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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?

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    leaveit_tubebeaver

    Posts : 2
    Join date : 2020-02-07

    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 on Fri Feb 07, 2020 9:18 am

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

    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 on 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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    Join date : 2020-02-07

    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 on 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
    Bob Latino
    Bob Latino
    Admin

    Posts : 2874
    Join date : 2008-11-26
    Location : Massachusetts

    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 on 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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    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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