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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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    Output Tube Arcing?

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    medphys


    Posts : 3
    Join date : 2020-04-05

    Output Tube Arcing? Empty Output Tube Arcing?

    Post by medphys Tue Apr 28, 2020 11:03 am

    I recently finished assembling a VTA ST-70 kit and have been using it trouble-free for a couple of weeks. Yesterday, I was moving some components around and the RCA cable from the preamp output jacks popped out. When I went to plug it back in, the tip of one of the connectors touched the ground of the jack on the preamp and one of the EL34 output tubes on the ST-70 started glowing bright blue. I yanked the RCA cable out and the tube went back to normal but I powered everything down before reconnecting the cable. It all powered up normally and sounds fine but I am confused what happened. Was this caused by the input to the ST-70 shorting to the ground of the preamp and if so, why? Or was it just a coincidence and indicates a defective tube? The preamp is an Elekit TU-8500 with a tube output stage if that makes a difference. Thanks for any ideas!
    Bob Latino
    Bob Latino
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    Location : Massachusetts

    Output Tube Arcing? Empty Re: Output Tube Arcing?

    Post by Bob Latino Tue Apr 28, 2020 11:37 am

    medphys wrote:I recently finished assembling a VTA ST-70 kit and have been using it trouble-free for a couple of weeks. Yesterday, I was moving some components around and the RCA cable from the preamp output jacks popped out. When I went to plug it back in, the tip of one of the connectors touched the ground of the jack on the preamp and one of the EL34 output tubes on the ST-70 started glowing bright blue. I yanked the RCA cable out and the tube went back to normal but I powered everything down before reconnecting the cable. It all powered up normally and sounds fine but I am confused what happened. Was this caused by the input to the ST-70 shorting to the ground of the preamp and if so, why? Or was it just a coincidence and indicates a defective tube? The preamp is an Elekit TU-8500 with a tube output stage if that makes a difference. Thanks for any ideas!

    On the back of the parts list that comes with each VTA amp kit it mentions about not connecting or disconnecting interconnects with the amp ON. You disconnection was accidental but you should have shut the amp OFF before you made the reconnection of your interconnect. A disconnection or reconnection with the amp ON can cause a voltage spike which can take out a tube. If your amp still plays fine, then your reconnection with the amp ON probably did not cause any damage to the amp or the tubes.

    Bob
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    medphys


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    Join date : 2020-04-05

    Output Tube Arcing? Empty Re: Output Tube Arcing?

    Post by medphys Tue Apr 28, 2020 12:38 pm

    Thanks for the info, Bob. How is the voltage spike caused? I'm a newbie at tube circuits and I don't really know how they differ from solid state.
    Bob Latino
    Bob Latino
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    Posts : 3260
    Join date : 2008-11-26
    Location : Massachusetts

    Output Tube Arcing? Empty Re: Output Tube Arcing?

    Post by Bob Latino Tue Apr 28, 2020 1:57 pm

    medphys wrote:Thanks for the info, Bob. How is the voltage spike caused? I'm a newbie at tube circuits and I don't really know how they differ from solid state.

    You can send a voltage spike into the amp if the center pin of the interconnect contacts the input jack only on the center pin of the input jack without the ground on the outer shield of the interconnect connect to the ground on the amp.

    Bob

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