nasty 120Hz hum & scope trace question

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by OlAndrew, Aug 11, 2008.

  1. OlAndrew

    OlAndrew Member

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    I've got an old Gibson I use for a hobby, the thing has never been right . Heavily and badly modded when I got it, blown caps and trannies, the whole bag. 6eu7 preamps and EL84 outputs, push-pull. Reverb & trem, but that's irrelevant. Pair of diodes for rectifiers, with some small resistors and caps on them to kill switching noise.

    Got a bad 120 Hz hum, as soon as I come off standby. Scope shows grid input on the EL84's at just about flatline, there's a little noise on the screens, but not much, (huge choke in the power supply just before the screen, but after the power tube plate supply).

    We're looking at the typical setup, power trannie has two leads that go to the diodes, and a center tap from that winding to ground. DC + side of the diodes goes to + on an 80 uf cap, other end of cap grounded and connected to the centertap lead. Cap + hot lead goes to the center tap on output trannie primary, other output primary leads go directly to plates on the EL84s. Output secondary goes to the speaker, there's no ground at the jack, its isolated. (tried a ground there, no difference).

    Going hunting with the scope, we see a huge 120 Hz sawtoooth at the diodes, kinda what I'd expect. At the cap +, and at the plates, we see a much smaller, but identical sawtooth. At the speaker jack, we see a sort of rounded smoothed version of the sawtooth, fairly big, and that;s what I'm hearing.

    Now, two questions.... As I understand it, in a push-pull amp like this, the noise on the plates from the powersupply should be out of phase and cancel each other as far as output noise. Maybe not perfectly, but my rig isn't doing it at all, and it looks like the plate noise signals are IN phase. Am I crazier than usual, or is this quite weird?

    Second...that sawtooth at the cap + end and the plates....its the same waveform as at the diodes, just smaller. I'da thought it should be smoother and rounder and flatter than the waveform at the diodes. Tried different caps, alla same, no mo betta.

    Any ideas what I'm dealing with here, (besides my ignorance?) Could brute force it, with a mongo choke on the plate B+ line, but I'd much rather know what's really wrong and fix it properly.
     
  2. JJman

    JJman Member

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    Sounds like some wiring from the HV to the diodes is coupling onto the OP tranny primary. Although I don't think I've ever heard of this. Are these wires very close to each other?
     
  3. VacuumVoodoo

    VacuumVoodoo Member

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    Not weird at all. Sawtooth ripple you see at the plates is in phase but gets canceled in the transformer. Primary winding of the OT are arranged so that DC current flowing from CT through each half of winding i.e. to the respective plate causes equal but opposing magnetic flux in the OT core. If windings are perfectly symmetrical the net magnetic flux will be zero. Same mechanism applies to in phase ripple. Think of it as the halves of primary winding being wound in opposite directions. If there's a shorted turn in one of the primary sections this ripple cancellation will be severely corrupted.
    Tubes do pushing part while transformer pulls in push-pull arrangement.

    Both the transformer and tubes should be well matched for this ripple cancellation to be effective. Skewing bias on output tubes is one way to balance out mismatch in power tubes and OT.

    Are you talking about cap after the choke? If replacing the cap has no effect than the choke could have am internal short. Replace the choke temporarily with a 1-3kOhm resistor and see what happens.
     
  4. OlAndrew

    OlAndrew Member

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    OK. There's no coupling between the wires. This thing is a plumber's nightmare for wiring, but the pre and post filter cap wires a way way apart.

    Looks like I've got maybe a bad output tranny....puts out sound, but NONE of that canceling effect is happening, and the waves at the two plates appear to be IN phase not out, so there's no canceling there. Unfortunately, whilst my scope is dual channel, it won't display both traces at once. This thing has always put out way to much harsh edgy highs, despite some massive hacks to reduce the treble. Wonder if that's an output tranny thing too...

    [edit...DUH...the ripple SHOULD be in-phase, well ti is, but it's not canceling, hardly at all....gotta check the bias and currents, I guess, for a start...thing's cathode bias though....tried different tube sets, no change, so I didn't think tube matching had a lot to do with it. Oh well, onward through the fog.]

    I've got the London Power books, and he describes the bias balancing thing, I may try that, or a feedback thingy. This is an experimental rig, after all. Always better to start from a known good, rather than doing Harley fixes on iffy problems, though.

    The cap I'm talking about is the first one to the trannie and the plates. The power tree goes: Diodes, 1st cap to trannie and plates, choke, second cap to screens, resistor, third cap to pre-amp V2, resistor, 4th cap to pre-amp V1.

    Grounds on the caps are arranged by circuit, that is, plate cap is grounded at same point as the power tranny and the power tube anodes, preamp caps are grounded with the preamp anodes. Its 2 stars, really.

    I had thought that the filter cap would modify the waveform, not just reduce the amplitude of the ripple. I'll have to take the scope to some rigs I know are working quietly, to give me some comparison.

    Thanks for the explanations, with every explanation , I learn a bit more.
     
    Last edited: Aug 12, 2008
  5. Tone_Terrific

    Tone_Terrific Supporting Member

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    I'd clip in a cap in parallel to the first one to kill more ripple, maybe, if I read that right...but you probably tried that.
     
  6. VikingAmps

    VikingAmps Member

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    I would try jumping the power tube grids to see if the hum goes away. That's a big clue.
     
  7. Swarty

    Swarty Member

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    I'd measure the DC resistance between the CT and plates. If there's a difference of more than about 10% you've probably got some fried windings.
     
  8. donnyjaguar

    donnyjaguar Member

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    All of the above, and check for whipped EL84. I've seen and heard exactly what you described and it was the tube.
     

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