Crumbling resistors

Discussion in 'Amp Tech Center' started by Paul G., Feb 12, 2020.

  1. Paul G.

    Paul G. Friend of Leo's

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    Working on a friend's AB165 Bassman. After recap, add bias pot to bias balance, clean and tension all sockets, clean all pots and jacks I ended up with a good sounding amp, but there was a good amount of hiss, varying with the volume. Swapped out some of the 12AX7s and the noise changed but didn't go away.

    Went to swap out the plate load resistors. When I heated the joint and tugged at the resistors, 2 of them just crumbled into dust. Never saw that before.

    Now the Bright channel is dead quiet, but still some noise in Bass channel. I'll try to swap tubes again now with the new resistors.
     
  2. Dacious

    Dacious Poster Extraordinaire

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    Yep. Had that happen to a 68 Vibrochamp. Always pays to check by gently applying pressure.
     
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  3. RottenTheCat

    RottenTheCat Tele-Holic

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    Resistors are futile
    --- The Borg
     
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  4. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Super common thing in old Fenders.
     
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  5. RottenTheCat

    RottenTheCat Tele-Holic

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    I don't specifically recall "turning to dust" sort of crumbling, even on power supply or screen grid resistors. Seen lots n lots that were simply "cracked in the middle" and the same amount probably, that didn't look cracked, but "acted" cracked - especially with the ol' chopstick test. I don't recall the plate load resistors being particularly vulnerable. Usually it was the screen grid, grid stopper, then the power supply dropping/isolation resistors, but I guess anything can happen.
     
  6. muchxs

    muchxs Doctor of Teleocity

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    I wish I was still here to see it. Centuries in the future the last Fender amp with untouched solder joints and all original parts is in a nitrogen filled case in a museum somewhere. Curator decides they're going to take it out, poke and prod it for whatever reason...

    They break the seal. All the resistors turn to dust like vampires exposed to sunlight.

    :lol: :lol: :lol:
     
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  7. SoK66

    SoK66 Tele-Afflicted

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    Although visual heresey, for player amps it pays to replace the 1/2w carbon comp plate resistors with 1w metal film. His & crumble wories are over. My seemngly endless revival project '66 BFDR got a big helping of NOS Allen Bradleys. I was tempted to do the 1w metal film on it but decided I'd likely be tarred & feathered if I did.
     
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  8. zook

    zook Friend of Leo's

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    And also Vogons
     
  9. schmee

    schmee Doctor of Teleocity Silver Supporter

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    I have acquired and fixed two "broken" Bassman heads for very low prices in the past. Both only had bad CC resistors on the Power Tubes. Replace all the 100k CC resistors on the board, those are noisy. I think there's like 8 of them. IMO the CC resistor is like the wooden wheel. Should have been gone decades ago. Even the new ones I bought are often 20% out of spec to start. :rolleyes:
    [​IMG]
     
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  10. Paul G.

    Paul G. Friend of Leo's

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    The amps sounds great, but unacceptably noisy. Making a list of parts now, I'm gonna quiet this puppy down if it kills me.
     
  11. Uncle Daddy

    Uncle Daddy Tele-Afflicted

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    I did just that on my last PR build. Quieter than the Red Oktober and sounds fantastic. I hate white noise.
     
  12. Lowerleftcoast

    Lowerleftcoast Tele-Holic

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    '67 bassman same issue. It even had a 1Meg on the input jack crumble.
     
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