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Loss of preamp output - any ideas?

Discussion in 'Amps and Cabs [BG]' started by Ian Perge, Dec 14, 2005.

  1. Ian Perge

    Ian Perge Supporting Member

    May 11, 2001
    Evansville, Indiana
    I'm looking for any and all ideas to try before I get warranty support, so toss out any ideas, not matter how out there or silly they might sound.

    Signal chain

    About a week ago I suddenly got what can only be described as a "loss of preamp output" from my SWR Mini Mo' preamp, where I have to crank the gain, master outputs, and amp volume to get any decent volume... along with the accompanying static/hiss that you'd expect from such. I've narrowed it down to the pre and not the amp or cabinets by the simple fact that the same effect happens when I plug into the "headphone out" and get the same results without any usage of amp or speakers. Here's a list of all the other troubleshooting I've done:
    1. Bass output: I've replaced the batteries in two of my active basses with no difference. There's also no change using a passive bass.
    2. Bad instrument cable: I've tried several different cables with no change, and have tested them all on my Tascam CD-BT1 trainer with positive results.
    3. Tube problems: I have no telltale signs of tube failure such as noise or microphonic signal ("glass tinkling") before failure. I'm aware that they can fail without signs, but generally 12AX7s are rated for 1-3 years of moderate usage, and I'm only at 17 months of low (non-gigging, home practice only) usage.
    4. Bad power: I'm aware that my complex doesn't have the best wiring for high-power usages such as pro-level amplification (for example, at moderate volume the lighting would occasionally decrease/flicker about 10%) but I've never noticed any sound issues at my end and believe the draw to be much more on the amplifier end. I've also relocated my rack to another outlet with the same sonic results.
    As I said before, I'll take any ideas for research/test no matter how large or small they may be. I'm generally loath to bring it in for servicing, especially at the holidays (which I'd wait until they pass before doing so) and would rather do as much troubleshooting as possible for my own possible solution as well as to rule out ideas for a service center.

    Any thoughts?
  2. Try putting a signal into one of the effects returns, see if you get good output from that. Work your way back through the various outputs to see where the problem crops up. That proves the problem is prior to that particular effects return if it works. See if you're getting a decent signal out of the effects sends and tuner out. You know theproblem is after the last effect send/output that works.

    That preamp has lots of potential outputs/inputs. That should give you an idea what stage the problem is in. Its after the last good output from the efx sends starting from the input and going forward, and before the last good input working your way back from the pre output.

    Also tubes are cheap. Put a new tube in it. If it doesn't fix it, now you have a spare tube in case you ever break down on stage. Also check the speaker cord, or swap with a new or known new one.

    Bad power is highly doubtful as a culprit.

  3. Eric Moesle

    Eric Moesle

    Sep 21, 2001
    Columbus OH
    Small world. Last weekend my Eden Nav lost signal to its preamp outs, both the XLR's and the 1/4" outs, though the meters show signal just fine.

    I just dropped it off at the local well-respected service shop, I'm hoping its probably a solder connection on the board that cracked from the cold-to-heat-to-cold of the Ohio winter. I'll let you know what the diagnosis of mine was in a day or two.