The Dynaco Tube Audio Forum

Would you like to react to this message? Create an account in a few clicks or log in to continue.
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


3 posters

    KT120 power tubes blowing fuses and bias resistors in the ST-120

    avatar
    alexbramford


    Posts : 5
    Join date : 2021-03-07

    KT120 power tubes blowing fuses and bias resistors in the ST-120 Empty KT120 power tubes blowing fuses and bias resistors in the ST-120

    Post by alexbramford Sun Aug 22, 2021 9:31 pm

    G'Day All,

    As a first post and by way of introduction, I have been lurking for some time now, and I am pleased to be able to benefit from the knowledge of the folks on this forum.

    So I've earlier this year finished building the VTA ST-120 Amplifier, and I'm super pleased with it.
    I'm currently running it with the Brimar CV4003 in V1, and a matched quad set of Genelex Gold Lion KT88's.
    The sound is simply excellent (to my ears, and to my taste), with well-defined bass, definition and texture and "air" between the instruments in the mid-range.
    It's such a great amplifier and it's a major step up in sound quality from the Denon PMA-655R I've been listening to for so long.

    This week I thought I'd try the Tung-Sol KT120 to see what they sound like.
    I've read that other folks are running the KT120s in the ST-120... but alas I am having no success with this setup.

    I can get the KT120s in the left channel to bias correctly at the recommended .500 volts, no fuses or bias resistors blown.

    The first time placing the KT120's in the right channel, there was a low hum in the right speaker, a blue flash in the right rear KT120, and the 10 ohm 2W bias resistor blew (right, front).
    With excellent guidance from Bob Latino, I replaced the 10 ohm 2W bias resistor in in the right hand channel, and the amp played as usual on the KT88s.

    On the second attempt with the KT120s, the VARIAC fuse blew, and the amp went dead.
    Replaced the variac fuse, and the amp on KT88'a all is good again.

    Does this indicate that perhaps one or both of the RHS KT120's is "blown" (whatever that means)?
    And any advice on whether it's worth negotiating a replacement from the seller?

    Hoping someone can help out a noob...

    Many Thanks
    Alex Bramford
    peterh
    peterh


    Posts : 1599
    Join date : 2012-12-25
    Location : gothenburg, sweden

    KT120 power tubes blowing fuses and bias resistors in the ST-120 Empty Re: KT120 power tubes blowing fuses and bias resistors in the ST-120

    Post by peterh Mon Aug 23, 2021 3:13 am

    alexbramford wrote:G'Day All,

    As a first post and by way of introduction, I have been lurking for some time now, and I am pleased to be able to benefit from the knowledge of the folks on this forum.

    So I've earlier this year finished building the VTA ST-120 Amplifier, and I'm super pleased with it.
    I'm currently running it with the Brimar CV4003 in V1, and a matched quad set of Genelex Gold Lion KT88's.
    The sound is simply excellent (to my ears, and to my taste), with well-defined bass, definition and texture and "air" between the instruments in the mid-range.
    It's such a great amplifier and it's a major step up in sound quality from the Denon PMA-655R I've been listening to for so long.

    This week I thought I'd try the Tung-Sol KT120 to see what they sound like.
    I've read that other folks are running the KT120s in the ST-120... but alas I am having no success with this setup.

    I can get the KT120s in the left channel to bias correctly at the recommended .500 volts, no fuses or bias resistors blown.

    The first time placing the KT120's in the right channel, there was a low hum in the right speaker, a blue flash in the right rear KT120, and the 100 ohm 2W bias resistor blew (right, front).
    With excellent guidance from Bob Latino, I replaced the 100 ohm 2W bias resistor in in the right hand channel, and the amp played as usual on the KT88s.

    On the second attempt with the KT120s, the VARIAC fuse blew, and the amp went dead.
    Replaced the variac fuse, and the amp on KT88'a all is good again.

    Does this indicate that perhaps one or both of the RHS KT120's is "blown" (whatever that means)?
    And any advice on whether it's worth negotiating a replacement from the seller?

    Hoping someone can help out a noob...

    Many Thanks
    Alex Bramford

    Any tube that has encountered a "flashover" is suspect and should be replaced.
    Bob Latino
    Bob Latino
    Admin


    Posts : 3101
    Join date : 2008-11-26
    Location : Massachusetts

    KT120 power tubes blowing fuses and bias resistors in the ST-120 Empty Re: KT120 power tubes blowing fuses and bias resistors in the ST-120

    Post by Bob Latino Mon Aug 23, 2021 7:47 am

    alexbramford wrote:

    Does this indicate that perhaps one or both of the RHS KT120's is "blown" (whatever that means)?
    And any advice on whether it's worth negotiating a replacement from the seller?

    Hoping someone can help out a noob...

    Many Thanks
    Alex Bramford

    I can't say for sure but > if that 10 ohm 2 watt resistor that goes from pins 1 and 8 to chassis ground blows, this is usually a sign that the tube that was in that socket is bad. This can happen even with new tubes. Another possibility is that a pin on the socket is not making proper contact with the tube pin. A good candidate for the lost contact would be pin #5 where the bias voltage is injected into the tube to control the tube.

    Bob

    ttocs likes this post


    Sponsored content


    KT120 power tubes blowing fuses and bias resistors in the ST-120 Empty Re: KT120 power tubes blowing fuses and bias resistors in the ST-120

    Post by Sponsored content


      Current date/time is Mon Dec 06, 2021 3:51 pm