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Acoustic 361PP hum

Discussion in 'Amps and Cabs [BG]' started by FUNKonthewall, Nov 24, 2004.


  1. FUNKonthewall

    FUNKonthewall Nailing The Groove

    Sep 29, 2004
    Atlanta, GA
    Endorsing: Fodera Guitars, Aguilar Amps, Dunlop/MXR Accessories
    I just bought and cosmetically restored an Acoustic 360/361PP stack and I couldn't be happier with it except for the fact that there's kind of a bad hum coming from the power amp. I assume it is 60-cycle hum because the "pitch" of the hum is roughly a B flat and B flat is about 60Hz. Honestly, you can't hear any trace of it at performance volume, but I would still love to get rid of it nonetheless. I am by no means an electrician, but I would like to know what I have to do/replace to get rid of it. I tried searching, but I got no results, so I figured I'd start a new thread. Thanks for your help!
     
  2. Raven

    Raven

    Nov 14, 2004
    Jacksonville, Fl
    Have you checked your cables? Also does it hum even if the bass isn't plugged into the head? The head is a preamp, the power amp is in the cab, if the cab is the one I'm thinking of. The cable between the two has to be shielded, just like your instrument cable. Speaker wire won't work. Also, have you tried plugging in the power cord both ways? If the cord or plug has been replaced, its possible that someone crossed the line and neutral wires. If its a 3-prong plug, get one of those adaptor plugs so you can try plugging it in both ways. If none of these suggestions eliminate the hum, you may need to take it to a tech who you trust.
     
  3. FUNKonthewall

    FUNKonthewall Nailing The Groove

    Sep 29, 2004
    Atlanta, GA
    Endorsing: Fodera Guitars, Aguilar Amps, Dunlop/MXR Accessories
    Thanks for the input. I'm using shielded cables for all the connections. I know that it's something with the power amp (or it's power cable) because when I plug just the powered cab into the wall with nothing plugged into it, it still hums. I'm sure it's just because the amp's about 35 years old and it needs a tune-up. We've got a local tech who I've heard is very adept, so I guess I'll give him a try. If anyone's got any more info, feel free to post. Thanks!
     
  4. Rock&Roll

    Rock&Roll

    Jul 21, 2002
    USA
    Congrats on finally getting your 361 in your hands.

    As far as hum goes, I'm not so sure it can be completely eliminated. You can try the ground switch to see if that will help the hum, but I doubt it will ever be dead quiet. The other thing to do is look into getting a high class power conditioner.

    On my own amps I've had the power amps and pre-amps serviced by a really good tech shop about 2.5 hours away from here. It's worth the drive to get a tech who stands for quality work. When I got the amp back, the lower end bass (the really low stuff) was tighter and clearer which made slightly more present. All they did was replace the power transistors with new Motorola's and a few small caps on the circuit board. The big caps were fine.

    Despite all the positive changes of the amps being serviced, they never did loose their hum completely. Here's what I would do if I were you.

    ---First I would find a good amp tech. I HIGHLY STRESS the word good. I hate half assed electricians, esspecially when I'm paying them good money to fix my gear that needs to be reliable.

    If you get your amps serviced I would have them go ahead and replace the power transistors and have them do a detailed inspection of all the resistors, caps, transistors and the other components on the amp. Have them replace whatever looks a bit marginal. Have them readjust the bias when done.

    I would have them go over the pre-amp as well. Mine had cold solder joints in the head and various other problems that could not be heard.

    Last thing I'll mention is the power cords for these old amps. They came with 2-prong cords. Any good shop will automatically put on a 3-prong grounded power cord and attach the new ground conductor to the chassis. The power cord for the amp is attatched to the speaker 361PP cabinet. Since it has the special molex connector on it, I would take the amplifier's power cord with the amp that way the amp tech could put that connector on the new power cord. He can decide where to mount the ground conductor on the amplifier.

    After you do these things, if you still have hum, the next step is to go through your instruments and cables to double check that they are not the source. The next thing down the line after that is a good power conditioner. If none of those things can fix it, I would live with it. :(
     
  5. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    Chicago
    It's an old amp and if the electrolytic caps are original, it's high time that they be replaced. If the big ones in the power supply are dried out, the power supply could be lacking adequate filtering and the hum level would increase.

    Make sure all of the ground connections to the chassis, input plate, etc. are corrosion free.

    IIRC, the power cord is only two wire...try flipping that one around in the wall socket.

    Have fun!!!!
     
  6. FUNKonthewall

    FUNKonthewall Nailing The Groove

    Sep 29, 2004
    Atlanta, GA
    Endorsing: Fodera Guitars, Aguilar Amps, Dunlop/MXR Accessories
    I guess I'll just take it to a tech so he can check it out. I'll ask around and see who's good around where I live. Thanks for all of the info and help guys!
     
  7. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    Chicago
    Do you have a schematic diagram to share with the tech?
     
  8. FUNKonthewall

    FUNKonthewall Nailing The Groove

    Sep 29, 2004
    Atlanta, GA
    Endorsing: Fodera Guitars, Aguilar Amps, Dunlop/MXR Accessories
    Yeah. I've got the full instrucion manual and the full service manual with all of the schematics.