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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


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    VTA-70 Bias Resistor

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    Benevox


    Posts : 3
    Join date : 2021-11-23

    VTA-70 Bias Resistor Empty VTA-70 Bias Resistor

    Post by Benevox Mon Oct 09, 2023 12:38 pm

    I finally got around to installing the bias meter kit into a VTA-ST70 I've been using for awhile. The bias meters now read fairly low, about 300 mV, even all the way up on the pots. I feel like as I've modified the amp, I've placed more and more current demands on the bias system, running KT66 output tubes, the octal driver board, and now the meter system. Is it reasonable to assume I need to drop the value of R39 to get into a better place on the pots? Change for a 1K resister? Thanks for your help.
    Seamus
    Seamus


    Posts : 86
    Join date : 2020-03-17

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    Post by Seamus Mon Oct 09, 2023 2:16 pm

    The BIAS is a separate winding on the transformer, so should not change with circuit mods.

    What is the AC voltage @ D1 cathode and voltage @ each end of R39?

    If you replace the pots, multi-turn offer more precision and spread the wear area over a larger region.

    VTA-70 Bias Resistor Bourne10

    Bournes MultTurn Datasheet
    peterh
    peterh


    Posts : 1861
    Join date : 2012-12-25
    Location : gothenburg, sweden

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    Post by peterh Tue Oct 10, 2023 1:03 am

    Benevox wrote:I finally got around to installing the bias meter kit into a VTA-ST70 I've been using for awhile. The bias meters now read fairly low, about 300 mV, even all the way up on the pots. I feel like as I've modified the amp, I've placed more and more current demands on the bias system, running KT66 output tubes, the octal driver board, and now the meter system. Is it reasonable to assume I need to drop the value of R39 to get into a better place on the pots? Change for a 1K resister? Thanks for your help.

    Check the B+ voltage and the voltage at the bias smoothing cap. Too low B+ will make bias voltage low.
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    Benevox


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    Join date : 2021-11-23

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    Post by Benevox Tue Oct 10, 2023 3:27 pm

    As you have surmised, my voltages are all low. Rectifier may work, but not well enough.
    itrfguy
    itrfguy


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    Join date : 2020-10-26
    Location : Lehigh Acres, FL

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    Post by itrfguy Tue Oct 10, 2023 9:33 pm

    There's other posts that address changing resistors on the VTA driver board to get the bias voltage in the middle of the pots. I run the KT66 tubes in my VTA-70 and have had your issue. Here's what tubes4hifi sent me:

    For your reference, the following is what you would change given different tube options.
    R29, 30,31,32 (ST-70 w/ EL34): 150K
    R29, 30,31,32 (ST-120 or 70 w/ 6550): 100K
    Those are the 4 resistors to the outside of each bias pot

    If 150K are not doing the trick, then go to the 100K resistors. E = I x R, lower resistance, lower voltage drop across the resistor, more voltage to the bias test points.
    peterh
    peterh


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    Post by peterh Wed Oct 11, 2023 1:37 am

    Benevox wrote:As you have surmised, my voltages are all low. Rectifier may work, but not well enough.
    What is your B+ voltage ? If it's close to 300Volt you have a broken can cap ( first cap in the chain)
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    Benevox


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    Join date : 2021-11-23

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    Post by Benevox Wed Oct 11, 2023 11:10 am

    I think you're right. We'll see.
    itrfguy
    itrfguy


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    Post by itrfguy Wed Oct 11, 2023 3:21 pm

    Here's a previous post that'll help you troubleshoot your problem.

    https://dynacotubeaudio.forumotion.com/t171-st-70-quad-cap-long-post?highlight=multi+cap+voltages

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