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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


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    Added gold lion kt77 to my VTA st-70 and the lowest I can bias is .500 what can I do?

    leaveit_tubebeaver
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    leaveit_tubebeaver


    Posts : 2
    Join date : 2020-02-07
    • Post n°1

    Added gold lion kt77 to my VTA st-70 and the lowest I can bias is .500 what can I do? Empty Added gold lion kt77 to my VTA st-70 and the lowest I can bias is .500 what can I do?

    Post by leaveit_tubebeaver Fri Feb 07, 2020 9:18 am

    I'm using the recommended rectifier and my el34 have no problem with bias.
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    Bob Latino
    Bob Latino
    Bob Latino
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    Join date : 2008-11-26
    Location : Massachusetts
    • Post n°2

    Added gold lion kt77 to my VTA st-70 and the lowest I can bias is .500 what can I do? Empty Re: Added gold lion kt77 to my VTA st-70 and the lowest I can bias is .500 what can I do?

    Post by Bob Latino Fri Feb 07, 2020 9:41 am

    leaveit_tubebeaver wrote:I'm using the recommended rectifier and my el34 have no problem with bias.

    First > On section 3 of page 11 of the VTA ST-70 manual it mentions that KT77 output tube can be biased a little higher than EL34 output tubes. Use a bias setting of .450 VDC for KT77 output tubes. If you cannot get to .450 VDC on all of your KT77 output tubes, you will have to remove R39 which is now a 7.5K resistor and replace this 7.5K resistor with a resistor of a lower value. R39 is near the back edge of the driver board between the two small electrolytic capacitors that stick up from the bottom of the driver board. Try a 5K or a 3.5K resistor (1/2 watt) for R39 and see if your KT77's will now bias properly.

    Bob
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    leaveit_tubebeaver
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    leaveit_tubebeaver


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    • Post n°3

    Added gold lion kt77 to my VTA st-70 and the lowest I can bias is .500 what can I do? Empty Re: Added gold lion kt77 to my VTA st-70 and the lowest I can bias is .500 what can I do?

    Post by leaveit_tubebeaver Sat Feb 08, 2020 9:00 pm

    Thanks Bob. I bought this with the vta installed already and never thought to check the manual. Is running a 5u4gt not advised? I never feel like I see a straight answer, and with that tube I can get these tubes to 450. Is that rectifier just bad on the power trans over a long period of time? I have the original iron
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    Bob Latino
    Bob Latino
    Bob Latino
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    Posts : 3272
    Join date : 2008-11-26
    Location : Massachusetts
    • Post n°4

    Added gold lion kt77 to my VTA st-70 and the lowest I can bias is .500 what can I do? Empty Re: Added gold lion kt77 to my VTA st-70 and the lowest I can bias is .500 what can I do?

    Post by Bob Latino Sat Feb 08, 2020 9:07 pm

    leaveit_tubebeaver wrote:Thanks Bob. I bought this with the vta installed already and never thought to check the manual. Is running a 5u4gt not advised? I never feel like I see a straight answer, and with that tube I can get these tubes to 450. Is that rectifier just bad on the power trans over a long period of time? I have the original iron

    Yes - You can use a 5U4 as an alternate rectifier in the VTA ST-70 (or an original Dynaco ST-70). Yes - just rebias the amp as you have done ....

    Bob
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    • Post n°5

    Added gold lion kt77 to my VTA st-70 and the lowest I can bias is .500 what can I do? Empty Re: Added gold lion kt77 to my VTA st-70 and the lowest I can bias is .500 what can I do?

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