Potentiometer Question

Discussion in 'DIY' started by WntrMute2, May 13, 2018.

  1. WntrMute2

    WntrMute2 AK Subscriber Subscriber

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    I am planning on replacing a resistor of 220K ohms with a potentiometer to allow some adjustment in the input of a 6C6 tube. I've had recommendations that range from 47K to 1M. What I can't seem to find is the RANGE of a particular pot. For instance:
    https://www.mouser.com/datasheet/2/142/mw22b-1140575.pdf

    Ideas?
     

     

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  2. whoaru99

    whoaru99 Epic Member

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    The range of a potentiometer will be, generally, 0 to the full indicated value. In other words if you have a 470K pot, the minimum value will be 0 ohms and the max value would be 470K ohms.
     
  3. whoaru99

    whoaru99 Epic Member

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    potentiometer.png

    This is the schematic representation of the potentiometer you linked in. If this were a 470K pot, then it will always measure 470K (considering tolerance, of course) between terminals 1 and 3.

    With the adjustment fully CCW, resistance between 1 & 2 would be theoretically 0 (may actually be slightly non-zero) while between 2 & 3 would be 470K.

    With the adjustment fully CW, resistance between 1 & 2 would be 470K while between 2 & 3 would be 0 (same caveat as above).

    If it were a linear taper pot an adjustment of 1/2 way (1/2 way between fully CCW and fully CW) would result in approximately 50% of the rated value.
    If it were a standard log taper pot an adjustment of 1/2 way would be about 10% or 90% of the rated value, depending measuring 1-2 or 2-3.
     
  4. WntrMute2

    WntrMute2 AK Subscriber Subscriber

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    Boy that was simple! If I had only spent a minute measuring one!
    Thanks very much.
     
  5. C_D

    C_D New Member

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    Pay attention to the pot's curve, a k a taper. Controls which are used to adjust level (volume) need to be audio taper, a k a log taper, "A" suffix versus linear taper "B" suffix. Using linear in place of log will cause all the volume increase to occur in a small rotational range of the control. Linear taper pot can be made to behave similarly to log type by connecting a fixed resistor of 10 to 15% pot value between wiper and one leg. Lastly, when changing cathode resistors in power tube stages, one must pay attention to power rating in watts.
     
  6. WntrMute2

    WntrMute2 AK Subscriber Subscriber

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    Thanks guys. If I'm looking for a cathode resister hum-pot and wold like to go from a one-turn unit to a ten-turn unit to make the extraordinarily sensitive adjustment easier, do I need a audio taper or a plain linear pot? Same question for a pot I plan to use to adjust the amount of signal going to the top-cap of a 6C6 driver tube.
    The hum pot will be 50ohms, while the cap resistor will be around 50K to 1M. (I am unsure of the value I need)
     

     

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  7. C_D

    C_D New Member

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    Hum-adjust pot needs to be linear type, it is not a "volume" control for audio signal. When you say "...top-cap of 6C6..." I am guessing you mean the plate resistor, which also should be a linear pot. You may want to consider connecting a pot in parallel with a fixed resistor (try a pot 5 to 10 times the fixed value), so you can trim down the resultant value. It's a nice way to add flexibility and precision while assuring an open wiper does not cause other problems.
     
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  8. WntrMute2

    WntrMute2 AK Subscriber Subscriber

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    Thanks all. Some pots are on the way.
     

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