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Hum from TTE

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  • #1

    Hum from TTE

    Hello,

    I'm a proud owner of a Fulltone Tube Tape Echo (TTE) but have a little hum issue.

    I got the TTE in the U.S. but brought it back France.
    Since I already have a couple amps running on 110V, the TTE is running on the same 750W grounded step-down transformer, which measured output is 119V / 50hz.
    Neither amp hums without the TTE, but with it the Fender Super Reverb amp has some hum and the Deluxe has more, enough to be bothersome.
    Could a small DC voltage on the power circuit cause the TTE toroidal transformer to produce some hum?
    Another related note - there is a semi-loud <crash> noise when I switch pickups on my strat, like a capacitor discharging. Very annoying and sounds like something is really wrong. This never happened before using the TTE, and does not occur when I don't use the TTE.

    So... I measured the DC voltage on the TTE input line (with no output connected), at 0.5V. I could be wrong but I believe that's high.
    I also measured the DC voltage at the 220V receptacle at 0.05V.
    Is it possible this DC voltage is being carried through and maybe amplified to 0.5V on the input?
    The real question - is it this DC voltage causing the amps to hum and the strat to make noise between moving the pickups?
    If so, I was thinking of cleaning up the power by building this DC filter (http://sjostromaudio.com/joomla/inde...d=62&Itemid=27) to place before the 750W step-down transformer. Would that cure it? (Or am I way off.)

    The TTE is running on 120V, but I could change it (per the manual) to 240V. But I'm not sure that would help, and the change (in theory) should not really be needed since I should have good voltage (119V) already.

    I have tried other receptacles in the apartment, but so far the TTE causes hum no matter where I plug it in. Moving it away from the amps or around in the room makes no difference. I may bring the whole setup to my office to test there. I've also tried several guitar cables - no difference. Cables are not long. I don't have an oscilloscope.

    I've also written Fulltone but thought I'd ask here too.

    Thanks for your help and advice,
    Paul
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  • #2
    SOunds more like a ground loop to me. The AC potential of the ground common in two pieces of equipment can vary. And when they are not the same, if their chassis are connected together - as would be the case when signal cords connect them - this small voltage difference would flow between them.

    It can do this DESPITE being plugged into the same mains strip.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      Hi Enzo,
      That makes sense. I tried lifting the ground on the TTE, but the hum is the same. That's why I was thinking it could be DC voltage... What else could I try?
      Paul

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      • #4
        With the TTE independent of the amps - not connected - the input chord has 0.5V DC across it.
        When plugged into the amp, that, I believe, is causing terrible (loud) <crash> noise when switching guitar pickups, or adjusting volume or tone, guitar-side.

        Why does the input line have such voltage?
        What can I do about it?
        Again, it has this voltage even if the TTE is not plugged into the amp.

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        • #5
          Originally posted by pbiagi View Post
          With the TTE independent of the amps - not connected - the input chord has 0.5V DC across it.
          When plugged into the amp, that, I believe, is causing terrible (loud) <crash> noise when switching guitar pickups, or adjusting volume or tone, guitar-side.

          Why does the input line have such voltage?
          What can I do about it?
          Again, it has this voltage even if the TTE is not plugged into the amp.
          If the input tube shorts, it could place dc voltage on the grid of the tube, causing the symptom you describe. Try replacing the first pre-amp tube.

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          • #6
            And lacking a DC blocking cap on the inoput, the grid leak often can result in a similar voltage on the grid. That would be negative.
            Education is what you're left with after you have forgotten what you have learned.

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            • #7
              Thanks guys!
              I'll try replacing the tube in the TTE and see what that does.
              It is true that the loud scratchy noise started when I put "good" NOS tubes in the TTE.

              I'm not absolutely sure, but sounds like I may have two issues:
              1) DC voltage causing terrible noise
              2) hum on both amps, but mostly on the deluxe caused by ground loop/potential issue between the amp and TTE.?

              I understand the basics of the techy stuff, so really appreciate your all's guidance!

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