Nasty hum in my '74 V4B

Discussion in 'Amps and Cabs [BG]' started by MichelD, Apr 17, 2019.


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  1. BassmanPaul

    BassmanPaul Inactive

    Look at the output tubes to see if any of them have red areas on their plates when in use.
     
    bobyoung53 likes this.
  2. Sunnampeg

    Sunnampeg

    Nov 11, 2019
    UK
    no red areas , they are new.
     
  3. Sunnampeg

    Sunnampeg

    Nov 11, 2019
    UK
    it is main tranny ..
    but if i put a video cell close the hum remain the same
     
  4. BassmanPaul

    BassmanPaul Inactive

    Them being new makes no difference if there is a problem with the bias supply to one or more sockets.
     
    Sunnampeg likes this.
  5. Sunnampeg

    Sunnampeg

    Nov 11, 2019
    UK
    i dont know what is a Bias supply. anyway the amp is at the amp repair shop.
    I hope he s going to find the fault.
     
  6. MichelD

    MichelD

    May 19, 2014
    I'm eager to find out what the tech finds.
     
  7. Sunnampeg

    Sunnampeg

    Nov 11, 2019
    UK
    me too..
     
  8. BassmanPaul

    BassmanPaul Inactive

    Me three. :)
     
    Sunnampeg likes this.
  9. seamonkey

    seamonkey

    Aug 6, 2004
    It might be loose bolts holding the transformer.
    When tech service an amp they should check tightness on bolts, screws, ground lugs, jacks, etc.
    Just a thought
     
  10. BassmanPaul

    BassmanPaul Inactive


    Your amp is what's known as a fixed bias amplifier. the output tubes cathodes are grounded and a negative voltage is connected to the control grids to keep the tubes under control.This is called the negative bias supply. If that supply fails the output tubes run away and draw excessive current. So much so that the anodes/plates glow red and a loud hum is heard via the output. The tubes will eventually fail Under the stress.
     
    Sunnampeg likes this.
  11. Old Fart

    Old Fart

    Mar 11, 2011
    I am not an expert, but I think 7027A tubes are rated for 30 watts each and I think the folks who designed the V4 knew that.
     
  12. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    There is a difference between plate dissipation and audio power output. 2 different parameters that are only loosely related.
     
  13. Sunnampeg

    Sunnampeg

    Nov 11, 2019
    UK
    very very interesting observation just to be clear, on a fixed bias amp bias voltage should be always negative? is this right?by the way the amp has a quartett of TAD rca style black plate 6l6 long bottle.before had jj7027, but i ve never have an amp with 7027 or 6l6 , i know 6550 get a blue color inside when they light, both 6l6 and 7027 were had more orange red ish color.
     
  14. BassmanPaul

    BassmanPaul Inactive

    Yes the bias voltage should always be negative. A pet peeve: the tube you referenced is a 6L6GC. The letters are important 'cos there are a plethora of different 6L6 tube types. The 7027 is a super 6L6 tube. The orange glow is likely the heater. If the anodes/plates are glowing red the tube is drawing too much current likely from a too low bias supply. Many tubes exhibit the blue glow though some are more difficult to see than others. If you peer into the slots in the anode/plate of an EL34 you'll see it too.
     
  15. Sunnampeg

    Sunnampeg

    Nov 11, 2019
    UK
    so the tech said that its a cable that runs in front of EQ knobs and toggles switches PCB to the other PCB,also not enough current in the preamp section, those two things together cause this Hum, He proposes to add a dc trans for the preamp section .or check the main transformer, I don t know. I m not an expert. very tricky and confusing.what do you think?
     
  16. Sunnampeg

    Sunnampeg

    Nov 11, 2019
    UK
    thanks for your answer
    so the orange glow is a sign of any kind of sickness? low bias?
     
  17. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Well, the basic amp design amp itself doesn't seem to suffer from this problem, so your tech needs to identify the cause of the problem and correct it rather than modifying a known good design. That is simply a hack.

    What exactly is wrong with the DC voltage (current flows BECAUSE of voltage)?
     
  18. BassmanPaul

    BassmanPaul Inactive

    If you are talking about an orange glow in the centre of the tube's structure that is the heater and needs to be there as it supplies the electrons that make the tube work.

    A red glow on the anode/plate signifies too high a current flow within the tube.
     
    Last edited: Jun 8, 2020
    bobyoung53 likes this.
  19. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    I think what your tech is saying is that he wants install a dc heater circuit for the preamp. This can make an amp quieter. I seriously doubt that this is the reason for your excessive hum problem.
     
  20. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Unless that is the identified problem (which I doubt), it's nothing more than a tech hack-up IME.
     
  21. Primary

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