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Trace Elliot Head Issue

Discussion in 'Amps and Cabs [BG]' started by marob, Apr 7, 2009.


  1. marob

    marob

    Apr 18, 2005
    Wisconsin, USA
    On my recently acquired Trace Elliot AH250 GP12, I have noticed a problem. when first turning on the amp it displays 2 issues:

    1. it goes in and out of distorion, really ugly, static distortion

    2. if I roll the blend knob to the tube side, I get lots of 60hz hum

    After 2-3 minutes, both symptoms go away

    I am thinking a cold/broke solder joint, but the tube thing is weird.

    I don't have any experience with tubes, is that normal for a tube to hum until it heats up? Or could it be bad?

    Thanks
     
  2. pbagley

    pbagley

    Jun 2, 2008
    Ham Lake, MN
    I do not think it is normal for a tube to hum until warm. You could try swapping tubes. Ask your guitarist is he has a good spare 12AX7 you could try. After that it should be only 4 screws to get the head out of the case (and their 4 little covers). If a different tube fixes the problem, then you're good to go. A decent 12AX7 is between $10 and $20. Ask around for selection advice, you'll get a lot of opinions.

    While you're in there you should clean the tube socket. The static could be a dirty connector to the tube. A spray contact cleaner should work fine.
    Distortion while the tube is heating up is kind of expected. You should notice a ramp up of sound, getting louder and cleaner, and the tube warms up.

    If a tube swap does not cure the problem you may have a bad solder joint, a loose tube socket, or some other malfunction.

    Solder joint - look for the large capacitors in the power supply. These are most vunerable to shock due to the weight and only being anchored by the two solder joints. I just fixed a Fender Stage 100 with this problem.
    With the amp out of the case plug everything in except the power cord. Be careful not to touch anything (like bare wire, connectors, etc) inside the amp. Take off your watch and ring and any other metal on your hands before starting (they get really hot if they complete a circuit, and you don't want to know what 'gloved' means). Standard disclaimer about observing all safety procedures for servicing amps. If you don't know it's safe to touch, then don't touch it. The tube will get hot. Plug in the power. Turn the amp on. Wiggle the capacitors gently and see if there is a change in sound. Check the other components. The time tested method is to use a pencil, the eraser gently taps and wiggles the resistors and capacitors, maybe even the tube, to see what makes noise. You may also find a number of microphonic capacitors in the preamp this way.

    When you find the guilty component you can resolder the connections. If that don't work you can replace the component.

    Good luck.

    - Paul
     
  3. BassmanPaul

    BassmanPaul Gold Supporting Member

    Aug 25, 2007
    Toronto Ontario Canada
    I couldn't find the service manual for the 250 but if it's like the 300 it has a mosfet output stage one 12AX7 and the 12 band EQ. No your tube section should not hum although that symptom does describe a possible heater/cathode problem with the tube. Replace it and see if that helps.

    The distortion will be harder to track down. It could be that the power amp has a component that is temperature sensitive and is messing up the bias.

    Paul
     
  4. marob

    marob

    Apr 18, 2005
    Wisconsin, USA
    Well, after some more investigating, I've come up with more things.

    The distortion is accompanied by a weakening in the input signal. Regarding the green(weak), orange(good), and red(clip) LED input indicators - the green gets very faint and almost goes out when this distortion occurs. Now, I thought I better check my bass on my other amp just to be sure it's not at fault- will report later - EDIT: checked the bass, it is fine.
    The signal is also very weak overall. I need to turn the input gain almost all the way up to get the orange(good) LED lighting consistently. When I first started messing with this amp, I only needed the input gain about half way up.

    Lastly, I have been disconnecting the speaker cab when ever I turn on/off the amp, otherwise there is a loud pop, even with the output volume at zero.

    I will try to get my hands on a tube, but I really don't know many guitar players at this point.

    Thanks to the replies.
     
  5. marob

    marob

    Apr 18, 2005
    Wisconsin, USA
    Progress-

    I was doing the pencil test, and I discovered that when I rap the main board on the left side near the valve, the sound distorts and crackles each time I poke it. Tapping on the valve itself does nothing.

    So, something is being affected by the vibrations from the pencil taps...
     
  6. rdpbass

    rdpbass Supporting Member

    Oct 29, 2005
    Utah
    I have the same head and I have the schematic for it I just have to find it. I had to replace the (transformer coil) I think thats what you call it, a few years ago. I'll send it to you when I find it.
     
  7. marob

    marob

    Apr 18, 2005
    Wisconsin, USA
    Thanks, I'd really appreciate that.
     
  8. marob

    marob

    Apr 18, 2005
    Wisconsin, USA
    FIXED- one or more solder connections on relay 2 were bad. Re-soldered, works great now.

    Thanks for all the help!
     

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