'64 Concert--where to check bias

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by bluesy, Apr 7, 2008.

  1. bluesy

    bluesy Supporting Member

    Messages:
    224
    Joined:
    Sep 11, 2005
    Location:
    S.E.Michigan
  2. Blue Strat

    Blue Strat Member

    Messages:
    30,036
    Joined:
    Aug 2, 2002
    Location:
    Sterling, VA (not far from Washington DC)
    There's no straightforward, safe way of doing this in an amp without bias test points.

    I highly recommend getting yourself a bias probe. Since there is no bias pot in these amps, you may need to have one installed.
     
  3. bluesy

    bluesy Supporting Member

    Messages:
    224
    Joined:
    Sep 11, 2005
    Location:
    S.E.Michigan
    there's an adjustable pot thingy in there. But I'll gather that you're telling me, based on my obvious lack of experience, that buying the right tool is the best way to go. Just trying to save a few bucks if it could be done/checked without the tool(s).
     
  4. bluesy

    bluesy Supporting Member

    Messages:
    224
    Joined:
    Sep 11, 2005
    Location:
    S.E.Michigan
  5. slider313

    slider313 Silver Supporting Member

    Messages:
    7,481
    Joined:
    Jul 28, 2005
    Location:
    NC
    The '64 Concert is a blackface AB 763 and should have a bias pot. The '60-'63 brown Concert does not have a bias pot.
     
  6. bluesy

    bluesy Supporting Member

    Messages:
    224
    Joined:
    Sep 11, 2005
    Location:
    S.E.Michigan
    Not knowing one circuit from another, there's an AA763 and an AB763 circuit listed on schematicheaven. How do I tell if mine is an AA or an AB? And don't they both have bias adjustment pots? (based on the schematicheaven layouts?
     
  7. Blue Strat

    Blue Strat Member

    Messages:
    30,036
    Joined:
    Aug 2, 2002
    Location:
    Sterling, VA (not far from Washington DC)
    Yeah, I forget this is a 763 which does have a bias pot. I've seen mostly brown Concerts.

    Still should have a bias probe. You can get away with the least expensive one www.tedweber.com sells and use it with your DMM. It's about $30 I think.
     
  8. bluesy

    bluesy Supporting Member

    Messages:
    224
    Joined:
    Sep 11, 2005
    Location:
    S.E.Michigan
    I noticed at a glance; on the AA circuit pins1 and 8 are "jumpered" on the 6l6 socket, while on the AB there's a component spanning pins 1 and 5 on the 6l6's. Would this difference let me know if I have an AA or an AB? Would biasing be different between the two? Would a bias rite or bias probe tool work OK either way? Jeeeeezus, I have a lot of questions.
     
  9. Blue Strat

    Blue Strat Member

    Messages:
    30,036
    Joined:
    Aug 2, 2002
    Location:
    Sterling, VA (not far from Washington DC)
    Any bias probe will work fine regardless.

    The pin 1/5 thing might help you ID the circuit, but a lot of Fender amps don't completely follow the published schematics. Biasing is the same either way.
     
  10. bluesy

    bluesy Supporting Member

    Messages:
    224
    Joined:
    Sep 11, 2005
    Location:
    S.E.Michigan
    Thank you very much for the input. Seems like a bias probe is the way to go. Much safer, too.
     

Share This Page