Odd amp noise question

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by hipfan, Dec 9, 2004.

  1. hipfan

    hipfan Member

    Messages:
    2,302
    Joined:
    Nov 15, 2002
    Location:
    Evanston, IL
    I'm getting an odd noise from one of my amps, and I'm hoping there's a simple answer. Here are the facts.

    The amp uses EL34's, but can be biased properly for 6L6's. When there are EL34's installed, there is an odd, crackling noise that intermittently comes through the speakers on certain notes. The noise is detectable, but not too loud. Loud enough to be bothersome. The amp seems to still be putting out proper power, I think.

    At first I thought it might be a bum speaker or speaker connection, but the speakers work fine with other amps. I checked the tubes for microphonics, and found that when I nudged one of the power tubes a little, I could get the amp to hum and crackle a little. If I lean it back to the other side, the hum goes away. None of this seems to happen when I have used 6L6's in the amp.

    I'm thinking that the residual crackle at high volumes on certain notes could be a problem with the tube socket or something related. Should I go poking around with a chopstick and see if there is a problem solder joint? Has anyone experienced this kind of problem before?

    Thanks for any help!
     
  2. Wakarusa

    Wakarusa Member

    Messages:
    1,477
    Joined:
    May 7, 2004
    Location:
    Indianapolis, IN
    I'll take a crack at it. Have you tried the amp with a different set of EL34s?

    If moving the power tube around produces your problem I'd guess at a tube socket issue. You can try re-tensioning the socket with a dental pick (a process that I don't really subscribe to.. if the contacts are worn out I tend to just replace 'em). A simpler cause may just be oxidation in the tube socket. Get hold of some Caig ProGold, wipe it on the tube pins and cycle the tube in and out of the socket a couple of times and see if the problem goes away.

    The ProGold product is recommended because it will tolerate the heat and won't turn to varnish.
     
  3. hipfan

    hipfan Member

    Messages:
    2,302
    Joined:
    Nov 15, 2002
    Location:
    Evanston, IL
    Todd, thanks for the reply. Yeah, it does it with each set of EL34's I try. I think I've tried about 4 different pairs. The amp is only about 8 or 9 months old, so there *shouldn't* be an oxidation or wear-and-tear related problem, but I guess one never knows.

    I'll try to get hold of some of the ProGold stuff. Thanks!

    Tony
     
  4. John Phillips

    John Phillips Member

    Messages:
    13,080
    Joined:
    Mar 17, 2002
    Location:
    Scotland
    I'd take a bet that you have a poor connection on the tube socket - where the noise comes and goes if you move the tube - on pin 1.

    This pin is used for the EL34 connection but not the 6L6.

    It could be that the socket is not making good contact with the tube (in which case it needs retensioning) or a cold joint on the inside where the wire is attached. I'd actually guess the latter if the amp is fairly new.
     
  5. hipfan

    hipfan Member

    Messages:
    2,302
    Joined:
    Nov 15, 2002
    Location:
    Evanston, IL
    John - Thanks for chiming in. That makes sense. I'll see if I can take a look at the tube socket connections this weekend.
     
  6. hipfan

    hipfan Member

    Messages:
    2,302
    Joined:
    Nov 15, 2002
    Location:
    Evanston, IL
    Update - Turns out it was a resistor going from one of the pins, to ground. It was kind of dangling and only occasionally making proper contact. I got it resoldered, and everything seems peachy again. Thanks again for the expertise.
     

Share This Page