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    What transformer should I replace for running 6550 tubes

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    EugeneHiFi

    Posts : 12
    Join date : 2014-01-19

    What transformer should I replace for running 6550 tubes

    Post by EugeneHiFi on Thu Aug 21, 2014 1:45 pm

    I have the original stock ST 70 with the upgrade VTA board from tube4hifi that using 12AU7s. I like to try using 6550 tubes. Which beefier transformer should I order from Dynakitparts? Besides upgrading the transformer, what else I have to modify for the 6550 tubes to run with asking trouble?

    Thanks

    MontanaWay

    Posts : 767
    Join date : 2014-02-06
    Age : 58
    Location : Cameron, Montana

    Re: What transformer should I replace for running 6550 tubes

    Post by MontanaWay on Thu Aug 21, 2014 2:30 pm

    the way I understand is that you would probably want to upgrade your power transformer to the one used in the 'new' Tubes4HiFi ST70.
    It is a much beefed up unit, the original ST70 power transformers were already at their top limit.
    You may also wish to consider getting the 'bigger' multi cap as part of the power supply upgrade.
    Which rectifier tube are you currently using?
    I also understand that the 6550 and KT88's are quite similar in their specs, the Tubes4Hifi ST70 can easily drive KT88's so it should not present a problem to drive 6550's, you will need to re bias them though.
    So to sum it up, new power transformer, new multi cap, maybe upgrade rectifier tube, should be all that is needed.
    I am sure other members with more knowledge will chime in as well.

    Bob Latino
    Admin

    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: What transformer should I replace for running 6550 tubes

    Post by Bob Latino on Thu Aug 21, 2014 8:02 pm

    As MontanaWay has mentioned, you should not use 6550 or KT88 output tubes in an ST-70 with the STOCK original Dynaco PA-060 power transformer. The VTA ST-70 has an upgraded version of the Dynaco PA-060 power transformer. The way that you can tell if you have a stock ST-70 power transformer is to measure the height of the stack. The original Dynaco PA-060 has a stack lamination of 1 1/2 inches to 1 5/8 inches. Both the VTA PA-060 power transformer and the Dynakitparts version of the PA-060 have stacks over 2 inches and heavier duty wiring which can support higher current flows of 6550 or KT88 output tubes without overheating.

    Bob

    EugeneHiFi

    Posts : 12
    Join date : 2014-01-19

    Re: What transformer should I replace for running 6550 tubes

    Post by EugeneHiFi on Fri Aug 22, 2014 12:45 pm

    Thank you for the info. My ST 70 has the old version of transformer. I will order a new transformer and replace it before I switch over to 6550 tubes.

    Thanks. Very Happy 

    MontanaWay

    Posts : 767
    Join date : 2014-02-06
    Age : 58
    Location : Cameron, Montana

    Re: What transformer should I replace for running 6550 tubes

    Post by MontanaWay on Sun Aug 24, 2014 10:47 am

    EugeneHiFi wrote:Thank you for the info. My ST 70 has the old version of transformer. I will order a new transformer and replace it before I switch over to 6550 tubes.

    Thanks. Very Happy 

    I would also strongly recommend to upgrade the multi cap to 80/40/30/20 @ 525V, same one used in the Tubes4HiFi ST70/120!

    Bob Latino
    Admin

    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: What transformer should I replace for running 6550 tubes

    Post by Bob Latino on Sun Aug 24, 2014 1:37 pm

    MontanaWay wrote:
    EugeneHiFi wrote:Thank you for the info. My ST 70 has the old version of transformer. I will order a new transformer and replace it before I switch over to 6550 tubes.

    Thanks. Very Happy 

    I would also strongly recommend to upgrade the multi cap to 80/40/30/20 @ 525V, same one used in the Tubes4HiFi ST70/120!

    The quad cap as used in all the VTA amp kits in the last 2 years is a 550 volt rated Authenticap from Germany ..

    Bob

    EugeneHiFi

    Posts : 12
    Join date : 2014-01-19

    Re: What transformer should I replace for running 6550 tubes

    Post by EugeneHiFi on Mon Aug 25, 2014 1:35 pm

    Cool, I will order the new quad cap as well. Thank you for the suggestion.


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