Amp hum with no guitar

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by marinblues, Jan 28, 2005.


  1. marinblues

    marinblues Member

    Messages:
    1,875
    Joined:
    Dec 19, 2003
    Location:
    Italy
    Hi,

    my amp is producing a constant hum. These are the symptoms:

    1) hum with no guitar connected
    2) hum does not increase by increasing preamp gain
    3) hum does not increase when increasing pre-amp volume
    4) hum increase only when increasing master volume.


    What could it be? The output tubes? Grounding?

    thanks

    Marin
     
    ZoomP likes this.
  2. aeolian

    aeolian Member

    Messages:
    6,083
    Joined:
    Apr 5, 2004
    Location:
    Santa Cruz, CA.
    M.

    Did this suddenly start happening? Your amp is fairly new. Is it a 60 Hz or 120 Hz hum? Really strong or just more than before? Your amp has a SS rectifier right?

    Not likely to be output tubes. The hum comes from the AC line though the power supply. The filaments (heaters) of the tubes are usually run from AC and that can couple to the signal. The wireing that hooks them all to the power supply can couple to the signal cabling. A diode in the rectifier may have given out letting through AC. But it's more likely some capacitor that is supposed to filter the AC isn't doing such a good job anymore.
     
  3. marinblues

    marinblues Member

    Messages:
    1,875
    Joined:
    Dec 19, 2003
    Location:
    Italy
    Steve,

    The hum is kind of low freq, I'd say it's 50hz. I can't be sure, but I think I began noticing it since we moved into our new flat. There are no funny electronic devices around it so I thought that it could be a grounding problem.

    I don't know if the rectifier it SS.

    the hum is loud and annoying and has a constant level.

    thank
     
  4. aeolian

    aeolian Member

    Messages:
    6,083
    Joined:
    Apr 5, 2004
    Location:
    Santa Cruz, CA.
    I wonder if the grounding in your new place is bad and there's noise on the mains ground. Too bad you don't have a scope handy to check.

    Possible that one or more of the filter caps shorted out and are now passing AC.

    Maybe some others know about possible shorting or coupling in tubes. I don't know enough about the subject.
     
  5. VaughnC

    VaughnC Supporting Member

    Messages:
    14,814
    Joined:
    Jan 4, 2002
    Location:
    Pennsylvania
    Could be a preamp tube....sometimes they develop heater to cathode leakage.

    Could be a bad power supply electrolytic....sometimes they open, develop internal leakage, or short (a short should cause the fuse to blow). However, this is unlikely if the hum volume changes with the master volume control.

    Could be a bad internal ground connection/solder joint.

    Amp have reverb? Could be a reverb problem too.

    Obviously, the tubes would be the easiest thing to check first by substitution....from your symptom description, most likely the one(s) located in the circuit between the preamp volume and master volume controls (final preamp or phase inverter). Doubtful it's a power tube problem if the hum volume changes with the master volume control.
     
  6. marinblues

    marinblues Member

    Messages:
    1,875
    Joined:
    Dec 19, 2003
    Location:
    Italy
    Thanks guys. I'll recheck the tubes.

    However, I understand that there is some device which helps with poorly grounded power supplies?
     

Share This Page