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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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    After VTA-70 Board install problems

    sumner59081
    sumner59081


    Posts : 2
    Join date : 2017-07-05

    After VTA-70 Board install problems Empty After VTA-70 Board install problems

    Post by sumner59081 Wed Jul 05, 2017 7:37 pm

    Hello all,
    I've just installed a VTA-70 on my ST-70. I'm not getting the voltages I should and I'm not sure why. Can cap is brand new 80/40/30/20 with the 80 at the back of amp.
    Here's the numbers with rectifier tube installed power on:
    Rectifier tube pins 2 & 8 480v pins 4 & 6 360v
    Can Cap: All lugs are 485v even after the 2.2k resistor! Which I've checked numerous times and desoldered and re-soldered twice.

    So if I install just the rectifier and the 2 EL34 right side tubes and check the bias which is suppose to be .60v it runs away from me and I have to shut it down. I've triple checked the wiring against the schematic and everything is correct. I've checked the diode that replaced the selenium and it checks out good. I'm lost.

    Here's the picture from tubes4hifi's of the diagram.

    Any help is appreciated.

    After VTA-70 Board install problems Img_4911
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    Guest
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    Post by Guest Wed Jul 05, 2017 10:27 pm

    check the 10 OHM 'bias' resistors, one end to pins 1 & 8 on each EL84 socket, the other end to ground/chassis, also make sure that you have a good ground connection on those resistors.
    In general, make sure all connections to your star ground are good.
    Also note, your voltages will all be higher than normal operating levels since the power supply is not yet fully loaded, and only will be once all tubes are installed.
    Make sure that the ground connection for the can cap is good and solid as well.
    Also, check your BIAS supply, DC side, do this check without any EL34's installed.
    pichacker
    pichacker


    Posts : 103
    Join date : 2016-08-01
    Age : 60
    Location : Near to London - UK

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    Post by pichacker Thu Jul 06, 2017 3:17 am

    If I were you I'd leave the EL34's out of their sockets and check that you have a controllable -ve voltage at the grids. Without this the EL34's will be hard on and the 0.5v bias sky high. Not too good for their health either.

    Tad concerned that there is no voltage drop across the 2K2 after valve warm up as this indicates that the driver board is not pulling any current. When you say you've checked the diode that replaced the selenium rectifier this concerns me a little as the bias circuit is now on the board as per the diagram above. Does this differ on your circuit?

    If you can work out how to post photos then this would be best. Take a good quality one of the underside and then the guru's here can help out.
    sumner59081
    sumner59081


    Posts : 2
    Join date : 2017-07-05

    After VTA-70 Board install problems Empty Re: After VTA-70 Board install problems

    Post by sumner59081 Thu Jul 06, 2017 12:14 pm

    Thank you for the suggestions. I have indeed checked 10ohm resistors on each output tube several times and they test well within spec. Almost dead on actually.

    I forgot to mention the power tranny is a brand new PA060 from dynakitparts, also.

    The problem I have is some of the readings I need to check require all the tubes be installed. I can't do that because of the run away voltage issue.

    I'm thinking I could possibly have a bad tube socket, or possibly output tube (it would have to be all 4 since I've swapped them in and out). I know the 5AR4 is good because I swapped the one out of my Heathkit amp to make sure. And worst of all I don't have access to a tube tester anymore.

    It may be in my best interest to start from scratch with all new tube sockets and tubes.

    For my piece of mind and something I can confirm before I tear it all down, can someone verify I have this new cap wired correctly. I've read a few comments from different sites and there seems to be discrepancy in how it's suppose to be wired into the circuit. Photo attached of how mine is wired.

    After VTA-70 Board install problems Cancap10
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    Post by Guest Thu Jul 06, 2017 12:22 pm

    you do not need the power tubes in their sockets to take reasonably close voltage level readings. Did you check the BIAS supply on the DC side?
    The cap connections look good.
    But yes, do check pin tightness in your power tube sockets. How dirty are those socket pins?
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    nmchiefsfan


    Posts : 78
    Join date : 2012-03-21

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    Post by nmchiefsfan Thu Jul 06, 2017 2:35 pm

    Are there tubes in the VTA board when you measure the voltage after the 2.2k resister? Is your capacitor on the VTA board for the bias oriented correctly so that you get between -50 and -62 volts for bias at pin 5 of your tube sockets?

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