Ampeg SVT-IV Pro (USA) Mono Bridge Issue

Discussion in 'Amps and Cabs [BG]' started by lazerviking, May 13, 2017.

  1. I will try to keep this as simple/to the point as I can while providing as much info as possible!

    I am experiencing a somewhat similar problem mentioned in this older thread: SVT-4 Pro Problem... PLEASE HELP

    So here is what is going on with mine. I have an original, made in USA Ampeg SVT-IV Pro which has an output power of 1,600 watts @ 4 ohms in bridged mode. Which is perfect for my beast of a cab, the Basson 8x10 which is rated at 2,000 watts @ 4. Speakon input only.

    So power amp A of my head had issues, and power amp B sounded clean and clear, nice and loud. Gave it to a tech to fix and when I got it back, the problem switched: power amp A sounded good, B not-so-good. He had said that everything was seeming to be working. Using the amp set to stereo and connecting to two cabs, both via Speakon, it sounded bad. My tech pal suggsedt I used a patch cable to bridge the poweramp in/preamp out for amp B and this fixed the problem with running the amp in stereo: Each power amp into a cab, via Speakon, and it all sounded great. This tells us that one of the jacks for B side isn't working properly. That will be easy to fix. Another thing to mention is the stereo/bridge switch seems to be working as well. When engaging, I can hear a "click" in the speakers.

    Now the issue is with bridging. When the selector is pressed in for bridged mono mode, and using an NL4FX to NL2FX wired for bridge cable I got from Bayou Cables, there is hardly any volume. I crank up the volume and and it's very dirty sounding and not very loud. I am sure the bridged cable is wired properly.

    Is having the power amp in/preamp out for B patched causing an issue with the bridged/mono Speakon output from properly working? Does the way that bridge/mono work in such a way that patched B is preventing the current from working as it should? Is there an issue with the phasing unit? Is the Speakon output jack for bridge mono not working? The tech did say he is not super well versed in Speakon connections, so it may be possible he has overlooked something wrong with the Speakon output.

    Anything more detail/information anyone has to offer about this would be awesome. Thank you!
    Last edited: May 14, 2017
  2. BassmanPaul

    BassmanPaul Inactive

    SpeakOn cables are very, very simple to use in real life. That someone who called himself a tech can’t understand their use is surprising to me.

    i have a feeling that by just using the one pre-amp section you are bypassing the signal polarity reversal that make the two output stages 180 degrees apart as required in Bridge.

    Your amp indeed does have a problem that needs to be correctly diagnosed and remedied.

  3. Thank you for the input! I maybe should not have used the term "tech", as he doesn't do this as his main profession but has worked on some tough fixes in the past with success.
  4. I should also add that plugging straight into poweramp in of A is quiet, distorted but plugging into poweramp in of B sounds nice and loud.
  5. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    On many amps you only run the A input in bridged mode, and in some cases you need to turn the B channel gain control all the way down as well. Your use of the term "bridging" for whatever patch arrangement you are using is a little confusing as well. Yel_wink.gif