1. boombaxx

    Thread Starter New Member

    Jul 3, 2013
    Hi there folks i have a problem i am trying to resolve. I have a Marshall B150 bass amp that is not working properly. I thought it would be the preamp valve or glass fuse but it is neither. The amp has very little volume and emits a humming noise on switch on without anything plugged into it. What would be the likely cause of this?.
  2. tubeguy

    Well-Known Member

    Nov 3, 2012
    It could be a blown transistor and possibly related parts in the power amp. If so it's putting DC to the speaker, so don't leave it turned on like this with the speaker connected or you risk burning out the speaker.
    To verify this, disconnect the speaker and measure the DC voltage on the speaker output.
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
    I vote for a power supply cap or rectifier at power supply.
  4. #12


    Nov 30, 2010
    I vote for Gopher. Bad power supply capacitors are famous for causing hum and low power output.
  5. boombaxx

    Thread Starter New Member

    Jul 3, 2013
    Thanks for the answers i will check this tomorrow.
  6. Mike33

    AAC Fanatic!

    Feb 4, 2005
    If this amp has a tube output section, DO NOT DISCONNECT THE SPEAKERS AND TURN THE AMP ON!!! EVER!!!! EVER!! You probably WILL fry the $150, hard-to-replace output transformer if you do. Had to say that, or someone coming along may try the 'disconnect and read the DC' trick....yours is probably solid state.

    That said, this sounds like it MAY be your PS filter caps...when they age, they leak, and you get hum with no input (a noisy amp). How old is the amp? Does it EVER sound reasonable (good volume)?

    Now, the fact you have very low output isn't good - but it isn't terrible either! At least you have sound, so you know that *most* of the amp is working. Your trouble, if not the capacitors, could easily be a bad solder connection to a transistor, a burned out power transistor, or even some issue in the preamp part of the amp. All it takes is one diode, cap, or resistor to go back, and the output goes away. The only way to really know what's up is to use an audio probe and go thru it, trying to find an "off specifications" section. You can work in from the input, or from the output backwards (I'd go backwards). If you are pretty good with electronics, this is what you have to do. If not, please don't expose yourself to lethal voltages by poking around inside, ok?? Plus, one bad move and that amp is not going to be worth the cab it's in.

    Hummmmmm is generally related to DC leaking in from a cap somewhere, at least. Hope that helps. You kinda have to be there to get more specific!