Crackling noise on 1973 Super Lead 100 watt

Discussion in 'The Workbench' started by 1973superlead, Nov 1, 2012.

  1. 1973superlead

    1973superlead New Member

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    Hey everybody! I've been a member on here for a while but this is my 1st post. I don't know a lot about amps but have learned quite a bit just by reading the threads on here. My old Super Lead amp seemed to start humming a little louder than usual about 4 months ago, about the same time I got my brand new Vox AC30 C2. Maybe it was jealous? :Ohno: It shouldn't be because I still love it. Anyway, the hum was there whether I had my guitar plugged in or not. I decide to change out a couple of preamp tubes to see if that helped. The v3 tube was changed out about 1 1/2 years ago so I didn't see the need in replacing it. I also tried cleaning the sockets on all 3 of the preamp tubes by spraying DeoxIT D5 on the pins of the tubes and inserting them and then pulling them back out several times. I hope DeoxIT was ok to use. After cleaning the sockets and installing the new preamp tubes, the hum was still there and now I'm getting a crackling kind of sound but not all the time. Just when I play certain chords like an open A or if I hit a B bar chord on the 7th fret, it's really bad. Also, it seems to be ok when I'm using the lead pickup on my Strat. It's mainly when I'm using the rhythm pickup (the more warmer, bass sound) that I get the crackling sound and when I make that B bar chord on the 7th fret, it's really bad. Sometimes when I make that chord, I get a really loud noise, like a low pitched vibrating type of sound. I will have the power tubes replaced if that's what's causing the problem but I'd rather not do that right now if that's not going to solve this problem. If anyone has any ideas on what's causing this, I would greatly appreciate your input. Thanks!
     
  2. plexifier

    plexifier New Member

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    Are the filter caps original or older than 15 years? They might be the cause, especially the "low pitched vibrating type of sound" and "humming".

    Replacing filter and bias caps is standard maintenance. Either could cause what you've described.

    It may also be power tubes and they should be replaced if they are old or have a lot of playing time on them.
     
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  3. GIBSON67

    GIBSON67 Well-Known Member

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    Welcome to posting on the best forum!

    I would go ahead and change the PI tube just to eliminate that tube, also.
     
  4. 1973superlead

    1973superlead New Member

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    Yeah, I've had the amp since 1985 and I don't recall ever having the filter caps or bias caps replaced. The man that I used to take it to said he replaced the polarity cap about 3 years ago. He moved away from my area a couple of years ago. Now, I have to drive about a 100 miles or so if I need to have it looked at. But if that's what I have to do, then that's what I'll do. Wish I could work on it myself but I don't want to be killed. The power tubes do have some age on them too. Also, they're not a matched set. They're JJ's in 1 & 4 and Tesla's in 2 & 3. A guy in Nashville who is supposed to be an authorized Marshall repair tech replaced one of the JJ's last year and I thought it was kinda strange to just replace one power tube but it has sounded great for about a year until these recent issues came up. Thanks for your input Plexifier! :)
     
  5. 1973superlead

    1973superlead New Member

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    Thanks GIBSON67! This is a great forum! I did do some swapping around on my preamp tubes including the PI and still seemed to have the same result but I will try to go ahead and pick up another new preamp tube so that all 3 will be brand new.
     
  6. grainman

    grainman Active Member

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    im stock with hummin with my super lead also...its loud and i noticed it when i had my amp back from the tech wich changed the hum balance pot and few resistors ....no more pot is in there now just resistors....someone have a clue of what is going wrong in there? thanks
     
  7. m1989jmp

    m1989jmp Active Member

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    Replacing old filter caps is standard maintenance, do it regardless.

    Also, whenever there's a problem of this nature, someone will suggest power tube replacement; if nothing, to eliminate that possibility.
     
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  8. Kris Ford

    Kris Ford Well-Known Member

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    When my Super Bass started crackling, it wound up being a few resistors..
     
  9. anitoli

    anitoli Well-Known Member

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    Plate resistors especially carbon comp types can add much noise as they age.
     
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  10. neikeel

    neikeel Well-Known Member

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    It is unlikely to be cc plate resistors in a stock 73 SL, however you do see 10k and 8k2 ccs in the B+ dropping chain.
    From the description of resonant crackling you have to think of a failing connection somewhere either a solder joint or internal to a component or, more likely output valve bases (are yours black/brown phenolic or ceramic?). Also you have not said how old your output tubes are (or I missed that bit).
    Personally I would give the amp a good clean and service (I am not a random cap swapper either so would not replace them shotgun).
    Ymmv but I suggest a clean, reseat the octals with a tiny dab of deoxit gold on each pin, preferably new output tubes correctly biased and check all operating voltages correct, proper ground wire too (ideally omitting the death cap and switch).
    Feel free to post pics and update when you decide what to do.
     
  11. ampmadscientist

    ampmadscientist Well-Known Member

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    There is 2 problems:

    1. Filament Buzz : it's not the capacitors. People confuse these noises all the time.

    2. The bias is too cold. This is what causes cracking distortion.
     
  12. ampmadscientist

    ampmadscientist Well-Known Member

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    The hum balance control explodes.
    And so, the hum balance control should be removed, and replaced by 2X 100 ohm resistors.
    This would be the best way to prevent problems, that we know of (so far).

    BUT also: because the hum balance control doesn't really work at all...
    The "hum" which is filament noise - is caused by the position of the filament wires...

    The filament wire layout needs to be changed. This is what causes the "hum".....it's not really HUM at all...it's AC voltage bleeding into the audio path.

    And people blame capacitors for this all the time.
    It's not caused by capacitors.
     
    Last edited: Sep 16, 2017
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  13. grainman

    grainman Active Member

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    ...
     
    Last edited: Sep 16, 2017
  14. ampmadscientist

    ampmadscientist Well-Known Member

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    Try freeze spray...freeze components one at a time.....

    If you freeze the resistors one at a time, the cracked / faulty / noisy resistor can reveal itself.

    Carbon resistors are glued together...the leads are attached with glue.
    over time, the glue can crack due to constant cycle of heating / cooling.

    But do not depend on "ohm meter" reading to find a faulty resistor.

    When the resistor is under power, and heated up, the reliability can also change...
     
    Last edited: Sep 17, 2017

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