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tube amp make loud siren/hum like noise in pentode mode

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  • #1

    tube amp make loud siren/hum like noise in pentode mode

    I have newly bought JCM 25 style head (Max 25 Watts RMS) online which uses 2 x 6L6GC, 3 x 12AX7 (both JJ's). Whenever i run the amp in pentode mode with all the knobs down and just switch from standby causing huge siren/hum (https://soundcloud.com/anoop-sidharthan/pentodenoise) from speaker (Fender Super Champ 80W RMS 1x12 Cab 8 Ohms).

    But everything nice and good in triode mode.

    Did some trouble shoot by removing the pre-amp tubes - taking out and running in different combinations. And found that whenever V3 taken out the siren/hum disappears though no output to the speaker. But whenever tube present in V3 causing the noise (though tried different tubes in V3)

    Any suggestions please...

    Regards
    Anoop
    Tags: troubleshooting an amp
  • #2
    So that hum is with the guitar turned up and vanishes when you turn the guitar down? I ask because I can hear an audible "soft cut" of the noise that doesn't involve the click a switch (audible in the opening few seconds".
    "Take two placebos, works twice as well." Enzo

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "If you're not interested in opinions and the experience of others, why even start a thread?
    You can't just expect consent." Helmholtz

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    • #3
      Sounds like maybe the OT phase is backwards?
      Without schematic I can't really guess why it wouldn't do same feedback in triode mode, but maybe others more familiar with that mode could say.
      Originally posted by Enzo
      I have a sign in my shop that says, "Never think up reasons not to check something."


      ​

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      • #4
        I can't really guess why it wouldn't do same feedback in triode mode, but maybe others more familiar with that mode could say.
        Much less gain in triode mode. But we don't even know if the amp has global NFB. Without NFB loop no chance of positive FB with OT phase reversed.
        - Own Opinions Only -

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        • #5
          In the recording you can hear there is no snap as the noise fades (relatively slowly too) and the background room ambiance is still audible. I believe the poster stopped the noise at the end of that recording by turning their guitar down. Which mean either 'No guitar, no noise' or 'Grounded input, no noise'. Still not enough info for a full diagnosis and the OP could be a butterfly member. That is, they come out of nowhere, land briefly and leave something behind and then fly away forever.
          "Take two placebos, works twice as well." Enzo

          "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

          "If you're not interested in opinions and the experience of others, why even start a thread?
          You can't just expect consent." Helmholtz

          Comment

          • #6
            Though some leave better things behind than others...

            Jusrin
            "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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