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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


    Red Plate, V6 - two year old ST-120 with original 6550

    dalemurray
    dalemurray

    Posts : 47
    Join date : 2018-09-25
    Location : Wheaton Illinois

    Red Plate, V6 - two year old ST-120 with original 6550 Empty Red Plate, V6 - two year old ST-120 with original 6550

    Post by dalemurray on Tue Oct 06, 2020 1:41 pm

    I did the requisite search and found the 2w 10ohm resistor (R40-41) on V7 has failed.

    I assume I should test all the the remaining tubes to see if they show signs of damage, possibly indicate a larger issue?

    If I understand the correctly, the minimum I should do is:
    - Replace burned resistor
    - Replace burned tube

    Ideally I should:
    - Replace resistor
    - Check rectifier tube
    - Replace all four output tubes

    The amp has been solid for the two years I've had it and has a lot of hours on it.

    I think somebody turned up the Variac powering the amp. I friends daughter, 4 year old, was listening to music several weeks ago and I imagine she may have turned the "big beautiful knob". I cannot say for sure.

    When it happened I immediately cranked it down and killed the power but it seems like the dial was higher than it should be.

    So, lets talk tubes.
    I have been quite satisfied with the Sovtek 6550WE supplied with the amp.
    I am driving Goldenear Triton Fives.
    Any suggestions, and why?

    Thanks
    peterh
    peterh

    Posts : 1434
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Red Plate, V6 - two year old ST-120 with original 6550 Empty Re: Red Plate, V6 - two year old ST-120 with original 6550

    Post by peterh on Tue Oct 06, 2020 2:45 pm

    dalemurray wrote:I did the requisite search and found the 2w 10ohm resistor (R40-41) on V7 has failed.

    I assume I should test all the the remaining tubes to see if they show signs of damage, possibly indicate a larger issue?

    If I understand the correctly, the minimum I should do is:
    - Replace burned resistor
    - Replace burned tube

    Ideally I should:
    - Replace resistor
    - Check rectifier tube
    - Replace all four output tubes

    The amp has been solid for the two years I've had it and has a lot of hours on it.

    I think somebody turned up the Variac powering the amp. I friends daughter, 4 year old, was listening to music several weeks ago and I imagine she may have turned the "big beautiful knob". I cannot say for sure.

    When it happened I immediately cranked it down and killed the power but it seems like the dial was higher than it should be.

    So, lets talk tubes.
    I have been quite satisfied with the Sovtek 6550WE supplied with the amp.
    I am driving Goldenear Triton Fives.
    Any suggestions, and why?

    Thanks

    replace resistor (of course )
    replace at least the affected side with a matched pair, best is to replace all 4
    with a matched quad.  6550 from eurotubes.com delivers well matched tubes with
    warranty.
    I recommend replacement of all 4 as the amp has been in use for 2 years and you say it
    has been in heavy use. Tubes, especially power tubes, are consumable, better replace them
    before they start fail.
    BTW I would recommend KT88 as they will cope better with the gridresistor ( 270k) used
    in vta-120, it's a little high for 6550, KT88 works fine with 270k. Also available at
    eurotubes.com



    Last edited by peterh on Tue Oct 06, 2020 2:49 pm; edited 1 time in total (Reason for editing : Afterthought)

    dalemurray likes this post

    dalemurray
    dalemurray

    Posts : 47
    Join date : 2018-09-25
    Location : Wheaton Illinois

    Red Plate, V6 - two year old ST-120 with original 6550 Empty Re: Red Plate, V6 - two year old ST-120 with original 6550

    Post by dalemurray on Tue Oct 06, 2020 2:59 pm

    When I told my wife about diagnosis today she did not seem phased I would need to replace the tubes. Infact, she said "it might be a good idea to keep a couple extra on hand in the future." <-- I WIN!

    I am not wildly versed in tubes and the difference in sound between the various brands and 6550/kt88/KT120 etc.


