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How do you trouble-shoot hum in a tube amp?

Discussion in 'Amps and Cabs [BG]' started by MrBonex, May 4, 2006.


  1. MrBonex

    MrBonex

    Jan 2, 2004
    New Hampshire
    I need some sage advise. My Trace V6 is a little hummier than I'd prefer. How do I go about trouble-shooting a solution?

    Where is this type of problem most likely to occur? Preamp tubes? Power tubes? Somewhere else?

    It only happens when the standby is off and it's volume doesn't increase as I increase gain. So it's a steady level. It doesn't matter if there's anything plugged in. It's just there when you're ready to play -- and it's too loud for softer gigs. The tubes are about 4 years old -- and not gigged a lot (which says more about me than it does about the amp!).

    Thoughts? Need more info? Thanks!
     
  2. barthanatos

    barthanatos Insert witty comment here

    Feb 8, 2006
    South Carolina
  3. It depends on what kind of hum you're talking about. A lot of amps hum from the AC heater supply. For this reason many are also fitted with "hum balance" pots that allow you to minimize (and often completely silence) hum that comes from the heater lines. Look on the back of an old Ampeg V-4, for instance, and you'll see a little screw-set hum balance control.

    Most hum balance controls will be inside the amp, probably mounted on the PCB. But be careful, because companies often use the same type of pot they use for the BIAS adjust! I haven't been inside one of those V6's...but if you post a pic of the inside, myself or someone else may be able to point you to the hum balance, assuming it has one.

    Sometimes bad tubes can hum, especially pre-amp tubes. But it's not all that common to tell you the truth. Best way to find out is to just replace them.

    And finally, there could be an issue with the amp. Anything from a bad ground connection, to a cold solder, or a faulty component (partially shorted cap...).

    I'd start by looking for a hum balance pot, as that's a nice easy thing to figure out.

    -Nick
     
  4. If you're not familiar with the insides of tube amps, you're pretty much shooting in the dark, sometimes dangerously so.

    Since you later post that it is not affected by volume settings, chances are its in the power section itself. Filament hum IS usually affected by volume changes and the V6 lacks a hum balance control anyway.

    I'd say you need a bias adjustment. Amps that have bias problems will also hum, something no one else has suggested so far. I'm pretty sure that's your problem, especially since you mention it happens whennothing is plugged into the amp. Unless you know for certain how to bias the amp, do NOT attempt to fix it yourself; you could cause more, worse problems or hurt/kill yourself.
     
  5. MrBonex

    MrBonex

    Jan 2, 2004
    New Hampshire
    Thanks for the replies!

    It looks like I'm going to take it in to my amp tech. I'll start with a re-bias. That will certainly be cheaper than replacing six KT-88s!!!!

    If my power tubes were matched and biased four years ago, can that bias drift over time?

    PBG, I'm aware of residual voltage in tube amps and am not quite stupid enough to pretend I know what I'm doing in there -- even if I am a bass player! ;)
     
  6. Yep. [edit- clarification]- The bias voltage itself does not drift. The tubes themselves either increase or decrease in the amount needed to get them to properly operate as they age. In your case it sounds as though the bias voltage probably needs to be increased.

    That comment was mostly for the benefit of the flood of newbies here who seem hell-bent on performing a myriad of mods on their tube amps, whether they can safely do them or not. As a related aside, it seems TB's Amps forum has had a changing of the guard as it were, and a lot of the older posters have left and newer ones who can't/won't use the search function and latch on to the first piece of "advice" they read have replaced them.
     
  7. MrBonex

    MrBonex

    Jan 2, 2004
    New Hampshire
    PBG, thanks for the info.

    Even if I'm not a total newbie, I always think it's a good idea to let others know that amps are not without sneaky little suprises that can flatten one's ass. It probably can't be repeated too often.

    And besides the search function ignorance, there's always the Internet Misinformation Network (TM) that seems to be quite pervasive. :rolleyes:
     

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