Momentary hum on power-up

Discussion in 'Amp Tech Center' started by tillamook, Jun 12, 2021.

  1. tillamook

    tillamook TDPRI Member

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    Hello:
    About 15 seconds after I turn on my new Vox AC10 it begins to hum, the hum increasing and then decreasing over a period of 30 seconds, then everything is OK although it may be that the low level idle hum has increased a little over the 2 1/2 weeks I've had the amp.
    Is this likely a faulty tube problem? The amp shipped from Sweetwater to the coast of Oregon, it may
    have gotten bounced around a bit.

    Thanks in advance for any help,

    Bill
    Astoria, Oregon
     
  2. Jon Snell

    Jon Snell Tele-Holic

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    I would replace the two EL84s/6BQ5s with a balanced pair.
     
  3. Wally

    Wally Telefied Ad Free Member

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    Be watching your power t7bes when you fire the amp up. Is either one redplating at any point?
    I would not want any new amp that does this unless a tech checked it out. It well may be that the tubes are not draw8ng the same current and a new, matched set would cure the problem as Jon says….or it could be another problem that is causing the mismatched current draw. W
     
  4. tillamook

    tillamook TDPRI Member

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    Thanks Wally and John:

    I have a new tube set coming on Tuesday, I'll see what happens. I can't see the tubes without taking the back off so I think I'll wait
    until Tuesday to check for redplating.
     
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  5. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Maybe one of the EL84s takes a little longer to warm up? I'll be interested to hear the rest of this story!
     
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  6. Jon Snell

    Jon Snell Tele-Holic

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    Armed with the information given by tillamook, an unballanced valve is the most likely cause.
    It could be as simple as one has the heaters is in a slightly different position regarding heat conduction to the cathode and is perfectly acceptable as long as it catches up with the other valve soon after power up.
    If it was a new machine, I would insist of a set of balanced EL84s FOC as a replacement as it will be under warranty and effectively faulty out of the box.
     
  7. soundchaser59

    soundchaser59 Tele-Afflicted

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    This.

    And if I know sweetwater from previous experience they would not balk at such a request.
     
  8. tillamook

    tillamook TDPRI Member

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    I'm sorry to disappoint those waiting for an exciting resolution of the mysterious hum problem but I boxed up the amp today and it's on it's way back to Sweetwater.
    I was right on the edge of the return window and did not want to be without an amp while dealing with a potential repair or swap. It's a week and a half shipping time from Sweetwater to the Oregon coast.

    A side note: Sweetwater informed me that swapping the tubes myself would void the Vox warranty. I guess you could always be a sneaky criminal type and not tell them.

    I made the drive into Portland yesterday and picked up an AC15C1X (Alnico Blue loaded), Yowza!
     
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  9. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    That the story has a happy ending is good enough for me! Congrats on the AC15. :cool:
     
  10. schmee

    schmee Doctor of Teleocity Ad Free Member

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    Do they have a rectifier tube? A couple of my vintage amps do that until the rectifier warms up. Then they get silent.
     
  11. tillamook

    tillamook TDPRI Member

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    No rectifier tube in the AC10.

    Thanks, the AC15/Blue is perfect with my current favorite, a flatwound strung Gretsch hollowbody.
     
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