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The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


    Bias help. One side not adjusting to proper voltage.

    gjmess
    gjmess

    Posts : 9
    Join date : 2020-10-16

    Bias help.  One side not adjusting to proper voltage. Empty Bias help. One side not adjusting to proper voltage.

    Post by gjmess on Wed Oct 28, 2020 2:40 pm

    Been reading the forum again trying to get an answer to my problem.   Just installed the Bias circuit kit from Dynakit and I cant't get the right (side) bias pot to adjust to 1.56....gets to about 1.38 and then stops even though I still have adjustment left in the pot.  Left side works fine.  Bad pot?   Everything else seems fine.
    peterh
    peterh

    Posts : 1457
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Bias help.  One side not adjusting to proper voltage. Empty Re: Bias help. One side not adjusting to proper voltage.

    Post by peterh on Wed Oct 28, 2020 3:07 pm

    gjmess wrote:Been reading the forum again trying to get an answer to my problem.   Just installed the Bias circuit kit from Dynakit and I cant't get the right (side) bias pot to adjust to 1.56....gets to about 1.38 and then stops even though I still have adjustment left in the pot.  Left side works fine.  Bad pot?   Everything else seems fine.
    It could be the tubes at the right side that is weak. Have your swapped right and left side tubes ?

    If this is not the problem, pull the powertubes and the rectifier tube, turn on and
    measure the voltage at pin 5 of the powertube sockets. There should be a negative voltage
    here that changes when turning the bias pot.
    Does both sides behave equal ?

    gjmess
    gjmess

    Posts : 9
    Join date : 2020-10-16

    Bias help.  One side not adjusting to proper voltage. Empty Re: Bias help. One side not adjusting to proper voltage.

    Post by gjmess on Wed Oct 28, 2020 4:42 pm

    peterh wrote:
    gjmess wrote:Been reading the forum again trying to get an answer to my problem.   Just installed the Bias circuit kit from Dynakit and I cant't get the right (side) bias pot to adjust to 1.56....gets to about 1.38 and then stops even though I still have adjustment left in the pot.  Left side works fine.  Bad pot?   Everything else seems fine.
    It could be the tubes at the right side that is weak. Have your swapped right and left side tubes ?

    If this is not the problem, pull the powertubes and the rectifier tube, turn on and
    measure the voltage at pin 5 of the powertube sockets. There should be a negative voltage
    here that changes when turning the bias pot.
    Does both sides behave equal ?

    Swapped tubes...same result...negative voltage on all pin 5s and it does adjust....although the right pot makes no correction to the voltage for the last 1/4 of the turn...both with the bias and the negative voltage.
    peterh
    peterh

    Posts : 1457
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Bias help.  One side not adjusting to proper voltage. Empty Re: Bias help. One side not adjusting to proper voltage.

    Post by peterh on Wed Oct 28, 2020 5:45 pm

    gjmess wrote:
    peterh wrote:
    gjmess wrote:Been reading the forum again trying to get an answer to my problem.   Just installed the Bias circuit kit from Dynakit and I cant't get the right (side) bias pot to adjust to 1.56....gets to about 1.38 and then stops even though I still have adjustment left in the pot.  Left side works fine.  Bad pot?   Everything else seems fine.
    It could be the tubes at the right side that is weak. Have your swapped right and left side tubes ?

    If this is not the problem, pull the powertubes and the rectifier tube, turn on and
    measure the voltage at pin 5 of the powertube sockets. There should be a negative voltage
    here that changes when turning the bias pot.
    Does both sides behave equal ?

    Swapped tubes...same result...negative voltage on all pin 5s and it does adjust....although the right pot makes no correction to the voltage for the last 1/4 of the turn...both with the bias and the negative voltage.  
    Then check the pot by ohm reading from one end to viper. The pot might be bad.
    gjmess
    gjmess

    Posts : 9
    Join date : 2020-10-16

    Bias help.  One side not adjusting to proper voltage. Empty Re: Bias help. One side not adjusting to proper voltage.

