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Need help replacing a power cord in a tube amp.

Discussion in 'Amps and Cabs [BG]' started by Jazzbassman23, Oct 24, 2000.


  1. Jazzbassman23

    Jazzbassman23

    Apr 20, 2000
    Maryland
    I have acquired a 1969 Oliver B200 tube amp that needs its rotted two prong power cord replaced with a new three prong cord. Should I solder the ground to the chassis? Any thing else I should be concerned about so that I don't fry myself? Or is this something I should leave to a pro? TIA

    Dave
     
  2. pkr2

    pkr2

    Apr 28, 2000
    coastal N.C.
    JB:

    I'm not at all familiar with your amp. Does the amp have a power transformer in it?

    A two prong cord can indicate that it that it's a hot chassis or A.C./ D.C. circuit. If that be the case a ground wouldn't be a good idea.

    There were a lot of radios called the "All American Five" produced close to that period that were hot chassis designs. They were responsible for a lot of fatalities before the design was abandoned.

    I have never seen an amp of that design, but who knows? If it looks like the original cord on it, I'd install the new one exactly like the old one. Look for a U.L. label on it. Chances are good that it will be safe if there is a label.
     
  3. Jazzbassman23

    Jazzbassman23

    Apr 20, 2000
    Maryland
    It's a bass head. Oliver Jesperer was one of the prime movers and shakers in bass amp design at Ampeg and apparently was responsible for the B15 among others. As Ampeg began using transistors, he quit in disgust and started his own company. Pardon my ignorance, but there are two fairly massive "objects", one on each end, bolted to the chassis. Power transformer?
     
  4. Jazzbassman23

    Jazzbassman23

    Apr 20, 2000
    Maryland
    Hey, guys,
    Thanks for the replies. PBG, are you saying that the ground on a three prong plug would be for hum cancelling rather than safety? Cause if that's the case, it doesn't hum as it is now. It's just that the cord is kind of gross. So is that two votes to just replace it with the same type of cord that's currently there?
     
  5. pkr2

    pkr2

    Apr 28, 2000
    coastal N.C.
    One is probably the power transformer and the other the output transformer. Yes it should be ok to put a three wire plug power cord on it. Yes, the ground wire goes to chassis ground.

    If your amp has been long enough between servicings for the cord to become rotten or has been stored a long time you'll be lucky if the power supply caps aren't flaky("flaky" being a figure of speech). Pay close attention to hum level which can indicate a failing cap. If you should continue to use it with bad caps you'll almost certainly damage the pwr trans.

    If Psyco happens to read your post he can probably elaborate.

    Pkr2
     
  6. pkr2

    pkr2

    Apr 28, 2000
    coastal N.C.
    Psyco:

    I was interrupted by the phone halfway through my post. When I came back I just finished and submitted it. You posted while I was typing so yours hit the board first. Confusing?:)

    Pkr2
     
  7. pkr2

    pkr2

    Apr 28, 2000
    coastal N.C.
     
  8. Skip

    Skip

    Mar 22, 2000
    Bronxville, NY
    The main reason for a ground lift - as I understand it - is to stop any ground loop hum. This is caused by having different ground potential, and is rare if you are using power from one source. I've seen it happen in early A/V processors because the power ground and the cable ground are at different potentials. This can cause seriously annoying hum, and is solved by lifting the cable ground.

    PBG is right though, you should ground any high power device you use. I assume (but don't know) that the ground lift switch on an amp lifts the ground on the incoming signal - which is presumedly coming in at a much lower power level from the pre-amp or effects box.
     
  9. pkr2

    pkr2

    Apr 28, 2000
    coastal N.C.
    Live and learn. :) Tnx Skip
     
  10. Jazzbassman23

    Jazzbassman23

    Apr 20, 2000
    Maryland
    As usual, you guys have been a big help. It's an interesting amp (at least to me) in that it has two channels, each with two inputs. One side has standard eq with a separate treble boost, and the other with a bass boost. The treble boost side is putting out more volume with everything set flat. I'm going to get this thing cleaned up and give it a whirl. Our guitarist picked it up in a pawn shop for a pittance.