Andrew

I just got a marshall JCM 2000 head, and a JCM 900 lead 1960 cabinet used the other day, and i think the cable he gave me to plug in the head to the cabinet was a guitar cable, so I used it for a couple days, then I found out that there is a special cable to use, because using a guitar cable can be really bad for it, so I ended up replacing the cable.

But I'm still curious what using a guitar cable can do to it, and how to find out if the amp has been damaged at all. The guy had it for four years, and I'm worried if he used a guitar cable for the for years he's had it. The clean sounds find, but the distortion I think doesn't sound very good, and when i bought it i thought if the clean sounded good, I'd just have to work with the distortion to get it how i wanted, so I'm having troubles getting it how to sound good.

So, if anyone can answer how the guitar cable effects my amp, and how i can tell if it has, and some good distortion settings/pedal recomendations for distortion, that'd be greatly appreciated. I play Experimental Hardcore for the band I'd need good distortion with.

JohnH

If it worked and the cable did not fry and it still works fine as a guitar cable, then there would be no damage done.

The risk is this: The guitar cable only has a fairly small amount of copper conductor, since it doesnt need much to carry the small signal from the guitar. Under the high currents used in a speaker output however, it could heat up too much and either break or melt a solder joint at one end. Then you have a broken cable which would mean that no speaker is connected to the amp. For a tube amp that is bad since the output transformer is trying to push out alot of power, which has nowhere to go so it blows the transformer instead.

But if there hasnt been such a problem so far, then its probably not damaged, just use proper heavy speaker wire from now on. Its not very tech, even good mains flex is OK.

John

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Wilder Amplification

Transformers don't "push out" power. The speaker draws current from the amplifier through the transformer, and current flowing through the impedance of the speaker creates a voltage drop across it. Voltage x Current = Power.

When the tubes draw current through the transformer primary, it creates a magnetic field around the primary coil. This magnetic field magnetizes the core of the transformer, and the intensity of this magnetic field rises as the signal goes more positive, falls when the signal goes less positive, then reverses magnetic polarity and rises and falls as the signal swings more and less negative. This core ends up becoming a magnet with a magnetic flux that fluctuates with input signal frequency, which induces current in the secondary. This current gets drawn by the speakers.

When the signal switches polarity from positive to negative, this creates a back voltage which is felt by both the tubes and the output transformer itself. You also have a back voltage being generated by the speaker as it moves in and out across its magnet. The damping factor of the amp is the measurement of how well the amp dissipates this voltage.

The problem is that when there's no load, this back voltage rises beyond the limits of the transformer and the tubes themselves. This is harmful to the valves as well as the transformer itself, and it's that excessive back voltage that ends up killing them.

On top of the core of signal cable being too thin to carry the current, shielded cable also has a low value of capacitance. The center conductor acts like the inner foil while the shield acts like the outer foil, while the insulator on the center conductor is the dielectric. This low cable capacitance appears as a short circuit to higher frequencies, which can also be detrimental to the transformer as well as it does affect the load seen by the amp.

Speaker cable is just a pair of individual heavy gauge wires that are designed to carry the current being drawn by the speaker. They don't have a shield wrapped around them like shielded cable does so a capacitor is not formed by paired wire.

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Jon Wilder
Wilder Amplification

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