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Hiwatt 60-cycle hum

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by malabarmusic, Jan 21, 2005.

  1. Jan 21, 2005 #1
    malabarmusic

    malabarmusic Member

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    My '72 Hiwatt DR-103 has got a hum issue that is proving hard to trace.

    It's not too bad, except when all three volumes are cranked or close to it. At that point, moving the Bass knob past 11:00 brings up a low-level 60-cycle hum that gets progressively louder as the control is turned up. At max bass (not that I'd ever play it there), the hum is so annoyingly loud that it makes me wonder if something could be faulty.

    The amp has been gone over by a very good tech. The tubes are new and test fine. The bias is set just a tad on the cold side of "normal." All ground wires have been thoroughly traced and no loops found. All filter caps were just replaced with the proper values.

    Should I to learn to live with this hum or should I press on in quest of a "fix"? :confused:

    - DB
     
    malabarmusic, Jan 21, 2005
    malabarmusic, Jan 21, 2005
    #1
  2. Jan 21, 2005 #2
    John Phillips

    John Phillips Member

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    Replaced or not, it could be a faulty filter cap, or a bad ground connection on one. If so, it must be the one furthest down the B+ chain, since the hum is coming from the very start of the signal path, if it only occurs when the channel volumes are up.

    If not that, could it be a ground problem at the input jacks?

    I've never noticed really excessive hum like that from Hiwatts, usually they're pretty quiet.
     
    John Phillips, Jan 21, 2005
    John Phillips, Jan 21, 2005
    #2
  3. Jan 21, 2005 #3
    malabarmusic

    malabarmusic Member

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    I'll definitely look into the filter caps. I doubt it's the input jacks, because (I forgot to mention) that the hum is there even when nothing's connected to the jack.

    - DB
     
    malabarmusic, Jan 21, 2005
    malabarmusic, Jan 21, 2005
    #3
  4. Jan 21, 2005 #4
    KLB

    KLB Member

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    I can remember the particulars, but I recall that Ken Fischer discusses the routing of one particular wire being very sensitive in the 100w Hiwatt regarding noise. I believe this info appears in the middle section of one of the Weber tube amp books.

    Sorry I can't be more specific.

    Good luck!

    - Ken
     
    KLB, Jan 21, 2005
    KLB, Jan 21, 2005
    #4
  5. Jan 21, 2005 #5
    Swarty

    Swarty Member

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    As covetted as these amps are for the fine wiring and clean look, the bundling of all the wires together can induce noise. If the filters are OK and the tubes are matched the solution may be to rip apart the bundles in the early stages of the preamp. You may want to have it examined with a scope to see at what point the noise is being introduced. A non scope way would be to set the amp up to make the noise and start pulling preamp tubes (starting at the first stage) and see where the noise disappears.
     
    Swarty, Jan 21, 2005
    Swarty, Jan 21, 2005
    #5
  6. Jan 21, 2005 #6
    malabarmusic

    malabarmusic Member

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    And the answer is ...

    ... bad preamp tube (V1)! I was passing the ideas from this thread to my tech, and he had pretty much exhausted all other possibilities. The filters all tested fine, the jacks were as they should be, he had already moved around some wires to minimize problems, etc. The guy definitely knows his stuff, and seemed to have gone above and beyond. Finally he went back and pulled the tubes (all NOS that I supplied), replaced them with new ones, and presto! Hum disappeared. He then put back the old ones and swapped out one at a time until it was determined that V1 and only V1 was the problem. With my tube -- hummed like a Strat under a beer light. With a different tube -- voila! No more hum.

    Anyone want a lousy Phillips-Miniwatt 12AX7? Free to good home ... :)

    - DB
     
    malabarmusic, Jan 21, 2005
    malabarmusic, Jan 21, 2005
    #6

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