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Help with Humming AC15 Heritage Handwired

Discussion in 'Glowing Bottle Tube Amp Forum' started by philthejuggler, Oct 8, 2017.

  1. philthejuggler

    philthejuggler TDPRI Member

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    Hi all! First post here - I hope to become a regular!

    I have just picked up an AC15 50th anniversary TV combo.

    Sounds great, but hums quite a bit especially if the EL84 circuit is in triode mode - where should I look first for a cure? Valves? Capacitors?

    I tried in a filtered mains socket - no different. Doesn't make any difference if guitar plugged in or not!

    Thanks in advance!

    Phil
     
  2. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Have you tried swapping any tubes yet? That's always the first place to start with these guys.

    If you're comfortable doing your own tech work we can make some additional recommendations that require more invasive work, but swapping tubes is always the easiest solution.
     
  3. philthejuggler

    philthejuggler TDPRI Member

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    Many thanks - yes - I've built solid state amps before so happy with soldering etc and swapped out drivers on valve amps that needed tubes removed first so that should be no problem.

    The seller did say all the valves were replaced not long ago, but I guess one could be dodgy!

    I also don't know how much hum to expect, but I've had an AC4, AC15C1 and AC10 before and none of those hummed noticeably - neither does my HT40. I have heard this model is more prone to hum though.
     
  4. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Does the hum increase with the volume of the amp? If so, the hum is being amplified somewhere in the preamp. If not, there could still be issues with the amp, or it could be just the normal noise/hum floor.

    Is it enough hum to drive you crazy? When you play does it get easily drowned out?
     
  5. philthejuggler

    philthejuggler TDPRI Member

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    No the hum is independent of the volume of the amp - it is irritating because I often plan to use this amp for low volume practice at night when the kids are in bed. The previous owner put an attenuator in the amp, and this drops the noise floor significantly so that is plan 'B' if I can't find a specific fault.

    Does the fact the hum is independent of volume point to a specific problem?
     
  6. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    While it doesn't point to a specific issue it can help narrow down some things.

    I'd swap tubes (if you can) and then go from there.

    You can also try pulling tubes one by one, starting from the preamp section, and see if the hum stops whenever a particular tube is pulled. That would indicate the problem is in that stage of the amp or before.
     
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  7. philthejuggler

    philthejuggler TDPRI Member

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    Update!

    Short answer - amp is 'normal'

    Long answer - I took the amp to a valve amp specialist who fiddled and decided there was some 100Hz hum from the power supply and some 50Hz hum - probably from a design issue to do with grounding.

    I then drove to Vox amps & met the bloke who designed it. He said - 'normal - you could increase the power caps, but it changes the tone - live with it'.
     
  8. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Awesome - glad you were able to get a definitive answer. So what you're hearing is normal for that amp and you know where the noise floor is now.
     
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  9. Richie-string

    Richie-string Tele-Meister

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    Probably very near the top of my list for the most coveted/desired/wanted amplifier of all time. Any chance you may be able to take some pictures so the rest of us can sit and admire it's outstanding beauty?
     
  10. philthejuggler

    philthejuggler TDPRI Member

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    As lovely as it is I think I can't live without a master volume control - if you're seriously interested it is up on eBay for £900, but would accept £750 as a direct sale!!
     
  11. philthejuggler

    philthejuggler TDPRI Member

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    Sorry it's taken me a while....here are some pictures as requested!

    AC15H1TV-3826.jpg AC15H1TV-3829.jpg AC15H1TV-3830.jpg AC15H1TV-3836.jpg AC15H1TV-3838.jpg
     
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