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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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    Mark 3 fuse blows after hours of use

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    jrethorst


    Posts : 39
    Join date : 2009-02-01

    Mark 3 fuse blows after hours of use Empty Mark 3 fuse blows after hours of use

    Post by jrethorst Fri Jul 29, 2022 8:46 pm

    I just had my two Mark 3 amps restored by someone who seemed to know his stuff. But the fuses blew after hours of use. I turned them on, listened for quite a while, then left them on without an input signal while I did something else for an hour or two. Came back and both fuses were blown. What would do that?

    John R.
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    jrethorst


    Posts : 39
    Join date : 2009-02-01

    Mark 3 fuse blows after hours of use Empty Re: Mark 3 fuse blows after hours of use

    Post by jrethorst Fri Jul 29, 2022 8:49 pm

    Another issue -- one amp sometimes blows its fuse within a minute of turn-on. I noticed a little bit of hum from this amp as well.

    Thanks,
    John
    peterh
    peterh


    Posts : 1689
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Mark 3 fuse blows after hours of use Empty Re: Mark 3 fuse blows after hours of use

    Post by peterh Sat Jul 30, 2022 5:18 am

    jrethorst wrote:I just had my two Mark 3 amps restored by someone who seemed to know his stuff. But the fuses blew after hours of use. I turned them on, listened for quite a while, then left them on without an input signal while I did something else for an hour or two. Came back and both fuses were blown. What would do that?

    John R.

    6550 tubes of some brands has problems with overcurrent.
    6550 spec states that the grid resistor should be less then 50k. In MkIII
    it is 100k
    https://frank.pocnet.net/sheets/127/6/6550.pdf

    The end effect is that some tubes will start emitting grid current, and as this
    draws the grid more positive ( i.e. less negative) this accelerates the current in
    the tube, which increases temperature of the grid which in turn increases grid current.

    KT88 spec allows 100k while using fixed bias
    https://frank.pocnet.net/sheets/086/k/KT88.pdf


    What tubes do you have ?


    Last edited by peterh on Sat Jul 30, 2022 5:19 am; edited 1 time in total (Reason for editing : spelling)
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    jrethorst


    Posts : 39
    Join date : 2009-02-01

    Mark 3 fuse blows after hours of use Empty Re: Mark 3 fuse blows after hours of use

    Post by jrethorst Sat Jul 30, 2022 10:30 am

    peterh wrote:What tubes do you have ?

    KT88, new.

    Thanks,
    John
    peterh
    peterh


    Posts : 1689
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Mark 3 fuse blows after hours of use Empty Re: Mark 3 fuse blows after hours of use

    Post by peterh Sat Jul 30, 2022 12:07 pm

    jrethorst wrote:
    peterh wrote:What tubes do you have ?

    KT88, new.

    Thanks,
    John

    What make of kt88 ?
    Do you still have your previous tube set ? Will the amp stay stable with them ?
    And, how is the bias at 2 minutes after power on
    30 min and 60 minuters.


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    jrethorst


    Posts : 39
    Join date : 2009-02-01

    Mark 3 fuse blows after hours of use Empty Re: Mark 3 fuse blows after hours of use

    Post by jrethorst Sat Jul 30, 2022 10:00 pm

    > What tubes do you have ?

    KT88, new.

    > What make of kt88 ?

    RF Parts Company

    > Do you still have your previous tube set ?

    No.

    > And, how is the bias at  2 minutes after power on 30 min and 60 minuters.

    I'll check.
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    jimzim429


    Posts : 7
    Join date : 2017-02-08

    Mark 3 fuse blows after hours of use Empty Re: Mark 3 fuse blows after hours of use

    Post by jimzim429 Wed Aug 17, 2022 8:41 am

    Do the fuses blow showing spatter or do they sort of melt open circuit? Also has the power supply capacitance been increased greatly? Do you run a tube or solid state high voltage rectifier? Increased bias current can be an issue but there may be others.
    Bob Latino
    Bob Latino
    Admin


    Posts : 3172
    Join date : 2008-11-26
    Location : Massachusetts

    Mark 3 fuse blows after hours of use Empty Re: Mark 3 fuse blows after hours of use

    Post by Bob Latino Wed Aug 17, 2022 1:33 pm

    jrethorst wrote:I just had my two Mark 3 amps restored by someone who seemed to know his stuff. But the fuses blew after hours of use. I turned them on, listened for quite a while, then left them on without an input signal while I did something else for an hour or two. Came back and both fuses were blown. What would do that?

    John R.

    Anytime a Dynaco type tube amp blows a fuse for no apparent reason after being on for a period of time, try replacing the rectifier tube. Just because a rectifier tube "lights up" is no indication that that the rectifier is good and can produce enough DC long term for the amp.

    Bob

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