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    New ST 70 with volume fluctuations

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    fp2000

    Posts : 9
    Join date : 2017-08-23

    New ST 70 with volume fluctuations

    Post by fp2000 on Thu Sep 27, 2018 1:55 pm

    hi. I am kind of new to this forum. I recently built my first Vta st 70 from parts bought from Roy (board and components), dynakits ( transformers, cap can) and I supplied my own chassis.
    The thing turned out fantastic, the sound right from the start was amazing. But after about 40 hours or so I am noticing more and more often subtle temporary fluctuations in volume. This happens at full volume but also at low volume settings. Has anyone here experienced that issue?
    the amps is biased at 0.5 mV reading using KT88s.
    it is strange since I only noticed this yesterday, but it became more and more apparent. Could this be related to power supply ?
    I appreciate any help. thanks
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    peterh

    Posts : 1041
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: New ST 70 with volume fluctuations

    Post by peterh on Thu Sep 27, 2018 2:32 pm

    fp2000 wrote:hi. I am kind of new to this forum. I recently built my first Vta st 70 from parts bought from Roy (board and components), dynakits ( transformers, cap can) and I supplied my own chassis.
    The thing turned out fantastic, the sound right from the start was amazing. But after about 40 hours or so I am noticing more and more often subtle temporary fluctuations in volume. This happens at full volume but also at low volume settings. Has anyone here experienced that issue?
    the amps is biased at 0.5 mV reading using KT88s.
    it is strange since I only noticed this yesterday, but it became more and more apparent. Could this be related to power supply ?
    I appreciate any help. thanks

    I suspect bad connections / solders .
    If it is both channels at the same time the cause is something common, could be
    a central earth-point or simular.

    Poking around with a woodstick and try to provoke the problem.


    fp2000

    Posts : 9
    Join date : 2017-08-23

    Re: New ST 70 with volume fluctuations

    Post by fp2000 on Thu Sep 27, 2018 2:44 pm

    that's what I suspected. I will first check to verify that it happens on both channel at the same time. It just sounds as if it temporarily lost power. It could be a cold solder on the cap can... hopefully something easy to fix.
    I will go from the most basic parts of the circuit and onto the board
    thanks though
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    corndog71

    Posts : 673
    Join date : 2013-03-19
    Location : It can get windy here

    Re: New ST 70 with volume fluctuations

    Post by corndog71 on Fri Sep 28, 2018 9:13 am

    Check speaker connections too. Maybe swap the driver tubes around.

    fp2000

    Posts : 9
    Join date : 2017-08-23

    Re: New ST 70 with volume fluctuations

    Post by fp2000 on Fri Sep 28, 2018 2:49 pm

    so far the center tube swap fixed it. I will keep you posted. tganks for the ideas

    rjpjnk

    Posts : 117
    Join date : 2018-07-18

    Re: New ST 70 with volume fluctuations

    Post by rjpjnk on Mon Oct 01, 2018 10:09 am

    fp2000 wrote:
    the amps is biased at 0.5 mV reading using KT88s.

    .5 mV? Is this a typo?

    fp2000

    Posts : 9
    Join date : 2017-08-23

    Re: New ST 70 with volume fluctuations

    Post by fp2000 on Mon Oct 01, 2018 10:16 am

    sorry. 0.5 volts. divided by 10 ohms comes down to 50 mA per tube
    but everyone knows that right? lol

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