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Hum help.

Discussion in 'Amps and Cabs [BG]' started by merlin, Mar 18, 2001.


  1. Alrighty, i just changed the volume pot on my bass because it was scratchy. On the active pickups i noticed the soldering was a bit dodgy to my standards but what the hey.

    You may ask, "why's this in amps?", well the past few weeks i have noticed that i get a hum out of the amp/cab, especially at high volumes of say 8 or 9. I initially thought it may have been the active electronics in the bass (thus the opening bit) but its not. The amp isn't anything, some home made PA one with Altron written on it. That said it is also old, i am building a new one.

    I was wondering if there is anything i can do to cut down the hum and the terrible distortion i get at playing at high volumes?? Any suggestion in the short term while i am still building mine?

    Merls
     
  2. yawnsie

    yawnsie

    Apr 11, 2000
    London
    Errr... how are you sure it's not the bass? If the soldering is a little bit dodgy, then it might be worth trying it in another amp. If you have done that, and it definitely is the amp, try turning down the treble as well as the tone on your bass. If the amp has any settings like Presence or Gain, bring those right down too.
     
  3. As if it would be advanced to have Gain! lol.
    This just has the bare essentials of the pre-amp like Volume, Master Volume, Bass, Mid, Treble.

    I'll give it a go. Cheers.

    Merls
     
  4. JMX

    JMX Vorsprung durch Technik

    Sep 4, 2000
    Cologne, Germany
    Hm, is it tube or transistor?
    This might be helpful in a diagnose...
     
  5. its a transistor one. Its 200 Watts broken down into 4 channel out put at the back. I used it for a PA amp in an old band of mine, but then started using it as a Bass amp.

    Merls
     
  6. pkr2

    pkr2

    Apr 28, 2000
    coastal N.C.
    Merle, if the amp is old, chances are good that the power supply filter caps are getting tired.

    It. easy to check them by temporarily putting a substitute cap across the filter caps and see if the hum goes away. No need to disconect the old cap to make the test.

    Be sure the sub cap is discharged and the power is off when you make the temporary connections using test leads with alligator clips. Be SURE to observe the polarity of the sub cap- hook it up backwards and you can get a mini-explosion with lots of pretty white smoke! Guaranteed to get your attention.

    After the amp has been on long enough to warm up completely, feel the power supply caps and if you find one getting much warmer than the others, that's probably the one that's causing the prob.

    All electrolytic caps have a vent made into the plug that seals the can. Take a critical look at the plugged end of the cap and see if there is a little bulge about the size of a B-B where the electrolyte is trying to escape. That's a sure indication that replacement is in order.

    Whew, I bet that's more than you wanted to know. :)

    Pkr2
     
  7. Is the hum also there when you're NOT playing or just when U play with the volume cranked?

    If the first statement is true it's just a bad pot and a tiny bit of contact spray could be doing the trick.
     
  8. Gabu

    Gabu

    Jan 2, 2001
    Lake Elsinore, CA
    Hmm... I have to admit I have never owned a amp that didn't hum to some degree by the time you got up to 8.

    But in any case, what I wonder is if at some point it did not hum. If it did not hum at volume 8 on monday, and started to hum when you used it on wednesday then what happened to your amp on tuesday?

    That would be the approach I would take. What has changed? Undo that, then maybe your problem will be solved.
     
  9. Thanks guys. Because its not my amp i can't take the back off and do tests on it. I am pretty close to finishing the new one i amp building. its in two rack cases- one for the power amp, the other for the 2 channel pre-amp with effects.

    I want to be able to check just the power amp. its 100 watts RMS in 4 ohms. Is it ok to hook up a speaker of 4 ohms, not rated at 100 watts (too expensive) but say a car speaker or something and pass a signal through just to check it all works?? There would be no preamp, just direct into the amp.

    See it turns on, everything works, no typical smoke from my electronics! :p so i want to see if it is ok with a signal, it can sit there with no load or signal or day when turned on, but its a different story with a load.

    Cheers
    Merls