73' Champ tube noise

Discussion in 'Amps and Cabs' started by Seal, Apr 20, 2016.

  1. Seal

    Seal Supporting Member

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    I got a SF champ recently. It plays and sounds great but there is this kind of sound - I'm not sure how to describe it. It's not in the audio path and its still there when the amp volume is down, and it doesn't appear to becoming through the speaker. Its like once of the tubes is humming internally. It's a faint sound but enough to annoy the help out of me - I'm not playing the amp loud, maybe one 4.

    I took a sharpie (its non conductive) and kind of probe the tubes, seeing if there were any issues, and pushing on the base of the 5Y3 seems to change it a bit. The 6v6 and 5Y3 are old, so I was thinking maybe it's one of them.

    Has anyone experienced anything like this?
     
  2. zenas

    zenas Member

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    Old doesn't mean much with tubes. It does with electrolytic capacitors however. Cap can say Mallory? It's old.

    You need spare tubes anyway so buy some and try them. Get a real 5y3 not a newly made piece of crap. NOS is cheap so get a made in USA one.
    Also get a good 6v6 and single NOS USA ones are pretty cheap too. A Champ might fry a Russian tube.
    Only one 12ax7 about any thing will work but a good one doesn't hurt there either.
    Unless it's a Vibro Champ then there's two 12ax7s.
     
    Darkness likes this.
  3. Seal

    Seal Supporting Member

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    I meant old in that though it still seems to function alright, it may have an issue from years of use.

    Can cap is Mallory. I think it's the 5y3. I haven't heard a filter cap make a sound like that before, especially not in the audio path.
     
  4. redeyedjim

    redeyedjim Silver Supporting Member

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    As was said above, a USA made, NOS RCA 5Y3 will set you back $15 or less on eBay, so that's certainly worth trying. And yes, a 43 year old cap can and equally old electrolytic capacitors probably aren't helping, or providing as much filtering as they did when new. They could well be the culprit.

    Champs are a simple circuit, and as such each component in the circuit stands out more. Parts drift over time, so it's possible something has drifted out of spec. Also, a lot of owners abuse the hell out of their Champs, to the point where its common for the OT to be damaged, especially in a 43 year old Champ that's passed through many different hands. If a previous owner stuck a 6L6, EL34, etc. in it for a while the OT may have overheated and been damaged. It happens.

    Have you taken any bias measurements to see where the current tube is biased?
     
    Last edited: Apr 21, 2016
  5. zenas

    zenas Member

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    I'd replace all the electrolytics before I did anything. Give the new 6v6 bias cap some room between it and the 470 ohm resister.
    A lot of times the original tubes are still good in Champs because most of them sat in closets for years. Kids bought bigger amps as soon as they could or just stopped playing.
     
  6. Seal

    Seal Supporting Member

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    So It seems to be the 6v6 is rattling from certain frequencies. I recorded a riff on my looper that triggered the effect and then poked around and found that if I hold the 6v6 with my sharpie, putting a little tension on it, the rattling sound stops. Is there a tube damper or something I can use to keep it stable?
     
  7. zenas

    zenas Member

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    Could be a loose or dirty tube socket, a bad joint were the wires solder to the socket, or a bad tube. Tubes are easy to change so put your spare tube in it.

    Another old tube amp tip.
    Unplug it first! Then pull the fuse and make sure it's the right one, not a 20 amp "upgrade" or tin foil wrapped.
     

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