AB165 Bassman Bias Pot

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers T' started by Nolatone Ampworks, Jan 17, 2008.

  1. Nolatone Ampworks

    Nolatone Ampworks Supporting Member

    Joined:
    Jun 4, 2006
    Messages:
    2,642
    Location:
    Flowery Branch, GA
    Hey folks,

    It's my understanding that full sized pot that connects off the bias board is the bias pot. However, the schematic has a note that says:

    "* 4. When installing new 6L6's, reset hum baalance for min hum."

    Can someone please clarify what this thing is really for?

    Thanks,

    Paul
     
  2. teleamp

    teleamp Member

    Joined:
    Aug 22, 2006
    Messages:
    3,520
    Location:
    Central Texas
    I guess that was the low tech way to bias an amp back then. Its definitely the bias pot. I guess if the amp hummed after installing new tubes it was biased too hot. Makes some sense, to prove it out, set the bias on an amp by ear then measure, chances are that you got it really close by ear.

    MikeY
     
  3. Nolatone Ampworks

    Nolatone Ampworks Supporting Member

    Joined:
    Jun 4, 2006
    Messages:
    2,642
    Location:
    Flowery Branch, GA
    I just read about how this bias circuit works. It's a matching bias circuit in that you adjust the current on one tube to match the other non-adjustable tube.

    If this is true, what happens if you don't get the current level you want through the non adjustable tube?
     
  4. Blue Strat

    Blue Strat Member

    Joined:
    Aug 2, 2002
    Messages:
    27,682
    Location:
    Sterling, VA (not far from Washington DC)
    165's used a bias balance circuit, not "true bias". Best thing to do is convert the circuit to "true bias". Check the AA864 Bassman schematic for how this is wired.
     
  5. sickboy79

    sickboy79 Member

    Joined:
    Aug 17, 2006
    Messages:
    4,698
    Location:
    Chicago
    Agree 100%. This will allow you to properly bias the amp - as well as it improves the tone, IMO.
     
  6. Swarty

    Swarty Member

    Joined:
    Dec 16, 2003
    Messages:
    1,145
    Location:
    Michigan
    It's just a matter of moving a few wires.
     
  7. Nolatone Ampworks

    Nolatone Ampworks Supporting Member

    Joined:
    Jun 4, 2006
    Messages:
    2,642
    Location:
    Flowery Branch, GA
    Which circuit is newer? Is the AB165 older than the AA864?

    Trying to understand why Fender would do the Bias circuit that way in the AB165.

    Also, does a mod like this hurt the value of the amp?

    Thanks,

    Paul
     
  8. Nolatone Ampworks

    Nolatone Ampworks Supporting Member

    Joined:
    Jun 4, 2006
    Messages:
    2,642
    Location:
    Flowery Branch, GA
    I think maybe I just figured out the benefit of this particular bias circuit. Here's my SWAG:

    It allows you to balance unmatched powr tubes, which back then, suppliers matching tubes probably wasn't as big a thing as it is now, so they needed a convenient way to balance the current flow through the power tubes.

    Am I right?
     
  9. John Phillips

    John Phillips Member

    Joined:
    Mar 17, 2002
    Messages:
    13,113
    Location:
    Scotland
    Back then, tubes were much closer to the designed spec, but not usually sold as matched sets - and Fender certainly didn't buy them as such - so a bias matching control was more useful. Now, it's easier to get matched sets, but tube performance is all over the place, so an overall bias adjustment is more useful.

    In fact, the AB165's bias scheme is poorly thought-out anyway, and even if you wanted a bias matching (AKA bias balance), there are better ways of doing it, like the one Fender introduced with the next generation of amps where the arrangement is symetrical and adjusts one tube down as the other goes up, rather than keeping one fixed and adjusting the other. Why they did it like this I don't know - but they made several mistakes around this time... not all of which can be blamed on CBS, including this one - the circuit was finalised in January '65, the same month CBS bought Fender, so it must have been done by the existing design team.

    You want to fix it in any case...

    Modding this part of the circuit does not affect the value in any way, if anything it's regarded (rightly) as an upgrade.


    BTW, you can work out the chronology very simply - the three digits are the month and year, and the second letter is the circuit revision... so AB165 is the second circuit from Jan '65.
     
  10. Nolatone Ampworks

    Nolatone Ampworks Supporting Member

    Joined:
    Jun 4, 2006
    Messages:
    2,642
    Location:
    Flowery Branch, GA
    Great info John. Thanks. Now, would you indulge me in a little instruction. Here's what I think that circuit is doing, but I'm still not 100% refreshed on my theory that has been dormant since the 80's!

