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    VTA conversion went fine till I turned it on, low to mid volume sounds great, past mid volume starts to distort and crackle

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    donkim

    Posts : 5
    Join date : 2016-01-15

    VTA conversion went fine till I turned it on, low to mid volume sounds great, past mid volume starts to distort and crackle

    Post by donkim on Fri Jan 22, 2016 6:39 am

    Bought a used ST 70 and began upgrading to VTA board. Replace all sockets, quad cap, and replaced power xfmr. Using new el34s and gz34 rectifier. Biasing the amp went fine. Turned it on and played great on low volume, as I increased the volume past half way it starts to distorting. Any ideas?

    peterh

    Posts : 679
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: VTA conversion went fine till I turned it on, low to mid volume sounds great, past mid volume starts to distort and crackle

    Post by peterh on Fri Jan 22, 2016 8:16 am

    What's the typical voltages ( as per the st-70 manual ) on the el34 and gz34 ?

    corndog71

    Posts : 451
    Join date : 2013-03-19
    Location : It can get windy here

    Re: VTA conversion went fine till I turned it on, low to mid volume sounds great, past mid volume starts to distort and crackle

    Post by corndog71 on Fri Jan 22, 2016 10:11 am

    What are you using as a source, preamp?

    arledgsc

    Posts : 340
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: VTA conversion went fine till I turned it on, low to mid volume sounds great, past mid volume starts to distort and crackle

    Post by arledgsc on Fri Jan 22, 2016 10:56 am

    Did you replace the 15.6 ohm resistor with individual 10 ohm cathode resistors at the power tubes?

    Are you using low-gain tubes like 12AU7s instead of 12AX7s?

    donkim

    Posts : 5
    Join date : 2016-01-15

    Re: VTA conversion went fine till I turned it on, low to mid volume sounds great, past mid volume starts to distort and crackle

    Post by donkim on Fri Jan 22, 2016 12:14 pm

    Voltages for the gz are pin 2 438v, pin 8 378v, pin 4 370v and pin 6 374v. and el34 pin 3 416v, pin 4 417v, pin 5 40vdc, pin 6 40vdc and pin 8 .4 vdc. I am using a tuner with pre outs. Yes the bias resistors with 10 ohm resistors I am using 6sn7s on the low gain.

    corndog71

    Posts : 451
    Join date : 2013-03-19
    Location : It can get windy here

    Re: VTA conversion went fine till I turned it on, low to mid volume sounds great, past mid volume starts to distort and crackle

    Post by corndog71 on Fri Jan 22, 2016 1:45 pm

    donkim wrote:Voltages for the gz are pin 2 438v, pin 8 378v, pin 4 370v and pin 6 374v. and el34 pin 3 416v, pin 4 417v, pin 5 40vdc, pin 6 40vdc and pin 8 .4 vdc. I am using a tuner with pre outs.  Yes the bias resistors with 10 ohm resistors I am using 6sn7s on the low gain.

    Is the volume on the tuner or the amp?

    peterh

    Posts : 679
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: VTA conversion went fine till I turned it on, low to mid volume sounds great, past mid volume starts to distort and crackle

    Post by peterh on Fri Jan 22, 2016 1:50 pm

    donkim wrote:Voltages for the gz are pin 2 438v, pin 8 378v, pin 4 370v and pin 6 374v. and el34 pin 3 416v, pin 4 417v, pin 5 40vdc, pin 6 40vdc and pin 8 .4 vdc. I am using a tuner with pre outs.  Yes the bias resistors with 10 ohm resistors I am using 6sn7s on the low gain.
    These values seems to be on the spot.
    If all el34 has 0.4V on pin8 ( catode ) then your power supply seems ok.

    And it both channels that distort ? Then
    Do you have any dc komponent from your pre ? st70 and vta70 has no cap on input, dc here will upset
    the working point of the front tube.

    Next i'll check groundings and B+ connections to the vta board.

    donkim

    Posts : 5
    Join date : 2016-01-15

    Re: VTA conversion went fine till I turned it on, low to mid volume sounds great, past mid volume starts to distort and crackle

    Post by donkim on Fri Jan 22, 2016 2:12 pm

    volume in on the tuner. Will check the grounding and b+ connection on the vta.

    deepee99

    Posts : 1333
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: VTA conversion went fine till I turned it on, low to mid volume sounds great, past mid volume starts to distort and crackle

    Post by deepee99 on Fri Jan 22, 2016 7:30 pm

    You could just have a dirty volume pot. Have you tried tuner cleaner or DeOxit on it?

    sKiZo

    Posts : 1310
    Join date : 2013-04-01
    Location : Michigan USA

    Re: VTA conversion went fine till I turned it on, low to mid volume sounds great, past mid volume starts to distort and crackle

    Post by sKiZo on Fri Jan 22, 2016 7:49 pm

    Sounds reminiscent of some issues I had here when I tried adding a meter circuit and floated the ground on one side of each power tube pair. Basically ended up pushing one end and missing the complementary pull, and ended up with half a signal on each channel. OK at lower volumes, but would get all sorts of strange when pushed a bit.

    Double check the schematic and instructions, checking off each step.

    donkim

    Posts : 5
    Join date : 2016-01-15

    Re: VTA conversion went fine till I turned it on, low to mid volume sounds great, past mid volume starts to distort and crackle

    Post by donkim on Fri Jan 22, 2016 10:20 pm

    thank you will try starting from the beginning.

    arledgsc

    Posts : 340
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    Location : SF Bay CA

    Re: VTA conversion went fine till I turned it on, low to mid volume sounds great, past mid volume starts to distort and crackle

    Post by arledgsc on Sat Jan 23, 2016 10:16 am

    Check the negative feedback connection from the transformer outputs to the low-gain board.

    donkim

    Posts : 5
    Join date : 2016-01-15

    Re: VTA conversion went fine till I turned it on, low to mid volume sounds great, past mid volume starts to distort and crackle

    Post by donkim on Sun Jan 24, 2016 11:59 am

    arledgsc you were spot on the feedback connection was a terrible soldering on my part.
    Thanks a million.

    arledgsc

    Posts : 340
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: VTA conversion went fine till I turned it on, low to mid volume sounds great, past mid volume starts to distort and crackle

    Post by arledgsc on Sun Jan 24, 2016 1:17 pm

    Very Happy Sounded like too much gain was the issue. Congrats on your successful conversion!

    tubes4hifi
    Admin

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    Join date : 2008-11-30

    Re: VTA conversion went fine till I turned it on, low to mid volume sounds great, past mid volume starts to distort and crackle

    Post by tubes4hifi on Mon Jan 25, 2016 5:08 pm

    way to go, arledgsc, thank's for getting the right answer!!

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