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Amp Noise

Discussion in 'Glowing Bottle Tube Amp Forum' started by bluesky636, Oct 26, 2020.

  1. bluesky636

    bluesky636 Tele-Holic

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    One of my amps has started making a soft crackling noise. The noise sounds like a combination of static and rustling leaves.

    The amp in question is a Weber 5F1 Champ clone that I built about 5 years ago. It is modified with a 5F2A tone stack replacing the low level input jack. Speaker is a Weber 8" alnico.

    Troubleshooting to date is as follows:

    1. Replaced the 12AX7, 6V6GT, and 5Y3 tubes with new ones. No real change in the noise but the original 6V6GT apparently was on the way out as the new tube (JJ 6V6S) sounds louder and better.

    2. Checked all mechanical and soldered grounds, all are good.

    3. Touched up a few suspicious looking solder joints and replaced the wire to ground from the 12AX7 cathode resistor and bypass cap.

    4. Tubes are not microphonic. Tapping on the solder joints between the input jack and the grid input of the 12AX7 with a wooden chop stick creates some noise but I think that is to be expected.

    5. Cleaned volume and tone pot with Deoxit.

    6. Noise seems to be in the 12AX7 stage as it goes away when the tube is pulled or volume turned down.

    7. Tube sockets all cleaned.

    8. Noise seems to increase as amp warms up.

    All this has resulted in a noticeable decrease in the noise but it is still present. It is not hiss.

    Any other suggestions or thoughts?
     
  2. jackleg

    jackleg Tele-Meister

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    did you clean the input jacks?
     
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  3. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    So how would we further isolate the issue besides "in the 12ax7 area" ? I'd make a signal tracer and hook that up to the amp and use that to probe and listen for the noise. Educated guess would say it's plate resistors, but building a signal tracer and using it will level up your tech skills :D
     
  4. bluesky636

    bluesky636 Tele-Holic

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    Yes.
     
  5. bluesky636

    bluesky636 Tele-Holic

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    It's a 5F1. I can change the plate resistor in less time then it would take to build a signal tracer.
     
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  6. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Totally true, but still an invaluable tool to have if you plan on working on amps in the future.
     
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  7. bluesky636

    bluesky636 Tele-Holic

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    I have been building amps (5F6A, 5F1, Trainwreck Express) and modifying them (Champion 600 Reissue, Blues Deluxe Reissue) for several years and have a BSEE. I can trace a circuit easily but a 5F1 is not worth the time.
     
  8. King Fan

    King Fan Poster Extraordinaire

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    Hmm, making an audio signal tracer with alligator clips and a guitar cable can be a 10-minute job. If you don't like that advice, I'll respectfully ask what you think would work better?

    In terms of a random but logical next step, I'd clean and re-tension the socket springs. The noise you describe would be typical there...
     
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  9. bluesky636

    bluesky636 Tele-Holic

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    a. I have isolated the noise to the 12AX7 input circuit already.
    b. There are 3 resistors in that circuit. I can replace all three in 10 minutes. Flip a coin.
    c. I cleaned and tensioned the 12AX7 socket when I swapped tubes.
     
  10. bluesky636

    bluesky636 Tele-Holic

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    I decided to replace both plate resistors and the cathode resistor/bypass cap on the 12AX7.That seems to have done the trick. After a total of about an hour playing and just sitting idling there is just a barely audible hiss. I'm satisfied.
     
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  11. alnicopu

    alnicopu Friend of Leo's

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    Yup. If you used carbon comp resistors the snap crackle pop, bacon frying, leaf noise is indicative. Agreed, easier (and cheaper) just to swap all the preamp supporting components. FYI. I lowered the NFB resistor value a tad and lowered the noise floor in mine without noticeably affecting the tone. Just go up another notch on the knob to compensate if need be.
     
  12. Nickfl

    Nickfl Friend of Leo's

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    Then why did you ask? o_O
     
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  13. schmee

    schmee Doctor of Teleocity Silver Supporter

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    That leaves rustling is often a pre tube. But you've done that. It can be very hard to cure sometimes if not that. Often, by tapping on a tube with a fingernail you can hear microphonics... but it may not be the tube, something else in that path nearby instead. I once fought an amp with that and in the end replacing the 50 year old volume pot got rid of it!
     
  14. JWH7

    JWH7 Tele-Meister

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    you say its not a hiss sou
    Is the noise a" hum" that gets louder as the amp warms up? you say its not a hiss. I had Hum problem as the my amp warmed up and I did 2 things. First eliminated a possible "ground loop" feedback interference by isolating the amp from the rest of the potentials on the same ground or circuit. use a simple 3 prong - 2 prong adapter on the end of the amp power cord into the wall outlet. Hum got quiet you could barely hear it. next i swapped the Power tubes in different positions. you say its the pre amp tubes. swapped them around and the power tubes
    there is a very faint hum i can barely hear. if the 3prong - 2pron adapter works think about getting a HUM-X device. google it and check it out
     
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