1976 Ampeg V4B Question

Discussion in 'Amps and Cabs [BG]' started by bassmun, May 17, 2018.

  1. bassmun


    Feb 3, 2014
    So, I just purchased a 1976 Ampeg V4B. There are a couple things that need to be addressed, and I wanted to see if anyone else has had these problems or knows what they might be. The first problem is that the amp hums even when the volume is all the way down. Is that normal? I am not familiar with these amps yet. It appears that all of the Electrolytic caps have been replaced fairly recently. The can multi caps appear to be the originals. Not sure if perhaps replacing the can caps would help with the problem. I've tried adjusting the hum control on the back, and it seems that even at it's sweet spot it still hums pretty good.

    The other main issue is that the midrange pot seems to hum really badly when I cut or boost, when it's dead center it's fine (or doesn't hum as badly anyway). Would it be worth replacing the pot, or is something else most likely failing which is causing the hum?

    Thanks in advance for any help guys.
  2. beans-on-toast


    Aug 7, 2008
    My ‘74 V4B is dead quiet. You shouldn’t be hearing any hum.

    Bottom line, a tech needs to sort it out. There are so many possibilities and chances are, there is more than one issue.

    Before you change anything, I would check the tubes. The hum problem could be down to a bad tube. Since the mid adjustment affects the hum, perhaps a tube in the EQ stage has an issue. If the shielded wire going to the ext amp jack breaks at the jack, the amp can hum. There could be a bad ground. The list goes on....

    It is possible that the cap cans have expired. These amps run very hot and that takes a toll on the cap cans. But if the amp has had the other ecaps changed, you’d think that the cap cans would have tested good.
  3. BassmanPaul

    BassmanPaul Inactive

    Take a look at the output tubes when the amp is operating. Are there signs of any red spots on the plates?