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    Dynaco ST70 bias issue

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    wrayman

    Posts : 4
    Join date : 2019-10-16

    Dynaco ST70 bias issue Empty Dynaco ST70 bias issue

    Post by wrayman on Wed Oct 16, 2019 2:10 pm

    Hi all,

    I have the tubes4hifi Classic board installed. Excellent sound. Mishap as follows
    Installed quad of 6CA7's with the #6 pin intact.
    The V3 tube flared and died minutes after biasing to 40ma. Blew the fuse also.
    Reinstalled quad of EL34( no 6 pin). V3 tube lights up, but reads 0.00 on meter. All other tubes are able to be biased.
    As a bonus, there is an audible transformer buzz, not present before.
    Any ideas on what has happened ? I'm not technically inclined and will need a tech to interpret, LOL
    Thanks ! Mike
    peterh
    peterh

    Posts : 1228
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Dynaco ST70 bias issue Empty Re: Dynaco ST70 bias issue

    Post by peterh on Wed Oct 16, 2019 2:36 pm

    wrayman wrote:Hi all,

    I have the tubes4hifi Classic board installed. Excellent sound. Mishap as follows
    Installed quad of 6CA7's with the #6 pin intact.
    The V3 tube flared and died minutes after biasing to 40ma. Blew the fuse also.
    Reinstalled quad of EL34( no 6 pin). V3 tube lights up, but reads 0.00 on meter. All other tubes are able to be biased.
    As a bonus, there is an audible transformer buzz, not present before.
    Any ideas on what has happened ? I'm not technically inclined and will need a tech to interpret, LOL
    Thanks ! Mike
    The cathode resistor ( assuming you have individual bias resistors) is broken, happens easily when a tube flashes. And yes, it should break, it acts as a fast
    fuse to protect output transformer. Replace with similar.

    And Yes, the 6CA7 that "flared" is dead ! Don't try to use it again !!


    Last edited by peterh on Wed Oct 16, 2019 4:56 pm; edited 1 time in total (Reason for editing : Added comment about junking dead tube)
    Bob Latino
    Bob Latino
    Admin

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    Join date : 2008-11-26
    Location : Massachusetts

    Dynaco ST70 bias issue Empty Re: Dynaco ST70 bias issue

    Post by Bob Latino on Wed Oct 16, 2019 3:32 pm

    peterh is most likely correct. You may have blown on that socket the 10 ohm 2 watt resistor that goes from pins 1 and 8 to chassis ground. You can get 5 pieces relatively cheap on Ebay at the link below

    10 ohm 2 watt 1 % resistors on Ebay

    Bob
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    wrayman

    Posts : 4
    Join date : 2019-10-16

    Dynaco ST70 bias issue Empty Re: Dynaco ST70 bias issue

    Post by wrayman on Wed Oct 16, 2019 7:15 pm

    Thank you peterh and Bob. The resistor in question has very wonky measurements, so I'll replace it.
    Thanks for the resistor link Bob, I'll go shopping tonight. Smile

    Cheers, Mike
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    wrayman

    Posts : 4
    Join date : 2019-10-16

    Dynaco ST70 bias issue Empty Re: Dynaco ST70 bias issue

    Post by wrayman on Sat Nov 30, 2019 11:05 am

    Hello all,

    Updated re the transformer "buzz" which started after the bias resistor blew. Repaired the resistor issue, all was sonically well since, except for tranny buzz.
    The other evening this occurred:

    -Loud crackle heard through the speakers after system on for a couple minutes.
    -no sound within a couple seconds
    -transformer went silent, no buzz, but amp still powered up.


    All other components checked with alternate amp, no issues.

    Does the above sound like a dead transformer or something else ?
    Thanks for any thoughts !
    Mike
    peterh
    peterh

    Posts : 1228
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Dynaco ST70 bias issue Empty Re: Dynaco ST70 bias issue

    Post by peterh on Sat Nov 30, 2019 11:22 am

    wrayman wrote:Hello all,

    Updated re the transformer "buzz" which started after the bias resistor blew. Repaired the resistor issue, all was sonically well since, except for tranny buzz.
    The other evening this occurred:

    -Loud crackle heard through the speakers after system on for a couple minutes.
    -no sound within a couple seconds
    -transformer went silent, no buzz, but amp still powered up.


    All other components checked with alternate amp, no issues.

    Does the above sound like a dead transformer or something else ?
    Thanks for any thoughts !
    Mike

    check voltages and follow the fault-diagnostic procedure in the manual.
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    wrayman

    Posts : 4
    Join date : 2019-10-16

    Dynaco ST70 bias issue Empty Re: Dynaco ST70 bias issue

    Post by wrayman on Tue Dec 03, 2019 7:19 am

    thank you peterh.

    I'm sadly lacking in tech skills and cannot do much to check possible problems. Biasing was a big step for me, after years of owning a self biasing amp, LOL

    I may have found a local person who can help, it'll be a few weeks before he can check it out. Until then, a trusty NAD power amp is pressed into service.

    Cheers ! M

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