    Given the following schematic:

    http://www.el34world.com/charts/Schematics/fender/BASSMAN_AB165.pdf

    Bias voltage coming off the diode, I presume the cap to the right (tied to ground) is for filtering, correct?

    Now, moving on into the 10k POT...
    Obviously the upper 6L6 is the adjustable one.

    I've been theorizing how the lower tube's bias supply works.

    The grid on the lower 6L6 gets it's voltage across the 10k and 220k resistors, but it's not clear to me what the role of the 10k resistor tied to the left side of the 10K pot (which also taps in between the 10k and 220k resistors).

    Can you elaborate on that? The AA864 bias circuit is more straight forward to me.

    Thanks,

    Paul
     
  11. John Phillips

    John Phillips Member

    Joined:
    Mar 17, 2002
    Messages:
    13,113
    Location:
    Scotland
    What you effectively have with the two 10K resistors is a 20K pot in parallel with the actual 10K pot, permanently set halfway. This lets you adjust the upper power tube either hotter or colder than the lower one.

    This is plain bad design - even if it was a good idea to only give control of one tube, the correct resistor values would be 5K, to keep the two sides of the circuit symetrical. OK, the difference on the loading of the power tube grids is very small, but if you're going to do it, do it right... since it doesn't take any more complexity than to do it wrong.

    In some ways the best scheme would be to fit another 10K pot and have independent control of the bias for each tube, but in practice it's best to use at least roughly matched tubes, so you don't really need more than a single overall bias.
     
  12. xtian

    xtian Member

    Joined:
    Dec 15, 2010
    Messages:
    841
    Location:
    NorCal
    Can't get my head around this statement, because the 165 and 864 schematics look identical to me. Can anyone point to a Bassman schematic that has a good, if modified, true bias adjustment that I can use to update my Bassman 50?
     
  13. TweeDLX

    TweeDLX Member

    Joined:
    Mar 24, 2008
    Messages:
    3,655
    Location:
    Port Angeles, Washington
  14. zzmoore

    zzmoore Supporting Member

    Joined:
    Sep 22, 2006
    Messages:
    5,579
    Location:
    USA, California, 94585
    Seeing John's name always makes me smile......
    Anyway... xtian - are you looking at the schems.? Do you see the extra 10k resistors on the AB165 drawing.?
     
  15. phsyconoodler

    phsyconoodler Member

    Joined:
    Jul 11, 2005
    Messages:
    4,355
    Location:
    calgary canada
    Just completely convert the bias supply to AA864 specs.Then check the bias voltage to see if it has a range from about -55v to -35v. If you have that range then you are good to go.
    You can also add another pot to convert it to bias adjust AND balance.Useful for some folks.

    Too busy building amps and not well versed in Fender's,eh?Good for you!
     
  16. xtian

    xtian Member

    Joined:
    Dec 15, 2010
    Messages:
    841
    Location:
    NorCal
  17. diagrammatiks

    diagrammatiks Member

    Joined:
    Jan 6, 2010
    Messages:
    2,629

    yes that is right. it's basically just a balancing circuit with bias voltage at the input and the output tubes at the output.

    turned one one it sends all of the voltage to one tube and the other way it sends all of the voltage to the other tube.

    although, there's probably a limiter in there to make sure that you can only send so much to one tube at a time.
     
  18. phsyconoodler

    phsyconoodler Member

    Joined:
    Jul 11, 2005
    Messages:
    4,355
    Location:
    calgary canada
    Quote:"there's probably a limiter in there to make sure that you can only send so much to one tube at a time."

    It's called a bias range resistor.The pot simply balances the two tubes,it cannot alter the bias voltage overall.Not a bad design,just missing the adjust feature.If you had both,the amp would be able to use a wide variety of tubes.
    The AA864 circuit pre-dates the AB165.
     
  19. 1moreknob

    1moreknob Supporting Member

    Joined:
    Mar 20, 2008
    Messages:
    246
    Location:
    Northampton, MA
    I recently acquired an AB165 and want to convert the bias to AA864 spec. Are there detailed instructions anywhere I can use?
    I see the difference in the Layout drawings, but want to be sure I am doing it correctly the 1st time.
    Thanks, Bob
     
  20. smolder

    smolder Silver Supporting Member

    Joined:
    Jun 16, 2008
    Messages:
    10,753
    Location:
    up in the rockies

Share This Page