Speaker Cables vs. Guitar Cords
A perennial question: "Can I use a guitar cord as a speaker cable?" The correct answer is "Don't ever do that!" Yet there always seems to be someone who says they tried it and nothing bad happened to them. Luck of the draw, but it's still just dumb luck... Here's why you should never use a guitar cord in place of a speaker cable:
If you dime a 50-watt amp into an 8-ohm speaker, you're pushing 2.5 amps through what had better be a speaker cable.
The ability of a cable to handle current depends upon the thickness of the wire and the length of the cable. Most guitar cords use 22- or 24-guage wire, whereas even inexpensive speaker cords use 18- or 16-guage wire (some go even heavier).
24-guage wire is conservatively rated to carry about 3 amps in open air - the rating is less when it's part of a cable. 18-guage wire is rated to carry about 16 amps in free air. That's why you see so many short household extension cords made from 18-guage wire. With 2.5 amps flowing, 24-guage wire won't noticeably heat up. And at bedroom volumes, when the power level probably doesn't exceed 5 watts, the current-carrying capacity of a 24-guage wire is plenty safe.
However, those thin wires flex and break. Have you ever had a guitar cord go bad on you? It was probably because the wires broke, either at the jack or inside the cable at a stress point. Before the cable goes completely dead, though, a lot of strands have to break. A 24-guage stranded wire might be made up of many strands of 34-guage wire. Right before the cable fails completely, one of those 34-guage strands will carry a guitar signal without problem. But that strand can only carry about 1/3 amp of current. Plug that between your amp and speaker, and chances are that the remaining strand will overload and fail. An open speaker circuit is bad for tube amps. It can cause the output transformer to fail.
So, do you really want to take a chance that your spare guitar cord is on its last strand, so to speak? Even if you have money to throw away on your amp tech, you've gotta think about the down time.
So, don't ever use a guitar cord as a speaker cable. And do check your speaker cables occasionally - if you find frayed connections, resolder or replace the cable.