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Did my ignorance fry my head?

Discussion in 'Amps and Cabs [BG]' started by Squidfinger, Apr 22, 2004.


  1. Squidfinger

    Squidfinger I wish I could sing like Rick Danko.

    Jan 7, 2004
    Shreveport LA
    I just found out a minute ago that you're not supposed to connect a head and cab with instrument cable. I did just that when I got my Peavey T-Max and 4X10 :eek: . I've only used combos before and just didn't know. The head is now fried and the cab sounds very distorted. Did I do that? :confused:

    *holds head in shame*
     
  2. Hurley

    Hurley

    Feb 12, 2004
    Cape Cod, MA
    I couldn't tell by the post if you used actual speaker cable afterwards. Did the head and cab sound weird with actual speaker cable? I don't know why it would damage the head or cab, but there could be a reason...I'm no expert. I would think it just fried the instrument cable.
     
  3. Ive been told that you arent supposed to, especially when running high powered heads, but Ive done it in a pinch, and never had a problem. The cables is the weakest link in the chain so I would definately start by checking that out first.
    Good luck, hope its something minor!
     
  4. Just checked my Ampeg owners manual, and it just says that you should use "high quality speaker cables" but it doesnt say anything about damage if you dont. Question: When you were playing it did it clip easily?
    BTW Squid: Im from New Orleans! (Been in Denver about a year now).
     
  5. Eric Moesle

    Eric Moesle

    Sep 21, 2001
    Columbus OH
    Doesn't using instrument cable as opposed to speaker cable decrease the impedance seen by the amp? Its like your amp is seeing a lower impedance than the cab's rating, which might be well below the amp's abilities, and therefore damage.
     
  6. Squidfinger

    Squidfinger I wish I could sing like Rick Danko.

    Jan 7, 2004
    Shreveport LA
    Yeah it did clip real easy. I haven't been able to try it with speaker cables since the head is completely FUBAR. I'm an electronics student though and sooner or later I'll get around to fixing it, I'm just too busy with my studies and work right now.

    Nice to meet another Louisiana boy Spanky :D ! Do you follow the NOLA metal scene at all? (Tungsten, Choke, THE ALMIGHTY CROWBAR!!!)
     
  7. The lighter-gauge instrument cable will technically have a higher DC resistance than heavier gauge cables. However, if running high current through an instrument cable, the instrument cable will heat up and can either burn up or short out. I suspect you may have shorted out the instrument cable, lowering the impedance seen by the head, and therefore possibly frying the head. :bawl:
     
  8. JMX

    JMX Vorsprung durch Technik

    Sep 4, 2000
    Cologne, Germany
    Now that's what I call an excellent headline. ;)

    It's not aimed against you, but I think it's a cool headline, can I borrow it for future use?
     
  9. Some lessons are learned the hard way. Sorry to hear that this happened to you. Fingers crossed that it's a minor problem.
     
  10. Benjamin Strange

    Benjamin Strange Commercial User

    Dec 25, 2002
    New Orleans, LA
    Owner / Tech: Strange Guitarworks
    This is not an impedance issue, it's a shielding issue. Instrument cables are shielded, speaker cable is not. Using a shielded cable to connect a head and a cab over a long period of time can cause the amp to overheat and blow. If you used one for a very long time that may have hurt your amp.

    There should be no impedance difference between a speaker cable and an instrument cable.
     
  11. Yep. Ive even jammed with some of the local heavy weights there. Grew up with a couple of 'em. The (tavern) Crowbar was one of my regular hangs for a long time. Kinda the "Secret" place the tourists dont usually find. The bands I hung around with most were Soilent Green, Suffocating Lindsay, Goatwhore, Down-Superjoint-Eyehategod-C.O.C-Pantera or whatever Phil, Pepper and Jimmy felt like doing that week.... :rolleyes:
    I played for Soul Ignition and the Quarter Dreadfuls (gigging) and jammed with countless others. If Denver ever goes sour Im headed straight for home!
    (Apologies for wandering off subject).
     
  12. Sorry basstriaxis, but you're not correct.

    Instrument cable is 20 or 22 gauge. A thousand feet of 22 gauge wire has a DC resistance of 16.5 ohms.

    Speaker cable would be 16 gauge or heavier, I like 12 gauge myself. A thousand feet of 12 gauge wire has a DC resistance of 1.62 ohms. The 22 gauge wire has 10 times more DC resistance. In addition, the plugs on instrument cables often have wimpy little connections that can't handle heavy currents.

