most prescription medicine or Advil bottles; fuse caps have a similar purpose.

Once you’ve removed the fuse cap, the fuse will usually be kept in place by spring pressure and remain in the cap. Your fuse will very certainly be a 0.25″x1.25″ (or 3AG) glass cylinder slow-blow fuse with a 1 to 3 Amp rating. Under normal circumstances, your fuse allows power to pass through a narrow filament within the cylinder. When the fuse no longer likes what it sees, it will safely blow inside the glass cylinder, disconnecting the electricity.

• INSPECT THE FUSE VISUALLY

If you have decent eyes and a well-lit environment, you can visually study the filament within the glass cylinder. Newer slow-blow fuses have an almost hair-thin filament, and a blown fuse will be slightly burned and fractured if you look closely. When a connection is present, the spring on older (vintage) fuses is outstretched, and when blown, the connection is more visibly removed (they actually will make a quiet snap sound when blowing).

• USE A MULTI METER TO TEST THE FUSE.

If you can’t see if your fuse is blown, use a multimeter to check continuity (the symbol for this is similar to a modern wifi symbol or a’sound/audio wave’ symbol) and touch the probes to both ends. You have a working fuse if you hear the continuity beep. If there is no beep, there is no connection, and the meter will read “OL.”

That’s all there is to it: those are the only two methods for checking a fuse. If you don’t have a multimeter and can’t see the filament, I suppose you might connect it to a potato or something, although I’ve never attempted it. To be honest, if you can’t see it and don’t have a multi-meter, I’m not sure what to tell you… Invest in a multi-meter. Grab a few new fuses while you’re at it.

If you’ve confirmed that your fuse is still good and has continuity, our article on typical tube amp faults will help you narrow down the precise problem your amp is experiencing.

• FUSE REPLACEMENT

If you suspect your fuse is blown, always replace it with a fuse of the same amperage rating. The fuse amperage is normally posted next to the fuse holder on the chassis. If the chassis does not have any fuse size information written on it, the schematic would be the next place to go. Finally, if the schematic isn’t available, the amperage is stamped into one of the fuse’s metal ends. There’s a chance the fuse that blew was an out-of-spec replacement that a prior mechanic or owner put in there in a pinch if you’re going by the fuse size stated on the fuse itself and can’t otherwise definitively check the fuse size either on the chassis or the schematic. Replacing a 1 Amp fuse with a 3 Amp fuse, for example, can be dangerous because the 3 Amp will not explode as soon as the amp designer intended in the event of a malfunction.

• WHAT IF YOUR AMP’S INTERNAL FUSE BURNS?

Almost all small single ended vintage amplifiers have only one fuse: the mains fuse. One or more additional fuses are frequently found inside the amp chassis of higher power, more sophisticated, or push-pull amplifiers. These will be inline fuses, either installed on the board or held in a specialized in-line fuse holder. These are intended to safeguard a specific section of the circuit. Only a section of your circuit will be affected if one of these fuses blows. As a result, your amplifier will continue to operate and the pilot indicator light will remain illuminated. It’s possible that the amp has stopped producing sound, or that it’s still producing distorted sound.

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Final Verdict

How to Checking and Changing the Fuse in Your Tube Amplifier ? It will be more difficult to tell if one of these fuses has blown from the outside of the amplifier. To check for continuity, you’ll apply the same methods and principles as before. Although some of these fuses are hard-wired into the amp’s circuit, the continuity check will still function when they are connected. Working within an amplifier’s chassis necessitates double-checking that the capacitors are properly drained and that the amplifier is, of course, unplugged. If you are unsure about carefully investigating the inside of your amplifier, this is a job you should delegate to an expert technician.