The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Dynaco VTA tube amp kits, all Tubes4hifi.com products and all Dynakitparts.com products

    New tubes, ST-70 transformers running hot

    Share

    breading

    Posts: 22
    Join date: 2010-01-07

    New tubes, ST-70 transformers running hot

    Post by breading on Tue Mar 09, 2010 8:21 pm

    Hello,

    I just swapped out my Ruby EL34s for Genalex KT-77s and they sound great, but my power transformer is running very hot. The output transformers seem warm also, but not nearly as hot as the output transformer. The transformers ran cool to the touch when I had the Ruby's in it. The unit is a stock ST. I have biased the quads at 1.25VDC.

    Any ideas?

    Thanks,

    Bob Latino
    Admin

    Posts: 1879
    Join date: 2008-11-26
    Location: Massachusetts

    Re: New tubes, ST-70 transformers running hot

    Post by Bob Latino on Wed Mar 10, 2010 4:27 am

    breading wrote:Hello,

    I just swapped out my Ruby EL34s for Genalex KT-77s and they sound great, but my power transformer is running very hot. The output transformers seem warm also, but not nearly as hot as the output transformer. The transformers ran cool to the touch when I had the Ruby's in it. The unit is a stock ST. I have biased the quads at 1.25VDC.

    Any ideas?

    Thanks,

    KT77's draw noticeably more current than EL34's. I am not sure how much more but if you search the internet you may find current draw comparisons between the two tubes. The stock PA-060 power transformer is really undersized. The lamination stack is only 1 1/2 inches high. Newer "upgraded" versions of the PA-060 have thicker gauge wiring and higher stacks which dissipate the heat better.

    You are wise to bias at 1.25 volts instead of 1.56. 1.25 volts gives about 40 milliamps of bias current per output tube instead of the 50 milliamps you would get with the 1.56 volt setting. If you biased your KT77's at 1.56 VDC your power transformer would run even hotter. If you intend to run the KT77's long term you want to think about replacing your power transformer with an upgraded version of the original PA-060.

    Bob

    breading

    Posts: 22
    Join date: 2010-01-07

    Re: New tubes, ST-70 transformers running hot

    Post by breading on Wed Mar 10, 2010 1:38 pm

    Hi Bob,

    Thanks for the input.

    Since the ST70 runs so hot when biased at 1.25 VDC, is it possible to bias it even lower to achieve a cooler running operation? If so, does it affect performance? Finally, will "running hot" affect other components of the amp, for example, the output transformers, etc.

    Thanks for your help.


    Bruce

    Bob Latino
    Admin

    Posts: 1879
    Join date: 2008-11-26
    Location: Massachusetts

    Re: New tubes, ST-70 transformers running hot

    Post by Bob Latino on Wed Mar 10, 2010 1:56 pm

    Hi Bruce,

    I wouldn't go below a bias of 1.1 VDC on a stock ST-70 which will give about 35 milliamps per each output tube. Going below 35 milliamps bias current on a EL34 or KT77 will start to take these tubes out of their proper operating range. A 6L6 or the new Russian 6P3SE, however, is plug compatible with an EL34 and will operate nicely in the 30 to 35 milliamp range.

    Bob

      Current date/time is Tue Jul 22, 2014 6:20 am