Ruby Amp Motorboating

Posted by alectron on April 19, 2011 at 1:02am

Pretty self explanetory, my ruby amp starts motorboating when I turn up the volume.  The setup is crammed inside a small speaker cabinet, and the pc board is fully soldered but the rest of the connections (volume pot...) are all with alligator clips since I can't drill through the box yet.

(motorboating is where it's like a continuous ocatve effect, right? cause that's what it's doing)

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Replies

  • Servaas (Der Groovemeister) April 21, 2011 at 7:34pm
    You can connect ANY speaker to a Ruby. It will however sound best with an 8 ohm. You can even connect a Marshall 4x12 cab without problems (I did!).
  • Chuck Dubman April 21, 2011 at 5:52pm
    I wonder if the LM386 doesn't have an overload protection circuit that might be clipping the output.  Maybe the amp wants a different speaker?
  • Servaas (Der Groovemeister) April 21, 2011 at 9:21am
    Sounds like a short or bad connection. Check all your solder joints on the pcb. Make sure no shorts between tracks. Maybe reheat every joint to assure it is connected right.
  • alectron April 21, 2011 at 1:10am
    Alright, some more debugging

    moved the 9 volt battery closer to the pcb, no change, works perfectly.

    same with speaker

    same with control panel (plank of wood with pots and jack and switch on it (when I moved I got a tiny quiet little clickish type thing then some hum, then I moved it a bit more and it was fine))

    But then, I move the pcb, eeeeeeeeeeeeeeeeeeeeeeeeevvvvvvvvvvvvvvvvvvzzzzzzzzzzzzzzzzzzzzzzzzzzzzrrrrrrrrrrggggggggghhhhhhhhhh (oscillations)

    I think it might be the LM386 that I stupidly soldered in place. I dropped and it might have knocked something loose but I don't know if that can damage it. I'll try and get a video soon.
  • Servaas (Der Groovemeister) April 20, 2011 at 7:38am
    Can you post some pictures? Perhaps we can see what goes wrong.
  • alectron April 20, 2011 at 12:08am

    I just spent a little bit of time debugging it and I figured out what was wrong.

     

    My crappy soldering job created shorts when I crammed it in the cabinet.  I fixed it up and I can get it  to work well but whenever I move the components it starts oscillating (not motorboating) like crazy.  I haven't  been able to test it really well (which component movement creates the oscillations) since I don't wanna bust the LM386 because it's soldered in place (no socket, facepalm). It doesn't seem like anything would be microphonic, but I've heard bad vacuum tubes can become microphonic, but this is solid state. Keep in mind when I had it on a breadboard I stuffed it in a box to test different speakers and it worked perfectly.

  • Servaas (Der Groovemeister) April 19, 2011 at 11:36am

    I deleted my reply. But it seems you can only edit. So please ignore this post.

    sorry!

  • eric gittins April 19, 2011 at 8:13am

    i don't know the ruby amp, but have built several valve amps...

    but motorboating is not an octave type effect.  it's a low frequency oscillation - like putt-putt-putt.

    the octave type effect you mention, i have heard of on modded tube screamers etc. caused by changes to the feedback/clipping circuit.

    have you swapped out your IC? not all chips are created equal, and for a buck a piece, it's good to have a few kicking around to try.

    also maybe check your voltages right at the IC pins, just to make sure the chip is getting the power it needs to work properly, just a thought.

     

  • John W April 19, 2011 at 1:37am

    Don't turn it up so loud.

     

    No, seriously, which pot are you turning up?  Sounds like you might have a feedback loop where there should not be one, OR the input level coming in to the ruby amp is way too hot. 

     

    Disclaimer:  I've never debugged a ruby amp, specifically, but based on some other similar amps I've worked on those are the first few things I would try.

This reply was deleted.