DIY mistake: I screwed up my combo amp...

Discussion in 'Amps and Cabs [BG]' started by DaleyKD, Jul 26, 2013.

  1. DaleyKD


    Oct 19, 2010
    Anna, TX
    Ok, please don't flame me. I wanted to do a silly DIY project and I think I screwed up my Fender BXR60 in the process.

    I decided that I wanted to separate the head from the cab, and to allow the head to have a 1/4" out and the cab to have a 1/4" in. The current setup is two wires going from the cab to the amp, connected by a 3-pin Molex.

    Currently, the cab works great. I hooked it up to another amp and it sounded fine. So I 'successfully' have a 1/4" input jack on the back of the cab.

    However, something's wonky with the head. If I cut off the end of a 1/4" instrument cable and put Molex connectors on the wires, then connect the other end into the cab's input jack, it seems to work fine.

    It's when I solder the Molex connectors on, cut off the other end of the instrument cable and solder a 1/4" jack to that; then I use a brand new 1/4" instrument cable (as a patch cable) from the out of the head to the in of the cab that I get a little sound for a moment, then it gets scratchy, soft, and eventually seems to fizzle out.

    My goal:
    head ==> 1/4" out ==> 1/4" patch cable ==> 1/4" in ==> cab

    What could I have done in the soldering process or in the adding a 1/4" jack to have screwed this up?

    Thanks in advance,
  2. B-string

    B-string Supporting Member

    Soldered the "+" from the amp to the sleeve (ground) connection, using an instrument cable for speaker line use and maybe not insulating the jack from the amp's chassis.
  3. P Town

    P Town

    Dec 7, 2011
    You have used the term: "instrument cable" to describe the cable you are using to connect the speaker to the amp.

    If you are actually using an instrument cable, that is the problem.

    You could damage your amplifier using instrument cable to connect the speaker to the amp!

    An instrument cable is a shielded cable, with a small inner conductor surrounded by an outer shielding conductor.

    Remove any of this type of cable if it is connected between the amp, and the speaker.

    Replace it with speaker cable, which will consist of two conductors large enough to handle the power the speaker will dissipate.

    Not sure of the power you are dealing with, but the smallest you would use for a short, (two, or three foot long) cable, on a 100 Watt setup should probably be 16 ga, or (16ga as the very smallest).

    I typically use12 ga wire for speaker wire, (which is larger than is needed for any amp, and speaker I use), mostly for the more robust mechanical properties the larger wire provides.

    Edit: B-String answered you while I was typing this.
  4. DaleyKD


    Oct 19, 2010
    Anna, TX
    My first attempt at it was using regular 14 ga speaker wire (like you use for home audio). I had soldered it to the Molex connectors and the 1/4" jack. I also tried changing polarity by flipping the Molex connection. One of the polarity combinations gave me "AWESOME" distortion, the other combination gave me the aforementioned symptoms.

    I, too, thought maybe it was because I wasn't using actual speaker wire; however, when I connected another amp (Marshall JCM2000 DSL100) to the cab, I used the EXACT same instrument cable and it worked great.

    I have a 25' speaker cable that I can try, but if that doesn't work, any other ideas?
  5. B-string

    B-string Supporting Member

    Isolate the jack from the chassis of the amp (3/8" fiber shoulder washers IIRC) and STOP USING AN INSTRUMENT CABLE for speakers.
  6. DaleyKD


    Oct 19, 2010
    Anna, TX
    Well, I thought it was something with the output. Turns out, (I think) that it was because the polarity on my 120v was backwards.

    So far, it sounds good. I'll post again if it is still a problem.

    (I'm using a speaker cable now.)
  7. Rick Auricchio

    Rick Auricchio Registered Bass Offender Supporting Member

    That doesn't make sense. If the amp is having trouble when you reverse the AC plug, it is probably an electrocution hazard. Check the wall outlet with an outlet tester (and any power strips or extension cords you use.)
  8. ddnidd1

    ddnidd1 Supporting Member

    It should be DIY MistakeS
  9. Munjibunga

    Munjibunga Retired Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    Whether it's the cause of the immediate problem or not, DO NOT use instrument cable for the connection. You may have already cooked your amp by using it the first time. Instrument cables are designed to carry very low (milliamp) current at voltages below one volt. Speakers can draw several amps of current at tens of volts (hundreds in high power situations). The current load of a speaker can melt and fuse the wires in an instrument cable, creating a short (zero-ohm load) across the output of the amp, thereby frying it like a piece of chicken.
  10. Dave W

    Dave W Supporting Member

    Mar 1, 2007
    Westchester, NY
    Just out of curiosity, why do this mod?
  11. This does not makes sense because AC power does not have a polarity. The "A" in AC stands for alternating and that is because it is a sine wave. Half of its period is positive voltage and the other half is negative.
  12. DaleyKD


    Oct 19, 2010
    Anna, TX
    OK; you guys are all correct. I thought it was because I had switched the hot and the common, but it turns out it was because I had accidentally moved the jack so it wasn't touching the chassis. As soon as I tried to put the jack back in the drilled out hole in the chassis, it started making awkward squawk sounds. B-string was right:

    I will try to find some washers so metal doesn't touch.
  13. nojj

    nojj Guest

    May 20, 2013
    they make isolation jacks.
    I say use one of those, or sure as sh-ootin' the jack will touch the chassis again.
  14. Although your description of the hot lead's power is correct, you are incorrect about polarity. Reversing the hot and neutral leads on AC power is, in fact, referred to as reversing the polarity. It may be difficult to defend the use of the term as far as it's actual definition, but it is, in fact, the industry standard term.
  15. DaleyKD


    Oct 19, 2010
    Anna, TX
    Thanks! I had grown up around my father (an electrician) and that's what he always called it. However, Victor's description was accurate and totally made me question my sanity.
  16. DaleyKD


    Oct 19, 2010
    Anna, TX
    Can you show me one? I'm not sure I've seen one.
  17. No problem.

    In fact, the two-prong Edison plugs with one blade larger than the other (so you can control which conductor is connected to the hot lead) are referred to as "polarized plugs," even though the English of it may not strictly make sense.

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