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BF Bassman hiss

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  • #1

    BF Bassman hiss

    I recently bought 67 Bassman with the AB165 circuit. I already modded the power section (including bias) to AA864.
    There is way too much hiss (no hum) for my taste. This hiss occurs also when the vol pots are completly down. Already changed all the tubes. Other than that the amp sounds great.

    What I am trying out is replacing all 100k plate resistors with metalfilm. Also I will replace resistors that are in the path with the filter caps with Metallfilm Metalloxid. Any ideas other than that? What could be the typical reason for hiss in those amps?

    I am just finished my parts lists for ordering. And I don't want to order twice if possible
    Thanks alot.
    Last edited by shocki; 07-12-2014, 08:54 PM.
    Tags: None
  • #2
    Hey Shocki,

    I reckon it's something after the volume controls... don't know if there's a "typical" reason for this, but... check the channel mixing resistors - should be 220K, maybe 470K. I have a 67 AB165 that would make hissing/crackling noises on warmup; replacing those two resistors eliminated 95% of it. As always, check and replace them one at a time - no need to shotgun these amps, and it could be any one or a combination of the old CCs.

    Does yours have the 220K feedback resistors from plate-to-grid on the power tubes? Mine were removed; otherwise my amp is stock, well, it has a Twin OT... I love it! Good luck!

    Justin
    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    • #3
      yes it has the feedback resistors. I will try replacing the mixing resistors.

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      • #4
        Have the filter caps been replaced?An amp that age should be re-capped.The fact that it is there with the volume off tells me it is coming from the output section.

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        • #5
          No its not a hum. The filter caps are already replaced. All tubes are replaced. Its a hiss. My Deluxe Reverb also has a bit of hiss. But this 3 times as loud I would say.
          What I will do now. When I pull the preamp tubes and leave the PI tube inside there is no hiss. I think it is between the vol pot and the PI somewhere.

          1) clean the tube sockets with contact spray
          2) replace the plate resistors
          3) replace resistors close to the filter caps

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