    I've been very satisfied with the 6550WE but not afraid to try something else.
    peterh
    peterh

    Posts : 1434
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Red Plate, V6 - two year old ST-120 with original 6550 Empty Re: Red Plate, V6 - two year old ST-120 with original 6550

    Post by peterh on Tue Oct 06, 2020 3:04 pm

    dalemurray wrote:When I told my wife about diagnosis today she did not seem phased I would need to replace the tubes. Infact, she said "it might be a good idea to keep a couple extra on hand in the future."  <-- I WIN!

    I am not wildly versed in tubes and the difference in sound between the various brands and 6550/kt88/KT120 etc.


    I've been very satisfied with the 6550WE but not afraid to try something else.
    Good. Ask eurotubes for a quote of 4 matched kt88
    I think the GZ34 and the 3 pretubes will be alive and not need replacement ( yet)
    As pretubes ECC82 from eurotubes will fit just fine when the day comes ( later)
    The first sign of trouble with pretubes might be hum from leakage filament<>catode ( a
    buzzing sound )
    avatar
    Dale Stevens

    Posts : 154
    Join date : 2014-07-06
    Age : 71
    Location : Loris, SC

    Red Plate, V6 - two year old ST-120 with original 6550 Empty Re: Red Plate, V6 - two year old ST-120 with original 6550

    Post by Dale Stevens on Tue Oct 06, 2020 5:43 pm

    Dale, as Peter said; KT88 , I do the Genatex Gold Lion . What you had is sorta normal, oh well.
    Matched pair at min, quad would be best. ALWAYS, keep a complete set of tubes for trouble shooting. Our (mine ) goto guy for tubes is Jim McShane @mcshanedesign.net. Check him out. Hopes this helps, Dale Stevens
    corndog71
    corndog71

    Posts : 757
    Join date : 2013-03-19
    Location : It can get windy here

    Red Plate, V6 - two year old ST-120 with original 6550 Empty Re: Red Plate, V6 - two year old ST-120 with original 6550

    Post by corndog71 on Tue Oct 06, 2020 8:33 pm

    I had Tung Sol 6550 in my VTA 120 and thought they sounded great.  
    +1 Jim McShane.
    Bob Latino
    Bob Latino
    Admin

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    Join date : 2008-11-26
    Location : Massachusetts

    Red Plate, V6 - two year old ST-120 with original 6550 Empty Re: Red Plate, V6 - two year old ST-120 with original 6550

    Post by Bob Latino on Wed Oct 07, 2020 8:17 am

    peterh wrote:
    BTW I would recommend KT88 as they will cope better with the gridresistor ( 270k) used
    in vta-120, it's a little high for 6550, KT88 works fine with 270k.


    The VTA ST-120 does not have 270K grid resistors. The VTA ST-120 has 100K grid resistors which are appropriate for either 6550, KT88 or KT120 output tubes. It is the VTA ST-70 that has 270K grid resistors. These 270K resistors are appropriate for the EL34 output tubes normally used in "ST-70" type tube amps.

    Bob

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    peterh
    peterh

    Posts : 1434
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Red Plate, V6 - two year old ST-120 with original 6550 Empty Re: Red Plate, V6 - two year old ST-120 with original 6550

    Post by peterh on Wed Oct 07, 2020 12:29 pm

    Bob Latino wrote:
    peterh wrote:
    BTW I would recommend KT88 as they will cope better with the gridresistor ( 270k) used
    in vta-120, it's a little high for 6550, KT88 works fine with 270k.


    The VTA ST-120 does not have 270K grid resistors. The VTA ST-120 has 100K grid resistors which are appropriate for either 6550, KT88 or KT120 output tubes. It is the VTA ST-70 that has 270K grid resistors. These 270K resistors are appropriate for the EL34 output tubes normally used in "ST-70" type tube amps.

    Bob
    Nice to hear, then KT88 is within limits.
    dalemurray
    dalemurray

    Posts : 47
    Join date : 2018-09-25
    Location : Wheaton Illinois

    Red Plate, V6 - two year old ST-120 with original 6550 Empty Re: Red Plate, V6 - two year old ST-120 with original 6550

    Post by dalemurray on Wed Oct 07, 2020 10:29 pm

    Bob, what is your opinion?
    Should I just buy four more tubes and new 10ohm 2w resistor and get on with it?

    Is there something else I should check before powering up with new tubes?

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