    Post by gjmess on Wed Oct 28, 2020 6:23 pm

    peterh wrote:
    gjmess wrote:
    peterh wrote:
    gjmess wrote:Been reading the forum again trying to get an answer to my problem.   Just installed the Bias circuit kit from Dynakit and I cant't get the right (side) bias pot to adjust to 1.56....gets to about 1.38 and then stops even though I still have adjustment left in the pot.  Left side works fine.  Bad pot?   Everything else seems fine.
    It could be the tubes at the right side that is weak. Have your swapped right and left side tubes ?

    If this is not the problem, pull the powertubes and the rectifier tube, turn on and
    measure the voltage at pin 5 of the powertube sockets. There should be a negative voltage
    here that changes when turning the bias pot.
    Does both sides behave equal ?

    Swapped tubes...same result...negative voltage on all pin 5s and it does adjust....although the right pot makes no correction to the voltage for the last 1/4 of the turn...both with the bias and the negative voltage.  
    Then check the pot by ohm reading from one end to viper. The pot might be bad.
    ok....thanks
    gjmess
    gjmess

    Posts : 9
    Join date : 2020-10-16

    Bias help.  One side not adjusting to proper voltage. Empty Re: Bias help. One side not adjusting to proper voltage.

    Post by gjmess on Thu Oct 29, 2020 9:16 am

    gjmess wrote:
    peterh wrote:
    gjmess wrote:
    peterh wrote:
    gjmess wrote:Been reading the forum again trying to get an answer to my problem.   Just installed the Bias circuit kit from Dynakit and I cant't get the right (side) bias pot to adjust to 1.56....gets to about 1.38 and then stops even though I still have adjustment left in the pot.  Left side works fine.  Bad pot?   Everything else seems fine.
    It could be the tubes at the right side that is weak. Have your swapped right and left side tubes ?

    If this is not the problem, pull the powertubes and the rectifier tube, turn on and
    measure the voltage at pin 5 of the powertube sockets. There should be a negative voltage
    here that changes when turning the bias pot.
    Does both sides behave equal ?

    Swapped tubes...same result...negative voltage on all pin 5s and it does adjust....although the right pot makes no correction to the voltage for the last 1/4 of the turn...both with the bias and the negative voltage.  
    Then check the pot by ohm reading from one end to viper. The pot might be bad.
    ok....thanks
    Removed to pot....8.29K and the wiper goes from 6.1 to 8.29k Looks the the other pot is about the same although I only removed the one not both.
    peterh
    peterh

    Posts : 1457
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Bias help.  One side not adjusting to proper voltage. Empty Re: Bias help. One side not adjusting to proper voltage.

    Post by peterh on Thu Oct 29, 2020 10:57 am

    If the removed pot behaves correct when measures from both sides ( one end to viper, then other end to viper) then adjust the surrounding resistors.
    As the voltage is to high ( to low current in the tubes) you should reduce the bias voltage.
    Identify the 10k fixted resistor that connects from one end of both bias pots to ground, solder
    another 10k resistor across it, giving the effective resistance 5k.
    This will affect both sides, turn down bias pot before power on.
    gjmess
    gjmess

    Posts : 9
    Join date : 2020-10-16

    Bias help.  One side not adjusting to proper voltage. Empty Re: Bias help. One side not adjusting to proper voltage.

    Post by gjmess on Thu Oct 29, 2020 4:37 pm

    peterh wrote:If the removed pot behaves correct when measures from both sides ( one end to viper, then other end to viper) then adjust the surrounding resistors.
    As the voltage is to high ( to low current in the tubes) you should reduce the bias voltage.
    Identify the 10k fixted resistor that connects from one end of both bias pots to ground, solder
    another 10k resistor across it, giving the effective resistance 5k.
    This will affect both sides, turn down bias pot before power on.
    well that did it....thanks for the help.

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