    And the shielding has ablsolutely nothing to do with this. Shielding is imperative for low voltage, low current applications such as instrument cable. I could make a speaker cable out of 12 gauge shielded cable without any significant problems (although for long runs, the increased inductance and capacitance from the shielding could be undesirable because of its effect on the signal). From a power standpoint the shielding doesn't matter. Speaker cables aren't shielded simply because it's not necessary--the voltages and currents are so high that any EMI/RFI noise is unnoticeable.
     
  13. Benjamin Strange

    Benjamin Strange Commercial User

    Dec 25, 2002
    New Orleans, LA
    Owner / Tech: Strange Guitarworks
    Oh. Learn something new every day. I've read amp manuals from various manufacturers and they have mentioned that shielded cables are bad for amps. From now on I am going to get all my info from TB instead of manufacturers. Pleh. Do any of them give real info anymore?
     
  14. Munjibunga

    Munjibunga Total Hyper-Elite Member Gold Supporting Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    Speaker cable is at least 16 gauge, but 14, 12, or 10 is better. It's made to handle the high current flow between the head and cab. For example, if your amp is putting out 600 watts into 4 ohms, P=I^2 R, so 600/4=I^2, and I=12.2 amps.

    Now an instrument cable is usually 22 or 24 gauge (20 if you're lucky), and just can't handle 12 amps without melting. So what most likely happened is you fried the cable, it shorted across the output of your amp, reducing the impedance to 0 ohms, and cooked something inside your amp.

    So for those of you who say "... Ive done it in a pinch, and never had a problem ...", you are just the beneficiary of dumb luck. Don't "do it in a pinch." Just don't EVER do it. Of course, you've learned that very well by now.
     
  15. Try reading this information from the Procosound cable site.

    http://www.procosound.com/downloads/speaker_guide.pdf

    They also refer to using shielded speaker cables specifically to solve extreme interference problems.
     
  16. We need to get Bob Lee from QSC (nice rhyme huh?) to help us out.

    I think what may have contibuted to this is the fact that an instrument cable uses the shield a the ground return conductor. At high frequency the capacitance of the cable can present a very low impdedance load to the amp and it goes into oscillations that aren't audible because they are of such high frequency. This causes the amp to fry. I'm guessing that even if you did melt the speaker cable it would probably cause an open circuit, not a short. That's my theory anyway.
     
  17. Yeah, Id like to hear Bob Lees input on this too. But to err on the side of caution, Im on my way to Guitar Center right now and Im buying some really good speaker cables!
     
  18. Benjamin Strange

    Benjamin Strange Commercial User

    Dec 25, 2002
    New Orleans, LA
    Owner / Tech: Strange Guitarworks
    You just erred by going to Guitar Center. :smug:
     
  19. I agree that high frequency oscillations can certainly fry an amp. Some esoteric amp designs, and some early amp designs, are much more susceptible to this than others. However, most modern amps--if reasonably well designed--shouldn't go into high-frequency oscillation even if presented with a mildly reactive load. The shielding from an instrument cable isn't all that much capacitance, at least none of the ones I've measured. How about speakers with crossovers? The capacitors and inductors of the crossover IMHO would present far more of a reactive load than the few picofarads of capacitance of shielded cable.

    As far as cables, while any store sells them, I prefer to make my own for long or heavy cables. You can buy 12 gauge cable and connectors from Parts Express and make them, generally saving a few bucks in the process. (Certainly true for PA speaker cables).
     
  20. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Yes, that is true. Most instrument cable is maybe 50 or 100 "picofarads" per foot. That is pretty darn low, a picofarad is a millionth of a millionth of a "farad" the base unit of capacitance.

    Geeeminy...not that I don't like Parts Express, etc, but.....

    Trundle on down to your local hardware store, and get as much as you need of round black extension cord cable. You can get it in 18 ga, 16 ga, even 12 ga, they usually have big reels of it. I know the big stores have it, like Lowes, etc.

    Usually it will be 25 cents (US) per foot or less. Then you can make your own cables (using Speakon if possible) of any length you want.

    That cable is tough, you can run over it with carts, slam it in doors, etc, and it will usually be OK. Each wire is insulated, then the wires are bundled with some fillers and an outer jacket is put over the whole works.

    Don't ever get that stuff that looks like lamp cord unless you just plain have to. It isn't durable at all.