Question about using a trim pot to control 2 power tubes

Discussion in 'Fisher' started by thornev, Jan 8, 2019.

  1. thornev

    thornev Well-Known Member

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    gadget - What do you mean "10k on the bottom"? The bottom of what?
     

     

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  2. thornev

    thornev Well-Known Member

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    If that's true then one trim pot side is connected to the 10K resistor, but where is the other side connected? And why a 10K resistor connected to R52 but not one for R53?
     
  3. gadget73

    gadget73 junk junkie Subscriber

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    No. Leave R51 (33k) as it is.
    C- comes from the junction of R51 and C33, same as it does now.
    R52 (22k) gets replaced with a 10k fixed resistor and a 25k pot in series
    or for a less touchy adjustment, use an 18k fixed resistor and a 10k pot,
    20k resistor + 5k pot would also work. Even less touchy, but less adjustment range too.

    the idea is that you want something that adds up to about 22k between a fixed resistor and 1/2 the pot travel.



    basically the pot ends up controlling current flow and thus voltage drop through R51. If the pot dies, you get no voltage drop, so the tube grids get -26 instead of a situation where you get no bias and the tubes fry. I just happen to like that setup better because I've had problems with old pots getting crusty and losing connection while adjusting bias before. Not a lot of fun trying to adjust the bias on something and suddenly your brand new output tube set flashes bright red because the pot hit a dead spot.
     
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  4. thornev

    thornev Well-Known Member

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    Thanks, gadget. Last question... Where does the trim pot wiper lead go? The trim pot side leads go to the 10K and the 33K (R51), right?
     
  5. UncleBingo

    UncleBingo AK Subscriber Subscriber

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    1,809
    If you're asking about the diagram shown in post #22. 1 leg goes to the 10k resistor. The wiper and the other leg on the pot are connected together and go to the same place - the R53 junction.
     
  6. UncleBingo

    UncleBingo AK Subscriber Subscriber

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    1,809
    If using the type in post #13, the solution is shown a couple posts below it: "Yes, the 25K is a potentiometer of some type. The wiper of it connects to the C- point at the junction of R31 and R32."
     

     

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  7. thornev

    thornev Well-Known Member

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    Uncle Bingo - If you read post 24, apparently neither of the diagrams is what gadget intended.
     
  8. UncleBingo

    UncleBingo AK Subscriber Subscriber

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    You'll have to wait for Gadget, I guess.
     
  9. gadget73

    gadget73 junk junkie Subscriber

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    Wiper ties to one end of the pot. If the pot works in the opposite direction from how you want, just change which end the wiper ties to.
     
  10. 1rebmem

    1rebmem AK Subscriber Subscriber

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    Like this?

    Screen Shot 2019-01-09 at 8.33.55 PM.png

    I see what you were after....
    If the wiper of the pot goes open, C- floats up to full negative bias.

    Jef
     
    Last edited: Jan 9, 2019
  11. thornev

    thornev Well-Known Member

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    That 10K pot bottom lead has to go somewhere, Jef. Gadget said it ties to the wiper. I don't know how that is displayed in a schem. Maybe turns right to the 18K? Damn, and I just bought 25K pots today.
     

     

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  12. 1rebmem

    1rebmem AK Subscriber Subscriber

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    Gadget, This is why I drew it with one end of the pot not connected. If the pot wiper opens then current flow thru R51 ceases.
    If I tie the wiper to one end of the pot, current flow continues if the pot wiper opens and the bias voltage won't go full negative as desired.

    Seems to me one end of the pot should have no connection? Yes....no?

    Jef
     
    Last edited: Jan 9, 2019
  13. 1rebmem

    1rebmem AK Subscriber Subscriber

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    Not necessarily......
     
  14. 1rebmem

    1rebmem AK Subscriber Subscriber

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    You can still use the 25K pot.
    I'll redraw it.
     
  15. thornev

    thornev Well-Known Member

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    I just found a stash of 10K pots (3/4 turn). And I have some 1/4 watt 20K resistors.
     
    Last edited: Jan 9, 2019
  16. gadget73

    gadget73 junk junkie Subscriber

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    if you tie it, the bias will go negative enough. Basically as long as it doesn't go to zero volts and instead goes more negative you're good. but yeah, post 30 has the idea.
     

     

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  17. thornev

    thornev Well-Known Member

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    OK so then physically it goes like this... The 10K trim pot's wiper lead and side lead are connected together and they connect in the hole for the removed R52 that is closest to R32 and C-. The other trim pot side lead is not connected anywhere. It's taped up. The new 20K resistor (that's what I have in stock) has one lead connected to the removed R52 hole that is farthest away from C- and the other 20K lead is connected to the trim pot wiper (which is connected to one of the trim pot side leads and is also connected to the R52 hole closest to R32 and C- and to the R51 hole closest to C-).

    Another way to put this is that the trim pot wiper and side lead, which are tied together, are connected to BOTH R52 and R51 holes closest to C-. And the new 20K is connected between the wiper and the R52 hole farthest away from C-.

    Do I have it correctly?

    Thank you so much, Everyone, for your help ! I have learned so much in 1 year which is when I started this journey with my first Fisher. Thorne
     
    Last edited: Jan 9, 2019
  18. thornev

    thornev Well-Known Member

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    857
    Location:
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    I edited post 37 many times so don't rely on the text in an email if you are subscribed to this thread.
     
  19. 1rebmem

    1rebmem AK Subscriber Subscriber

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    This represents the drawing in post #30 except for one resistor and the pot which were changed to represent what you have in "stock", per post #35.

    Screen Shot 2019-01-10 at 6.56.31 AM.png
     
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  20. thornev

    thornev Well-Known Member

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    Thanks, Jef. Looks like I have a weekend project !
     

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