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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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    KT-120 electric shock

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    alexbramford

    Posts : 5
    Join date : 2021-03-07

    KT-120 electric shock Empty KT-120 electric shock

    Post by alexbramford Tue Aug 31, 2021 5:20 pm

    After building and running my completely awesome ST-120 amp for almost 6 months now with no issues, and in stable configuration on combinations of 4 x Sovtek 6550s , or 4 x KT88's, various 12Au7's  (all at 118 volts on variac, 0.5 V bias, no blown fuses, etc.) , I was quite surprised to receive a nasty bite when I "inadvertently" grasped the base of a newly rolled KT-120 between thumb and index finger.

    Given what I believe is a correctly constructed amplifier, and correctly manufactured output tube, I'm assuming a number of things:

    1) there should be no risk of electric shock from the metal base of an output tube
    2) there should be no voltage or current present between the metal base of the output tube and the chassis
    3) conversely, if there is voltage present, then there is an issue with the power tube?

    For comparison, I measured voltage / current flow / continuity on correctly functioning KT88's and Sovtek 6550s from the base to chassis, and found none.

    Apart from the obvious lesson that was learned here, am I correct in assuming that this is not the expected behavior of a brand new Tung-Sol KT-120 output tube, or from power tubes in general?
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    Jim McShane

    Posts : 230
    Join date : 2011-10-19
    Location : South Suburban Chicago

    KT-120 electric shock Empty Re: KT-120 electric shock

    Post by Jim McShane Tue Aug 31, 2021 6:41 pm

    Many metal base tubes have a connection from the metal base to pin 1. If you use your Ohmmeter and measure the resistance from the metal base to pin 1 you'll find there is continuity - meaning the KT-120 metal base is indeed connected to pin 1.

    While pin 1 is not connected to any of the internal tube elements, if you use pin 1 as a tie point then whatever you've connected to pin 1 is also connected to the metal base. If what you connected has high voltage on it the tube base will have high voltage on it!

    It's good practice not to use pin one as a tie point for that reason.

    Dave_in_Va and EWALAA like this post

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    alexbramford

    Posts : 5
    Join date : 2021-03-07

    KT-120 electric shock Empty Re: KT-120 electric shock

    Post by alexbramford Thu Sep 02, 2021 10:14 am

    Thanks for your informative response, Jim.

    Voltage appeared at the base  / pin 1 at the same time as the 10 Ω  2 watt  resistor from pin 8 to ground  blew.
    And I did a continuity test and confirmed that pin 1 is indeed connected to the metal base, by design.

    Now it remains for me to diagnose what caused that new KT120 tube to flash over in the first place...

    I'm new to this game, so it's been a good learning experience.
    Bob Latino
    Bob Latino
    Admin

    Posts : 3074
    Join date : 2008-11-26
    Location : Massachusetts

    KT-120 electric shock Empty Re: KT-120 electric shock

    Post by Bob Latino Thu Sep 02, 2021 10:53 am

    alexbramford wrote:Thanks for your informative response, Jim.

    Voltage appeared at the base  / pin 1 at the same time as the 10 Ω  2 watt  resistor from pin 8 to ground  blew.
    And I did a continuity test and confirmed that pin 1 is indeed connected to the metal base, by design.

    Now it remains for me to diagnose what caused that new KT120 tube to flash over in the first place...

    I'm new to this game, so it's been a good learning experience.

    Alex,

    Even new tubes can be bad on arrival. Most tube vendors pack the tubes very well but sometimes stuff happens during shipment that the customer is not aware of ? Vibrations on the truck that carry the box ? The box was thrown from one UPS or USPS employee to another employee while loading the truck. I have received a bad tube in a box of new tubes before. It happens ?

    Bob

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