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Balance pot questions

Discussion in 'Glowing Bottle Tube Amp Forum' started by jtcnj, Sep 8, 2017.

  1. jtcnj

    jtcnj Tele-Meister

    Age:
    53
    353
    Feb 2, 2015
    Brick, N.J. USA
    EDIT: probably belongs in Amp Tech Center

    Silkyn Super 50.
    I would like to add a balance pot.

    1. what type, value, and rating for the pot?
    I'll have to get in there and see what voltages I have across those resistors. Double that for the wattage should be ok, yes?
    I could guess based on the size of the resistors, or is that not an acceptable practice in this situation.

    2. where should I insert this pot?
    I am thinking replace one of the 220k with another value and a pot.

    silkyn bias section.png
     
    Last edited: Sep 8, 2017

  2. Ten Over

    Ten Over Tele-Meister

    175
    May 13, 2015
    Central California
    There is essentially no current flowing across the 220K grid leak resistors. No current means no voltage change when the values are altered.
     

  3. jtcnj

    jtcnj Tele-Meister

    Age:
    53
    353
    Feb 2, 2015
    Brick, N.J. USA
    Now that you mention it, thats something I should have caught, so the wattage is not an issue here.

    ok, so if the bias pot varies the grid voltage at idle, how and where would i place an adjustment point on just 1 tube's idle?
     
    Last edited: Sep 8, 2017

  4. jtcnj

    jtcnj Tele-Meister

    Age:
    53
    353
    Feb 2, 2015
    Brick, N.J. USA
    I'm tempted to swap that 50k pot bias pot out for a smaller value; too touchy on tiny turns.
    But, I'm running the stock 26w 5881/6l6.
    I'll need some range to properly bias a 30w pair I supppose.

    Or, is the way to do this to end up with 2 bias pots - 1 for each tube?

    EDIT: Not much came up the first time i searched this, not sure what I typed.

    I found a few examples to convert to 2 bias pots; I realize 1 affects the other.
     
    Last edited: Sep 8, 2017

  5. Ten Over

    Ten Over Tele-Meister

    175
    May 13, 2015
    Central California
    Attached is an example of a split bias that has minimal interaction between the two pots.
     

    Attached Files:


  6. Silverface

    Silverface Poster Extraordinaire Ad Free Member

    Age:
    65
    Mar 2, 2003
    Lawndale CA
    have you looked at schematics for any of the Fender amps that have a bias balance (not hum balance)? Any of those should get you in the ballpark.
     
    RLee77 and Wally like this.

  7. Ten Over

    Ten Over Tele-Meister

    175
    May 13, 2015
    Central California
    AB568 or AC568 are the Fenders that have balance pots.

    Attached is an example of an adjustable bias with a balance pot. It will probably give you about the same bias range as you had originally.
     

    Attached Files:


  8. Silverface

    Silverface Poster Extraordinaire Ad Free Member

    Age:
    65
    Mar 2, 2003
    Lawndale CA
    Those are the ones I was talking about - I figured they were easy enough to find.

    In my case I rarely install balance pots - I prefer the individual bias system like Rob posted. Seems more logical to me if two pots are going to be used anyway. And I've never installed it but I've worked on a couple of amps with 4 power tubes and 4 bias pots. That allows some interesting tube combination experiments as long as you keep a close eye on plate voltage.
     

  9. Ten Over

    Ten Over Tele-Meister

    175
    May 13, 2015
    Central California
    Amazingly, I found the transformer specifications for the Silkyn Super 50. I did a breadboard version and my Balance Bias example has a bias range that is slightly larger than stock and the stock range is included in the Balance Bias range.

    They are easy enough for you and me to find because we already know where to look. For those less familiar, the circuit numbers will cut the search time way down.
     

  10. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    This is the silverface style bias + balance circuit but note the pot has a center tap:

    [​IMG]
     

  11. Ten Over

    Ten Over Tele-Meister

    175
    May 13, 2015
    Central California
    That's fine as a retrofit for a Fender with "Output Tubes Matching", but the main problem with other implementations is that the center-tapped pots are a little difficult to source.

    The tact in general has a flaw in that changing the balance also changes the main bias setting and changing the main bias setting changes the balance. If you wanted a particular idle current on both sides, you would have to fiddle with it for awhile.

    Bias Bogus Balance.gif
     

  12. jtcnj

    jtcnj Tele-Meister

    Age:
    53
    353
    Feb 2, 2015
    Brick, N.J. USA
    Thanks for the feedback. I dont know why I had trouble finding more on my own, must have missed something.

    I think its going to be 2 bias pots and tweaking back and forth.
    Its kind of like steering a boat.
    The tendency is to over steer and then over correct but you get a feel for it.
     

  13. Bendyha

    Bendyha Tele-Afflicted Ad Free Member

    Mar 26, 2014
    Northern Germany
    You have a firm not far from where you are, that not only seems to have CT pots in stock, but will actually make any sort of custom pot you may desire.

    State Electronics
    36 State Route 10, STE 6 • East Hanover, NJ 07936-0436
    973-887-2550 • Toll Free 1-800-631-8083 • Fax 973-887-1940
    http://www.potentiometers.com/
     

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