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Electric Discharge from my Tube Amp???

Discussion in 'Amps and Cabs [BG]' started by jokerjkny, Jul 11, 2005.


  1. jokerjkny

    jokerjkny

    Jan 19, 2002
    NY / NJ / PA
    hey all,

    so i was setting the bias on my Demeter power amp, and i got a small discharge of current! while touching the bias point for power tube no. 1 with my meter, i saw a jump of current (snap crackle pop!) hit my screwdriver as i went to turn the trim pot. no harm done, thankfully.

    BUT NOW, i get an ever so low level hum when i crank the volume wide open. i dont think it'll be an issue for me live gigging, but i dont remember that hum being there before the "event". its also not present in my DB728 or 359. just fan noise.

    tried it with different basses and preamps, and the hum is ever present. it does, however, go away when i unplug the input to the Demeter. i'm gonna try swapping out the preamp tubes, and hopefully, i wont have to spring on another set of 6550's or send it back to Demeter. :(

    anyone else have any ideas?
     
  2. 44me

    44me

    Jun 17, 2002
    Bedford, NH USA
    Perhaps a bypass capacitor was damaged by the arc.
     
  3. jokerjkny

    jokerjkny

    Jan 19, 2002
    NY / NJ / PA
    :(
     
  4. tombowlus

    tombowlus If it sounds good, it is good Gold Supporting Member

    Apr 3, 2003
    North central Ohio
    Editor-in-Chief, Bass Gear Magazine
    Let me know if you ever resolve this. I have had an issue with my 300M. When I plug in my VTBP-201s, I get a loud hum and no signal. The loudness of the hum is unaffected by the volume setting on the preamp or on the amp. When I use my 201s with another amp, it does not do this. When I use my DB 680 or 659 with the 300M, it does not do this. But, when I use my Alembic F-1X with the 300M, it also shows this same behavior.

    I have biased the tubes in my 300M. I have tried different power cables, speaker cables, instrument cables, cabinets, basses, electrical outlets, and even used some Furman AC conditioners. No change.

    I have sent numerous e-mails to Demeter, and Adam Fifield seems to be trying to get an answer for me out of Jim, but so far, no dice. :confused::meh:

    It would be nice to at least hear what the Demeter preamp sounds like with the Demeter amp!

    Tom.
     
  5. jokerjkny

    jokerjkny

    Jan 19, 2002
    NY / NJ / PA
    oi...

    btw, tom, what do you have yours biased? mine's at a simple 300-305 mV.
     
  6. It MAY be similar to an issue with SVT's. The jacks on SVT's have lock washers which not only secure the jack to the chassis, but they also serve as a ground connection to it. If your bias pot uses a similar grounding scheme and the connection gets loose, it won't pass signal, be it bias current or input.
     
  7. Maybe try isolation washers in the rack? I'm thinking maybe a ground loop through the rack rails? Possibly the pre's that don't do it are painted, non-conductive, and the ones that buzz are just bare metal, or chipped paint to connect chasis together?

    Only think I can think of. Or try putting preamps in another rack as a test, seperate from power amp, see if they still hum with only the cable providing a ground path for the signal.

    Randy
     
  8. tombowlus

    tombowlus If it sounds good, it is good Gold Supporting Member

    Apr 3, 2003
    North central Ohio
    Editor-in-Chief, Bass Gear Magazine
    Good idea! Of course, I tried that, too, and no help. :rollno:
     
  9. tombowlus

    tombowlus If it sounds good, it is good Gold Supporting Member

    Apr 3, 2003
    North central Ohio
    Editor-in-Chief, Bass Gear Magazine
    I just followed the manual, so yup. Same as yours.